FREQUENTLY ASKED QUESTIONS

Optical Performance

Rattling of the lens

Actually, the lenses in our EDF refractors sit inside an inner cell that is then inserted into the main cell with cork spacers all around and in the back. The lens itself is held in alignment by the inner cell, and the elements cannot move with respect to each other. The whole assembly sits inside the outer cell and rests on 6 small cork spacers (visible if you look through the front of the lens). The cork spacers in the back will give a little with time, and this allows the lens + inner cell assembly to move forward slightly when the scope is held upside down. The retaining ring in the front of the cell is there merely to keep the inner assembly from falling out, and has no alignment purposes. It could theoretically be dispensed with if the scope was never turned upside down (similar to a Dobsonian mirror which is placed on its cell, but not held down rigidly with any kind of retaining clips). If it bothers you, there is a rear retaining ring that can be adjusted to eliminate the play, but it must be carefully done so that there is no pressure at all on the lens "sandwich" otherwise you risk introducing strain into the lens, and astigmatism into the image. Simply remove the lens cell from the front of the tube, place upside down on a table, loosen the 3 screws on the retaining ring, turn the ring until the cork makes contact with the glass, re tighten the 3 screws, which locks the ring in place, and re-insert the lens cell into the front of the telescope tube. Takes less than 5 minutes, does no good, but will eliminate the clicking.

On the subject of optical testing and rating:
Much controversy now exists between claims from various manufacturers about the wave front rating of optics, especially APO lenses. Manufacturers use interferometers to compare the wave front errors of a finished optic against some reference standard. In the case of the interferometer, it is a reference sphere of known high quality which is used to form interference fringes with the optic under test. When testing a mirror, it would not matter what wavelength was used, since mirrors are totally achromatic. In the case of lenses, it matters greatly what wavelength is used, since there is typically only one point in the wavelength range where the lens was nulled or figured by the optician. Testing at another wavelength almost always results in a lower wavelength rating. There are numerous methods for measuring and interpreting the results, so that testing the same optic on several different interferometer systems can result in different wavefront numbers. Typically, an optician will adjust the interferometer to display 6 to 8 lines across the aperture. Computer software then measures the deviation of these lines from parallelism, and assigns wavefront errors to the interferogram. These are sometimes further broken down to elements such as spherical error, astigmatism, coma, etc. This is repeated a number of times with the fringes tilted to various angles, and the reference optics, mirrors and beamsplitters may be rotated to eliminate any possibility of local errors being added to the test optic. The results are averaged in order to get a more accurate and realistic picture of the aberrations. These averaged results usually have the same RMS rating, but may result in better P-V ratings due to the cancellation of systemic errors. There are some systems that use hundreds and even thousands of very tightly spaced fringes (Zeiss interferometer system in particular), so that there will be absolutely no gaps across the aperture that is not measured. The RMS ratings will again be similar, but the P-V will be overly pessimistic. I now quote from the "Bible" of optical shop testing. Malacara states: "The P-V error must be regarded with some skepticism, particularly when it is derived from a large number of measured data points, as is the case with phase shifting interferometry. Even relatively large wavefront errors often have little effect on the optical performance if the error involves only a very small part of the aperture. Because the P-V error is calculated from just two data points out of possibly thousands, it might make the system under test appear worse than it actually is. The RMS error is a statistic that is calculated from all of the measured data, and it gives a better indication of the overall system performance". I cannot speak for the method used by other manufacturers to rate their optics, but the following is the method I use. The optic is normally nulled or figured to the best possible inside/outside diffraction pattern on a double pass autocollimator using a green light pinhole laser source and a separate white light source. The pattern is checked for roundness (absence of astigmatism and coma) and smoothness (absence of roughness and zones). The reference element is then inserted into the optical path, and the corresponding fringes are captured on the computer using a small CCD camera. I use QuickFringe software developed in Canada and recommended by Peter Ceravolo, who also made my reference elements. These elements were certified by equipment at the Canadian Bureau of Standards. Many passes are taken to nullify the effects of dust particles and slight air disturbances in the optical path and the results are averaged. If the optic passes the 1/10 wave P-V criteria, it is ready for coating and assembly. If not, it continues to be figured and tested until it does pass. The test data in the form of the Zernicky Polynomials is stored on the computer, along with the serial number and other pertinent data, so that if a question arises in the future about some lens, it can be looked up in the database. In day to day testing, the optician can pretty quickly tell whether a set of fringes will meet the performance goals or not. The fringe patterns that are recorded on the computer screen will not be absolutely clean, even if the optic under test is perfect. There always exists dust particles on the reference elements and autocollimating mirrors, as well as on the beam splitter cubes and laser collimating optics. Since interferometers are analog devices, this is akin to the clicks and pops that appear on vinyl phonograph records. This "noise" can cause the software to add spurious data points to the fringes where none should be, and this will normally lower the P-V rating, but again, the RMS is unaffected. In order to get a fair rating for the optics, I average multiple passes, something that Peter Ceravolo has recommended to do. Just as we would not downgrade the performance of the Chicago Symphony for every little recording noise, so I do not downgrade the performance of our optics because of interferometer noise. In fact, Quick Fringe software allows for the averaging of the various interference fringe runs, and then synthesizes a clean pattern that can be displayed in the final report. Some may call this a "fake" interferogram, but it is based on real data and is more representative of the actual performance than any one fringe pattern. Anyone who has watched in frustration the dancing Airy disc interference pattern in his telescope on an unsteady night can appreciate that the actual performance of the optic is not represented by a single snapshot of that pattern. Rather, if an average of many patterns is combined, the true picture will emerge. May I summarize then, that the P-V wavefront ratings given by various manufacturers can differ considerably for the same optic. It depends on the test method used, and whether or not a single set of fringes is measured, or an average of many is used, and whether the software creates 100 test points or tens of thousands. They can vary also for RMS, if the optic is tested at different wavelengths, and has refractive elements in it. This is the case with short focus refractors, SCTs, less so with Maksutov systems. Variation can crop up if the optic is not supported properly, whether it was allowed to stabilize after being placed on the interferometer (the warmth of your hands will significantly distort the figure of the lens for some time) and whether the air is stable in the test setup. False readings can be obtained if rigorous test standards and procedures are not used. It seems then that these numbers are useless to the final user. They are, however powerful tools to the experienced optician, even if they are less than enlightening to the final user. Rather, he should be concerned with whether the manufacturer has manufactured the optics well,  than whether they meet or exceed a certain number. The test certificate is just a piece of paper, after all, and cannot gather light and resolve fine planetary detail. Secondarily, customers can and will misinterpret test data. Unless you have been on the front lines and actually used an interferometer to test and figure an optic, it would be difficult to know what all the numbers and squiggles on that piece of paper really mean. For these reasons, Astro-Physics does not supply a test report, even though each lens'  test data is recorded and stored at our facility.

More on the subject of optical testing and rating:
The question comes up a lot: what does 1/5 wave P-V, 1/10 wave P-V, RMS, 95 Strehl, etc actually mean in performance? For that reason I have posted two images of an actual lens under test during the figuring stage.
Two interferometer test data for lenses of .95 and .99 Strehl ratio.

.95 Strehl ratio (above)
.99 Strehl ratio (above)

A little explanation of these images. All tests were done in green light, recorded with a black and white video camera and captured with Quick-Fringe software. They are artificially colored red presumably because all interferometers are assumed to be run with red lasers and so the Quick Fringe software authors naturally chose red as their screen colors. Nevertheless, they were taken in green light. The first image shows one of our lenses as it comes off the polishers with no figuring. It shows classic 5th order spherical with an overall correction of 1/5 wave P-V, .035 RMS and .95 Strehl. It has approximately 1/8 wave astigmatism also. On the right is the star test pattern in double pass autocollimation. Our interferometer shows both star test patterns and interferograms simultaneously. The star test pattern shows the error twice as large as would be seen at the eyepiece because of the double pass (the light goes thru the lens twice for higher sensitivity). The second image shows the same lens after hand figuring to bring it to our spec of better than .02 RMS (1/50 RMS) with astigmatism also improved. The lens measures .99 Strehl with 1/11 wave P-V. You can see that the .95 Strehl has differing inside/outside patterns (upper is inside, middle is focused Airy disc, bottom is outside focus), whereas the .99 Strehl has closer to equal patterns. The Airy disc of the .95 Strehl has some stray light surrounding, where the .99 Strehl lens has
very little if any stray light. Again, note that these errors are all twice as bad as when the lens is used on the night sky - this is a double pass test.
The main difference in the two lenses is the amount of stray light which cuts contrast and causes bright stars to bloat in a CCD image when you stretch the image to bring out fainter details. The difference for me is the amount of hand correcting required to get there from a machine polished lens.
This is a test, an actual test (of a real lens).


Roland Christen

Out of Focus Color

I've read Roland's excellent essay on "Star Testing Complext Optical Systems" and "Notes on Star Testing Refractors". But what does in means when one APO shows no colors in focus, but shows colors outside of focus while another APO show no colors at all in or out of focus?

Showing color outside of focus means that the basic color correction is not quite flat. However, for any given configuration, whether doublet or triplet, there is a tradeoff between perfect color correction and spherochromatism. Some designers prefer better sphero-chromatism over perfection of color error. Better spherochromatism means that the image formed by the different colors when in focus will be just a tad sharper and crisper.
While color correction can be seen relatively easily by racking the focus in or out, spherochromatism is hard to measure without using narrow band filters and setting the optic up on a measuring bench. I would worry less about out of focus color and concentrate on the image in focus showing good contrast and crisp definition.

Roland Christen


Atmospheric refraction
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When I look at the moon, there is a brilliant yellow-green fringe around the perimeter which changed to blue as the moon was moved through the field of view. In other words, if the limb was in the upper part of the FOV, it was yellow-green and in the lower part of the FOV, it was violet. The same was true for Jupiter, except that the image of the globe was yellow-green one one side and violet on the other (just the extreme edge). This was at any magnification (different eyepieces) First, rule out atmospheric effects by going to a star straight up. Use a good eyepiece at medium to high power. Do not use a prism diagonal. Place the star in the center of the field. Is there a color separation (different color on one side of the star vs. the other)? If not, your refractor is not at fault. If there is, your lens could be tipped. Collimate the lens cell using a Cheshire eyepiece. There are other possible causes of color separation (lateral color). Low-power, wide-field eyepieces are notorious for adding lateral color if the image is off-center. Atmospheric refraction adds color to objects as high as 45 degrees off the horizon. The closer to the horizon, the more lateral color. Prism diagonals will add color, especially if they are not well-centered (cheap imports may have this problem).

I have a couple of questions concerning the oil in the apo triplet design.
What is the durability of this design? Very durable.
Is the oil sealed more or less permanently or is there any maintenance needed as the objective lens ages? Thank you so much for your time.The oil is sealed and will not come out unless the glass is heated excessively in an oven (above 150 degrees F) for long periods of time. The glass in the lenses will never age or deteriorate. The oil can be replaced easily (like recoating a mirror) and the lens will be the same as new.

Astro-Physics mounts with Quartz Micro Drive Controller (400QMD, 600EQMD, 900QMD, 1200GTO)

I have one of your 900QMD series German equatorial mountings. I have tried to run this mounting using a newly purchased Universal AC/DC Adapter that has a 12V, 800mA max. output. When the 900 mounting's power cable is connected to this converter, the R.A. drive operates properly, but if I push the Dec. drive's button on the hand control, the power totally shuts down on the mounting. What's hapening? Is the 12V 800mA output from the AC/DC converter enough to power this mounting properly? (reword this one)
Universal AC/DC adapters do not have enough filtering on their DC outputs. They can put out 800mA, but the voltage drops below 90 volts 120 times per second, in effect resetting the electronics each time. You need a battery in the circuit in parallel or a large filter capacitor to hold up the voltage. Better yet, get a real power supply from Radio Shack or the one we offer from Pyramid with a good filtered output.

Polar Alignment scopes

How can I polar align in the day time?

By the way, Marj forgot to mention that these scopes will come well aligned, so there should be a minimum amount of touchup (maybe none) necessary.

In the next few days I will be testing the accuracy of these scopes and checking them against my well polar aligned mount. I will see just how accurately I can align my mount for field use. My goal is to come up with methods of alignment that are fast and accurate so that for
imaging you can be up and running quickly.
I have developed one method to use in the daytime that allows me to get essentially perfect polar alignment for CCD imaging. In the past I have wasted a lot of precious dark time trying to line up the mount with drift alignment. This is especially true in the summer when the sky does not get dark till after 9pm. The daytime drift method starts out with the scope in Park1 position. I go thru the Park1 - Park2 - Park1 sequence with the bubble level so I can set my altitude axis as close as possible. This is followed by a slew to the Sun (NOTE: DO NOT LOOK AT THE SUN WITHOUT A FILTER!!). The azimuth axis is then adjusted to center the sun. Slight touchup of the buttons may be required to accurately center the sun. Then I do #9Rcal to sync this position of the mount. Now I'm ready to find a bright star in the daytime sky. Using the Stars List, I slew to a right star and pick it up in a low power field. Right now Vega is ideal for that. The CCD camera is attached, and I choose my Ha filter. I purchased an inexpensive Ha filter from Edmund Scientific and fit it to my SBIG filter wheel in the Moon filter position. Not a great filter but gets the job done. This filter knocks out 99% of the bright sky and allows Vega to come through at 0.12 second exposure. I have also used Altair at .5 sec exposures. I do my focus and go directly to T&A in the CCDOPS program. I set it for perhaps 20 exposures with additional dark frames between each exposure. These darks are not needed for the image, but buy me extra time between actual image exposures. This way, it takes about 25 seconds for each image to appear, and allows me to see the drift in both axes. The T&A graph shows the direction of drift and the number of pixels for each exposure. After 3 or 4, I can see immediately how far off I am.I concentrate on each axis one at a time. The Dec axis drift is first adjusted by moving the azimuth adjusters approximately 1/4 turn. The star is then re-imaged (centered if necessary) and the drift in Dec is noted. It will either be better or worse. In my case, I know that an "up" drift in the curve means that I have to turn my Az adjuster counterclockwise. I have pasted a small pointer on the knob to make it easier to see small increments. It typically takes me 3 - 4 iterations before I get less than 1 pixel overall drift in a 5 minute time period (my focal length is 3700mm, so it is very sensitive to misalignment). In the meantime, I have ignored any drift in RA. By the way, it is important to turn off the "Relay" function in T&A so that the image is actually allowed to drift. When Dec is zeroed out, I do a similar adjustment to RA by adjusting the altitude axis. These adjustments are easy to do, and are straight forward. It takes a bit of practice to get used to the steps involved, but if everything works out and I don't get disturbed in the process, I can get very good alignment in about 1/2 hour, all the while the sun is up. Give it a try next time you go out. Good luck.

Roland Christen

Zeiss/ Baader Planetarium Binocular Viewer

How do I collimate my Baader/Zeiss Binocular Viewer?

Collimation is super easy. Place one pair of high-power eyepieces into holders, loosen 3 screws on left or right holder. Move holder left-right, up-down until the images merge. Use 4mm eyepieces for best alignment. Do not use high-power eyepieces to achieve high magnifications. Limit to 10mm eyepieces and use Barlow in front of Binoviewer to get high powers.


Jim, Martin, Bob, thanks for the help. Can I impose on you for some more guidance ? Bob was right on the nail - Yes, it was 9mm eyepieces that I was unable to merge ! I have a 6" 1200mm APO witha 1.25x corrector - I was going to add the 1.7x corrector and that would give me around 250x with the 9mm eyepieces. Well, I guess that that won't work now :-(


It should work. The problem with merging is that the eyecups have to be in exactly the same position for the two images to be in the same
relationship in both oculars. How do you know that the eyecups are in the same position? You move them yourself to be in the same position. There are 3 little screws that hold each eyecup on the binoviewer body. You can loosen them and move the eyecups around until they are exactly centered with respect to each other. I suggest that you start with one side and move the cup until the image merges. It is easy to do on a distant target in the daytime. If you run out of room to move one of them, tighten it down and move the other one. The Baader Binoviewer was made so it can be precisely collimated by the user. They are close from the factory but may not be exactly on. That is why there are 3 screws to allow the user to touch up the collimation.

One caveat. Even if the Binoviewer is exactly collimated, there may be a problem with shorter ocualars. This is because the optical axis of any ocular may not be exactly centered with respect to its barrel. In that case, just inserting the ocular in a different orientation may cause the image not to be centered. For that reason I recommend getting your power ahead of the ocular so that you don't have to magnify the image (and all decentering errors) after the Binoviewer. Not understanding these fundamental facts seems to be causing all kinds of confusion about merging images in a Binoviewer.

Roland Christen

Maxbright Diagonal

What is the accuracy of the Maxbright diagonal?

For the area actually used by a star image on-axis, the flatness is probably better than 1/40 wave P-V (peak to valley). The accuracy of the uncoated substrate over the entire surface from edge to edge is better than 1/10 wave, but the diagonal itself is oversize, and only 1.8" is actually used. Out of that 1.8" area, a star on axis occupies only about 0.25 inches. The accuracy of that 1/4 inch is extremely high. It is easy to confuse star diagonals with Newtonian diagonals, but their requirements are vastly different. The light of an on-axis star in a Newtonian diagonal can easily occupy the entire area of the diagonal. Thus, a Newtonian diagonal needs to be 5-10 times more accurate than a star diagonal to have the same effect on the image.

What are dielectric coatings (as used in the Maxbright Diagonal) and how do they differ from the standard magnesium fluoride coating?
Dielectric coatings make use of the wave nature of light to achieve reflectivity (or in the case of anti-reflection coatings, high transmission). There is no metallic layer involved. Normally, a thin layer of aluminum is evaporated onto the surface of the glass substrate. Since aluminum is soft, and can be rubbed off easily, there usually is a hard layer of quartz in the form of silicon monoxide added for durability. This combination works well enough for most applications. To get the highest durability requires that only oxide layers be deposited with no soft metals underneath. This is a dielectric coating. There are many layers involved, 50 layers, to get high reflectivity. They have to be put on accurately, otherwise there are gaps in the transmission band. It is very costly to do this, so dielectric coated mirrors are not cheap.

Why don’t you use dielectric coatings on the mirrors of your Maksutov-Cassegrains?
Dielectric coatings are not advised for large optic; the edges will curl up. In our diagonal, we have much oversized substrate that is rather thick and rigid. Only the center 1.8" is actually used. We had a 5" quartz diagonal coated, but the figure was ruined due to inadequate thickness (1" thick).

Astro-Physics GTO Mounts with Servo Drive

1. for stars near the celestial equator and eastern horizon, a declination drift south means I have to raise the polar axis.
OK
2. near the Celestial Equator and meridian, a declination drift south means I have to point my polar axis more to the west.

True.

Using these precepts I've just about eliminated dec drift in all portions of the sky; as I said the problem I'm having is in RA. If you eliminate Dec drift everywhere, you will introduce RA drift because you have compensated for atmospheric refraction near the horizon where the stars move at a different rate than sidereal, and at the same time you have introduced drift near the zenith where the stars move at the sidereal rate. In simple terms, if my drive is "running fast" near the meridian, what adjustments do I make to the elevation of the polar axis? I'm also curious as to the consequent effect on dec drift near the horizon. The drive is NOT running fast near the meridian. It is running at the sidereal rate, as are the stars. If you are imaging anywhere near the horizon, the sky is running slow as the object rises in the east or sets in the west. The closer to the horizon you get, the slower it moves. On the horizon, the object will appear to be approximately 1/2 degree higher in the sky than it really is. This means that it lags the sidereal rate by 2 minutes of time at the horizon. While the position of the object is compensated for in the keypad by calculating the atmospheric refraction and adjusting accordingly, the tracking rate is not changed in our present software. Doing a traditional drift alignment with stars near the
horizon will result in drift in RA over some portion of the sky. The best polar alignment will always be a compromise, so you have to determine where you want no drift. In my case, I usually image higher up in the sky due to severe light pollution at my site. If I am imaging low in the sky like for my recent M16 images, I will purposely offset the azimuth and altitude a small amount to zero out the drift. On my 1200 mount it amounts to a counterclockwise movement of the azimuth knob by approximately 1/2 a turn and a lowering of the altitude knob by about 1/10 turn. With my CCD camera and CCDOPS Track&Accumulate, these adjustments to the drift take about 20 minutes and I can then concentrate on
imaging that object over a period of several days. If you want to skip all over the sky on any given imaging session and shoot lots of objects, you will have to do the best compromise polar alignment and guide out the drift using the built-in guide chip. If you need to do unguided imaging and want to compensate for drift by changing the guide rate near the horizon in RA and Dec, you will have to wait just a bit longer. Several people are working on some slick new programs that will allow you to adjust the drift in both axes (essentially variable rate) for any portion of the sky. Once we get it working, we can add various goodies like automatic adjusting of the differential rate depending on position in the sky, etc.
I hope I have answered your question in enough detail.

Roland Christen

However, when my scope flips sides the dec accuracy is off, up to a degree.
I have adjusted my polar axis so I get optimal tracking where I tend to image which is a little south and east of the zenith.

Dec accuracy depends on the height of the altitude axis. The further from correct height, the more the Dec accuracy will suffer. For every degree you are off on the altitude axis, you will be off by 2 degrees in orthogonality. A second reason for Dec orthogonality is mirror flop. If the mirror in your SCT can slide sideways on the mounting tube, then it will do so when you flip sides. As an aside, a lot of these problems do not appear in fork mounted SCTs because the loading on the mirror is always in one direction. As soon as you mount a scope on a German Equatorial, then the mechanical requirements for the optical and mechanical integrity of the tube assembly become much more important.

Roland Christen

How could his polar alignment be off if a drift test shows it is not?

Drift and polar alignment are two different things. Drift is minimized when >the polar alignment is not quite perfect. With perfect polar alignment, drift
will not be minimized except for straight overhead. Everywhere else in the sky you will get either RA drift, Dec drift or both. None of this was very important until just recently when amateurs began using CCD cameras with tiny pixels that could pick up the smallest drift very quickly. In the old days of film photography the resolution was 10 times worse in most cases for you to ever notice a few arc seconds of drift. Also, anyone who did film photography seriously always guided either by hand or with a seperate ST4 guider. Now, people are jumping directly into imaging with high resolution CCD cameras without going through all the pains of film photography, and expecting perfection right off the bat. However, perfection is not easily realized without developing skills, and this includes setting up the mount for best results. Sometimes there is no easy solution to equipment shortcomings, in which case one needs to learn how to work around these problems. Mounts, no matter how well made, have periodic error that shows up in a CCD image. Telescopes, no matter how well made will have mirror flop, bending stresses, and other orthogonal issues that will limit both pointing and tracking accuracy. There may be software solutions like T-Point, PEM and variable tracking rates that can help. How much of these apply is each idividual's choice. Personally, I try to work around all these without adding to the complexity of the system with pile upon pile of added software. Others may disagree and want to use software solutions. I say - Go For It and let us know your results.

Roland Christen


When I connect the TIC cable from the ST-7 with the GTO mount, the mount starts moving in right ascension (eastward). What’s happening?

The circuit on the 1200 mount consists of 4 pull-up resistors attached to +5 volts on one side, and the sensing integrated circuit on the other side. The 4 inputs are, therefore, connected to the +5 volts by means of these pullup resistors. The circuit on the ST7 consists of 4 open collector transistors which have their emitters connected to ground. When any of these 4 transistors are energized, the corresponding collector pulls one of these input lines of the 1200 mount to ground, thereby energizing the circuit and causing the motor to turn. If any one of those 4 transistors in the ST7 is leaking current, it will pull that input to ground and cause the motor to turn. Also, if one of the 4 pull-up resistors is faulty, the input will become super sensitive and may energize even if the transistor is not on. To determine which is the problem requires measuring the input of the 1200 mount and the output of the ST7 with a voltmeter and ohmmeter. The following diagram shows the circuit. (based on John Kruis’ e-mail)
Add diagram which Roland prepared by hand. 400/600/900/1200 autoguider inputs, ST7/ST8 circuit and servo controller circuit., etc Needs transfer to Illustrator
. Now in Marj’s notebook.

Rephrase-  By the way, what exactly is the result of changing  the clock time in the keypad hand controller? I wonder if the mapping of the stars will not be disturbed.
The mapping of the stars is never disturbed by changing the clock time. What you have discovered is that your “local" time is not accurate to the actual universal time. The stars do not care about “local" time. The only place where local time is exactly accurate is in the middle of your time zone. If you reside at either end of your time zone, your actual local time will be off by ½ hour from the local time as reported by your radio station. One way to determine the actual time in any location is to check the universal time that appears on the hand controller with the actual universal time as reported by a time service. Lacking a suitable time signal, the other way is to use the park function. With the telescope calibrated to a known star, I would initiate Park 1. The scope will now slew to a horizontal position where the scope and counterweight shaft are parallel to the earth.  If my internal clock is off by ½ hour, the counterweight shaft will not be parallel to the earth. With a simple bubble level, I can then level it, read the difference in minutes on my RA setting circle (they are indeed still useful), and adjust the time in the hand controller accordingly. Now I slew back to the cal star, recenter it, park once more, and make a final adjustment in the angle and the clock. This way I have been able to determine that I am off by 18 minutes here in Rockford from the local time broadcast by the local radio stations. The reason is that Rockford is only about 150 miles from the eastern time zone in Indiana. The real Central Time Zone zero point is probably somewhere in Iowa. If I am using an AP900GTO, a C-11 OTA and a ST-8 and do a proper polar alignment, the calibration process using the ST-8 will overcome any problem with the scopes alignment with the mount.

Is this correct?The ST8 will compensate for any drift caused by misalignment on the pole.


Most advanced astrophotographers like to get bang on alignment by using the drift method. You can do this very quickly with the ST8. Simply watch the star drift on your computer screen. Drift in RA can be zeroed out by raising or lowering the altitude axis of the mount. Drift in Dec is zeroed out by adjustment of the azimuth axis left and right. After you do it a few times, you will be able to eliminate drift and get essentially perfect polar alignment in as little as 10 minutes. If I'm measuring things right it was about 12 arcseconds in 9 minutes.  I had no idea whether that was good or not.  I had an ST10-ME with the T211 guide chip, and measured it by guiding on a star using CCDSoft with the method described in Ron W's book.  The declination wandered above then below zero and never got outside 2 pixels which told me I had a good polar alignment.  The right ascension tracking had a hump that went 3 pixels above zero, then it steadily went down to about 15 pixels below zero.
What you are describing is periodic error PLUS drift error due to polar misalignment. Just because the Dec drift is close to zero does not mean that you are polar aligned. Zero dec drift means that your azimuth axis is close to correct. Drift in RA means that your altitude axis is too low or too high. To get a measure of true periodic error, you need to zero out your RA drift so that the beginning and end point are approximately on the same pixel. Secondly, one period is 6.38 minutes, so that is the period of time that must be measured. A 9 minute measurement does not measure the true PE of one worm cycle. If there were no RA drift, then you would see the pattern repeat over and over again every 6.38 minutes. With RA drift, there will be a steady downward drift, so you would have 30 pixels in 18 minutes, 60 pixels in 36 minutes, etc. This does NOT mean that you have a periodic error of 60 pixels!!

You are not the only one who has confused RA drift with RA periodic error. Attached is a chart showing the wrong way and right way to measure periodic error. The customer who sent me this chart assumed that the total error is all due to the periodic error of the worm and came to the conclusion that it was on the order of 15 arc seconds. In actuality, the PE he recorded was closer to 5 arc sec. To get the true error of the worm, you must either drift align, or subtract the component due to drift for 6.38 minutes as the upper data on the chart shows.
INSERT FILE: data/word/webpage/working on/PE chart1.jpg

To get zero drift in RA for a true measurement, you will need to drift align your mount so that a star stays put for at least 30 minutes, not counting the slow back and forth wander due to the worm error. This is not so easy to do, but is a necessity for doing any hi resolution imaging. I have a quick way to drift align using the camera in Track and accumulate mode in CCDOPS. I take maybe 20 exposures in quick succession, perhaps 5 seconds each exposure. I use the T&A graph on the screen to quickly zero in on the pole by watching the star drift both in RA and Dec. When the mount is properly aligned, the points on the T&A graph will all lie along the line and drift neither up nor down except for the oscillation about the zero line in RA due to periodic error. The camera must be oriented correctly east-west for this to work properly.

Roland Christen

Maybe you could be more specific about where to point the scope and how you know what adjustments to make.  I have the camera oriented so that Declination is aligned with my X-axis and RA with the Y-axis.  From everything I've read I thought that I was supposed to point to the meridian near the celestial equator and make azimuth adjustments to correct any declination error.  Then move to a star below 20 degrees to the east near the celestial equator and make altitude adjustments to correct any declination error.   Then repeat this process until the alignment converges to a good polar alignment.

That doesn't seem to be working for me.  It seems to get it close, but it never takes care of the RA drift.  If I make attempts to correct for the RA drift, it screws up the Dec drift.  I haven't found a good way to make adjustments that cause both RA and Dec drift to converge.  I got it where it was fairly good for both RA and Dec while pointing to the meridian.  Then I moved it to the east and the Dec was off.  I fix the Dec and the RA is off.  I think I'm going to order TPoint to see if that helps.


You must understand that at the level of accuracy that you are aiming at, pointing the scope at a star below 20 degrees will introduce very large errors in RA and Dec drift due to atmospheric refraction. The 20 degree method worked in the days of the star-on-crosshair-eyeball method, but fails in the age of super sensitive CCDs. A CCD will show considerable movement even when no apparent motion is detected on a crosshair. The only practical way to null out drift is to keep the scope pointed above 45 degrees, and even there the refraction error is not negligeable. In fact, I do my drift alignment within 20 degrees of the meridian, i.e. above 70 degrees. Getting T-Point can help with polar alignment, but will do nothing to eliminate atmospheric refraction.

Theoretically, pointing the polar axis exactly parallel to the earth's axis and driving the mount at the exact sidereal rate should result in no RA or Dec drift. Unfortunately, this is not the case due to atmospheric refraction effects. The best compromise, which will result in the best tracking, is to point the polar axis at the refracted pole, and to drive the mount at the King rate (built in to the sidereal drive). Since the idea is to eliminate as much of the RA drift as possible, I use the power of the CCD chip to show me the drift directly. I do this by imaging a starfield and using CCDOPS Track&Accumulate to show the second by second movement of the star in RA and Dec. In just a few iterations, I can eliminate Dec drift by azimuth adjustments (without affecting RA drift at all) and then adjust the altitude axis to eliminate RA drift (without affecting the Dec drift). This results in very accurate tracking in an area of the sky where most of my astrophotography is done. Going below 45 degrees elevation will naturally affect both RA and Dec drift because the sky moves at a different rate than it does at the zenith. I realize that most people are puzzled by this, but now that long focal length scopes with 6-9 micron pixel CCD cameras are being used, these effects will begin to dominate. Here is an article by Dr. Melsheimer (he designed the original LX200 electronics for Meade under contract): http://www.dfmengineering.com/news_internet_telescope.html.


How accurate do you need to be in this drift alignment?

Well, to measure periodic error, you have to eliminate the RA drift completely. To do imaging, you can have some drift if you guide with a guide chip. If you want to do unguided imaging, you have to limit your exposure to that period of time where the effects of drift and periodic error are below the level of the resolution of your system. So, the question to you is:

What of the three things above do you want to do?
1) Imaging?  That does not require ultra-precise drift alignment if you guide.
2) Measure periodic error? That requires at a minimum zero drift in RA for an accurate measurement. Zero Dec drift is not an absolute requirement. You will end up measuring exactly what we measured here in our lab setup- in your case about 4 arc sec.
3) Unguided imaging? You will be limited in your max exposure time by accumulated drift error (see article in the link). You can reduce the period error by a factor of 2 with careful PEM training, but the end result will be applicable only for a reasonably high elevation where atmospheric refraction is low.

With your C14 scope, you will find many other anomalies that upset your perfect tracking efforts. Just the tiny gravitational pull of the cords from the CCD camera will cause quite large guiding errors. If you don't believe me, just have someone gently touch or move the cords while you watch the position of a star in focus mode with a fast update time. It will jump all over the place as the cord is moved. The motion of your mirror as the scope moves across the sky will cause very large errors both in tracking and in pointing. Even if you manage to lock the mirror in place, the mechanical flexure of the tube/mounting plate will cause measurable errors. So, even if you could compensate for all the natural sky anomalies, you would still be left with the realities of an imperfect optical/mechanical system. The best advise is to get your tracking as close as possible overhead, guide with your auxiliary chip, and take some images. If you get to the point where you think you need better tracking than the SBIG guide chip can provide, then you might want to invest in the AO module from SBIG. Then all drift/periodic error/opto-mechanical problems will be moot.

And guess what? Even then, you will get oval stars sometimes. Even with absolutely perfect guiding, the stars will be strangely oval. Can you guess why? (I'll give you the answer in the next Email).

Roland Christen

I'm trying to dial in my polar alignment and have been spending three nights doing the drift method. I have a permanent setup, so I want it as good as possible. Doing the traditional drift method adjustments just doesn't seem to be taking care of the RA drift.

The traditional drift method uses a star near the eastern horizon. In the old days of crosshairs and eyeballs, this seemed to work ok. Now we are in the age of super sensitive CCDs where the distortion of the atmosphere due to refraction causes measurable errors in this traditional drift method. I have found that the fastest and easiest way to eliminate RA and Dec drift is to use the power of the CCD. I choose an area of sky within 20 degrees of thezenith toward the south-east. The camera is setup to take a dozen or more quick exposures in the Track&Accumulate mode of CCDOPS. The resultant chart
shows very quickly the direction and magnitude of the drift in both axes. Then, I make small adjustments in azimuth to zero out the dec drift, followed by adjustments of the altitude axis to zero out the RA drift. These adjustment are almost totally independent of each other, unless your initial polar alignment is very far off.

Track&Accumulate is a powerful tool for polar alignment, and it is only available in CCDOPS - nothing comparable in CCDSoft or MaximDL. If you do not have CCDOPS, you can do it a little differently in Maxim. You can go into focus mode, pick a star and update quickly. Magnify the image to max (1600x) on the screen. Place the cursor center over the star and watch the drift. Make the appropriate adjustments in azimuth and altitude until the drift becomes negligeable over time. It is a bit more difficult than T&A, but it can be made to work.

Roland Christen

Is there a way to send an external time signal from a computer (from a time service) to the mount?  Correct time is crucial as I am using T-Point to improve pointing and find it awkward to manually set the time. The mount clock is not always sufficiently accurate. I rarely get it right. I am one of those poor folks that has to leave the mount parked for a month or so before I get the time to use it. By the time I get back to it, it is off. This requires a new synch that pollutes the T-Point.I love my 900GTO. It is the last mount I will ever buy

Yes, you can set the clock externally using your computer rather than the keypad. Start your session with The Sky and update the time via modem from the National Institute of Science and Technology (NIST). Power up the mount without the keypad connected, and use "The Sky" (first ) to initialize the servo drive. Once the servo is up and running, you can connect the keypad. If you start with the keypad plugged in, the servo will get its time signal from the keypad, and the un park will be inaccurate by the amount of time that the keypad clock is off. By starting up with your computer, the servo will have the correct time according to your computer's clock. I hope this helps.Further question on using Tpoint and updating clock time.For this discussion, I will assume that you have a chip in your control box that is dated January 22, 2001 or later and your keypad is version 3.09 (according to my records you do). Anyone else who does not, may wish to upgrade. See the Technical Support section of our web site.  The control box software was designed to give you the ultimate in FLEXIBILITY depending on your setup. You can "initialize" the mount (date, time, location, time zone and park information, if any, is sent to the mount) with the keypad or software, such as The Sky . The important Auto-park feature of the chip in the control box will remember the position of the mount when the power is removed or interrupted, no matter where it is. You do NOT need to use the park positions of the keypad or park routine of The Sky, though you can if you want to. You simply remove the power and the mount will auto-park and the position will be stored to its memory- NO MATTER WHERE IT IS. This is what I recommend for your application:1. After you start your next observing session (go through the normal startup procedure beginning on p17 of the manual). Then, set auto-start to "yes" in your keypad. That way you will never have to go through the keypad startup routine again! You can easily change it back to "no", whenever you wish. 2. When you are finished with that session, simply move (either slew or use the N-S-E-W buttons on the keypad) the mount to any position you want. It does not matter (exception-please do not park in position where the counterweights are higher than the scope). When you disconnect the power, the position data will be saved to the chip in the control box of the mount. Alternatively, you can use the park routine in The Sky as discussed in our manual p52-53. This is useful if you plan to leave the power connected (not recommended since a lightening strike could damage your electronics). 3. Before you start up your next session, update your computer time through an update service (check Help in The Sky for suggestions). DO NOT PLUG IN YOUR KEYPAD. Instead, power up the mount (it will start to track at this point and adjusting its position data), then start The Sky. The Sky will send the date, time, location, time zone and park information, if any, to the mount. Whether you parked the mount with the park routine of The Sky or not, the mount will remember the last position and know precisely where it is. You are ready to go. No fuss, no bother. Extremely simple. If you want to use your keypad, plug it in and use any of the functions. It will not impact the time of the mount, since the mount was already initialized with The Sky. However, don't use the park function of the keypad since the goal is to park and unpark with The Sky so you can update the time of your computer for your next session.Question: As a further explanation, here is an answer to your specific question: It seems that the AP startup (in a non-park mode) wants to have you synch on a star or you can't get into other menu options (PEM, Drive rate, and so on).  Is this corrector am I missing something here?

No, you don't have to go through that routine. This is the beauty of the new software. When the keypad is in the auto-start mode (this feature set to "yes" in the keypad), all you need to do is reconnect power and the system will remember where you left off in your last observing session. If you are using the keypad , the Main Menu will appear on the screen, so you can go directly into the Objects menu and enter the desired objects to be viewed. This is perfect for permanently aligned mounts that have not been moved since the last viewing session. Auto-start and Auto-park are two of of the most important features of the control box chip upgrade. Please read the section in the GTO Keypad Manual (available in our Technical Support section) for further details- page 23. Please see discussion above and in the manual regarding The Sky. If you have auto-start set to "no", then you will get the startup men routine that forces you to choose a location and go through a start routine. This is recommended for people who are NOT permanently set up since their mount/scope will not be in the same position as the last session. This sequence is described on pages 17-22 in the manual. What did I do wrong? I certainly don't want to have the watch my mount when I slew using The Sky.Jumping back and forth between The Sky and the keypad can result in improper operation if you don't watch what you are doing. Most times this comes from trying to recal from one to the other program. I would advise using either one or the other, but not jumping back and forth when slewing to different objects. For instance, you cannot slew somewhere with The Sky, and then do a recal with the Keypad. The Keypad will recal on the last object that you entered with the keypad, not the last object you entered with The Sky. Stay in one program or the other, and use the keypad only for centering with the buttons if you are using The Sky for slewing to objects.

And, finally it seems as if the mount is running a bit fast as there is a slow but consistent drift in RA to the west-I can see this on a CCD chip over about 10 minutes.
I would suspect that your altitude axis is off. I have checked the crystal frequency of a number of servos, and they all fall within a very tight range, well within the specs for a crystal frequency standard. Most of them are 10 times better than spec, and the worst one was still 5 times better than spec. The gearing of the motor shaft is fixed, and if there were any errors of +- 1 tooth anywhere in the gearbox, you will have tremendous drift that you will see in a few seconds. Another thing to check is whether the drift occurs toward the zenith, or if you get drift at lower altitudes. At the lower altitudes I would expect drift due to atmospheric refraction. Also, do you know if your telescope optics are flexing or not? Using the keypad and software together The way the system is configured, the servo control is the center of the electronics. It receives information from outside sources to allow it to move to specific parts of the sky. One of these outside sources is the keypad, which is really a small hand held planetarium program, as well as a set of buttons that allows you to "steer" the scope in 4 directions. The other outside sources can be connected to the two com ports on the servo drive, and could be planetarium programs such as The Sky, the voice control program Digital Sky, scripted programs that use planetarium software for slewing, or custom software using the AP protocol to control the mount. All these sources are separate from each other, although they can be configured to talk to each other in limited ways. When you initialize the mount with the Keypad (either with Autostart on or by manual entry) you can send the mount to a known object, such as M57, and the keypad sends the RA and Dec co-ordinates of that object to the servo. It does not send any information about that object, such as the name or magnitude, to the servo, only RA and Dec numbers. When you link a second planetarium program, such as The Sky, to the servo drive, you will see the cursor move to the co-ordinates of M57 on the screen. The program does this by polling the servo drive (at intervals set by the user). At each poll command the servo sends only the RA and Dec co-ordinates every time it is asked for current position. The Sky then interprets these numbers and places the cursor on M57. When a program such as The Sky is used to slew the mount to a new object, for instance M27, the servo will receive from the computer the RA and Dec co-ordinates that correspond to M27. However, the information about these co-ordinates is not sent, so the servo has no idea that it is M27 that you are now on. All that the mount knows is the co-ordinates RA and Dec. The keypad will also not know that you are now on M27 because it does not receive this information from the original signal. The signal only contained RA and Dec, not M27. As far as the Keypad knows, it is on M57. You can set up the Keypad to follow the motion from The Sky (or any other program) by placing it in the digital circle mode, but it will not automatically look up these co-ordinates and match it to a known object in its database. (You can do a search function but this is a laborious process). If you use the Keypad to Recal on this new object from The Sky, it will do a calibration on the old object, and the servo will now have the wrong co-ordinates in its memory. Recal (button #9) on the Keypad is strictly a function that recalibrates the position of the last object in the Keypad memory to the current position of the servo. It is used for centering an object that you slewed to with the Keypad. If you wish to calibrate on a new object that you slewed to using The Sky, you can do that with either the Keypad "Sync" routine, or with the Sync routine in The Sky. You cannot do it with the keypad Rcal button. To sum up: The mount servo has the information of current position from any source in its memory. The Keypad can be placed in "Digital Circle" mode to follow commands from outside sources. The Keypad cannot be used to Rcal from an outside slew command. The Keypad can be used to Sync on an object from an outside slew command if that object is in the Keypad database. It's always had a condition where when the scope is moved via the hand controller towards the west and then stopped, the drive takes about 10 seconds to "catch-up" and continue moving the scope.  Moving the scope to the east does not cause that to happen.  There is nothing to fix per se. This is caused by a small amount of backlash in the RA worm gear. You can adjust the mesh of the gear to eliminate this. What happens is that you unload the driving tooth when you move the scope in the opposite direction. Then it takes some time for the worm tooth to make contact again with the opposite drive tooth since the motor is turning only at the sidereal rate. You can speed things up by pressing the 16x button briefly to bring the teeth into mesh, or you can tighten the gear mesh to reduce this delay.

I guess these are questions for Roland. Is there a limit to the number of corrections entered during a PEM recording run? If so how many?
In your opinion what is the optimum correction entry rate when recording, 1 second 2 seconds, longer, shorter? I have been trying to get a very good PEM recording, using the ST-V and the answers don't seem obvious, at least to me.
Jim


The PEM uses 256 memory positions over a 6.4 minute worm cycle. That means that a keystroke move will be recorded every 1.5 seconds. Within that 1.5 seconds the keystrokes are averaged so that one left and one right will cancel and no error adjustment will be recorded for that period. Any time you do a new run, the old one is erased. I don't know the optimum time setting. It depends somewhat on the stability of the sky at the time the errors are recorded. For instance, if the periodic error of the worm is 5 arc sec, and the instantaneous sky instability is 2-3 arc seconds, then entering data very rapidly on the order of
seconds will probably result in more periodic error rather than less. Perhaps the best way to train the PEM is to take long integration times, around 3
to 5 seconds to average out the seeing. Pick a night when the seeing is very good, and do your training high in the sky where the sky motion will be minimum.

Roland Christen

I just received my 1200 about 6 months ago and I am having problems with getting the dreaded yellow 'overload' light when manually (physically) moving the scope to the east only?

You may have a low voltage problem. Your power supply may be close to the cutoff point where the servo drive is detecting an undervoltage condition. When you move the mount agaist the motor direction, the extra load is dropping the voltage below the cutoff point and the yellow light comes on. Low voltage can be caused by a number of things including too long a wire between power supply and servo, wire too small, power supply too weak, or battery is not fully charged, or cannot hold a full charge due to age, etc.

Roland Christen

Do you have some way of turning the mount off and shutting down the camera after your imaging session in complete? Or do you have to get to the mount before the scope hits the pier?

Certainly. Use an old fashioned timer that turns the power off before the scope runs into anything. For large portions of the sky southward of straight up, the telescope will dive safely under the mount without hitting anything. In those cases they allow the scope to image all night till dawn. In the case where the scope might interfere with the pier, you can estimate how much time you have by using the right ascension setting circle. Start the scope close to the pier, note the position of the hour angle. Then slew to the object in the east and note the hour angle. Subtract and you have the number of hours of safe imaging. By the way, turning off the mount power with a timer will not result in a loss of position. The mount remembers where it was when power was removed and will start up from that position the next time you power up. You can come back next morning or evening, turn on the power, park the mount in your favorite position, or resume your normal observing functions. NO NEED to home the mount or start any sort of calibration run - assuming you have put your keypad in Autostart mode. Plug and Play at its simplest.

Roland Christen


My question is, if I slew to a star and then manually center it, will hitting Recal do the same thing as a Sync on that star now. The reason I ask is that after you slew to a star it doesn't seem like you can get to the "1 - Sync to current obj" option without exiting and scrolling all the way back to the star. If Recal will do the same thing, it would be much faster.

Recal is what you want to do when you are on a known object to which you have slewed. Sync is is used when you have manually moved to an
object or star from a different place in the sky and that object was not the last object entered. Rcal will "sync" on the last object entered and slewed to. Sync can be used on any object that you want to calibrate on, whether you slewed there with the buttons, or moved the scope by hand.

Roland Christen

I've had trouble with calibration for guiding and I just read most of the messages with "declination backlash" in them. I measured my backlash as 8 minutes and I don't really believe it. What is a typical value before and after adjustment ?

8 arc minutes of Dec backlash (480 arc sec) means that it would take 32 seconds to reverse the Dec axis at 1x guide speed. Is that really how long it takes for your mount to reverse?
Normally, it takes a few seconds for the Dec to reverse (1 second = 15 arc sec), and this is normal because the gearbox in the servomotor cannot reverse instantly. You can shorten this reversal time by dialing in a bit of backlash compensation. I would say that if your mount takes 32 seconds at 1x to reverse, you do not have proper worm mesh. The worm could be backed off by a large amount. Please note, however, that if it takes 2 - 3 seconds to reverse at 1x, it WILL take 4 times as long at .25x, i.e. it will take 8 to 12 seconds to reverse at that guide setting. Thus, it will no doubt produce poor calibration results when used at .25x. If it takes 32 seconds for your mount to reverse, then you cannot expect proper calibration results.

Roland Christen

formulate question

The way the system is configured, the servo control is the center of the electronics. It receives information from outside sources to allow it to move to specific parts of the sky. One of these outside sources is the keypad, which is really a small hand held lanetarium program, as well as a set of buttons that allows you to "steer" the scope in 4 directions. The other outside sources can be connected to the two com ports on the servodrive, and could be planetarium programs such as The Sky, the voice control program Digital Sky, scripted programs that use planetarium software for slewing, or custom software using the AP protocol to control the mount. All these sources are seperate from each other, although they can be configured to talk to each other in limited ways.

When you intitialize the mount with the Keypad, either with Autostart on or by manual entry, you can send the mount to a known object, such as M57, and the keypad sends the RA and Dec co-ordinates of that object to the servo. It does not send any information about that object, such as the name or magnitude, to the servo, only RA and Dec numbers. When you link a second planetarium program, such as The Sky, to the servo drive, you will see the cursor move to the co-ordinates of M57 on the screen. The program does this by polling the servodrive (at intervals set by the user). At each poll command the servo sends only the RA and Dec co-ordinates every time it is asked for current position. The Sky then interprets these numbers and places the cursor on M57.

When a program such as The Sky is used to slew the mount to a new object, for instance M27, the servo will receive from the computer the RA and Dec co-ordinates that corresponds to M27. However, the information about these co-ordinates is not sent, so the servo has no idea that it is M27 that you are now on. All that the mount knows is the co-ordinates RA and Dec. The keypad will also not know that you are now on M27 because it does not receive this information from the original signal. The signal only contained RA and Dec, not M27. As far as the Keypad knows, it is on M57. You can set up the Keypad to follow the motion from The Sky (or any other program) by placing it in the digital circle mode (press NEXT button when you are in Objects menu), but it will not automatically look up these co-ordinates and match it to a known object in its database. (You can do a search function but this is a laborious process). If you use the Keypad to Recal on this new object from The Sky, it will do a calibration on the old object, and the servo will now have the wrong co-ordinates in its memory.

Recal (button #9) on the Keypad is strictly a function that recalibrates the position of the last object in the Keypad memory to the current position of the servo. It is used for centering an object that you slewed to with the Keypad. If you wish to calibrate on a new object that you slewed to using The Sky, you can do that with either the Keypad "Sync" routine, or with the Sync routine in The Sky. You cannot do it with the keypad Rcal button.

To sum up: The mount servo has the information of current position from any source in its memory. The Keypad can be placed in "Digital Circle" mode to follow commands from outside sources. The Keypad cannot be used to Rcal from an outside slew command. The Keypad can be used to Sync on an object from an outside slew command if that object is in the Keypad database.

1) the clock though set to the time in the hand controller is off since I live so close to the time zone line?
2) should I adjust the clock on the hand controller so that my mount is level in its permanent location?
3) Is the clock critical in getting this procedure done in the daytime at any location and should be set back or forward depending on where you happen to be in relationship to the time zone?
Yes I do know that this is not how you do it for daytime but was curious if the mount should be level in park 1 position as standard


The fact that your mount does not return to a level position means that your clock is set to an average time for your time zone, not the exact time for your position. The mount is smarter than your clock, and more accurate because it looks at the stars, which are the reference points to which all clocks on earth are set. You can adjust your local time by the amount that the RA is offset (use the setting circles to estimate the amount you are off). After resetting your local time, you will see that the mount will park parallel to the earth.

Is it important for the mount to park parallel? Yes and no. Yes, if you want to re-create this park position in the future during the daytime with a Carpenter's level. No, if you don't mind that the mount's calculated meridian swap point will not be straight overhead, and that you will lose a few degrees of horizon on one side.

Roland Christen



Solar Accessories

What is a telecentric Barlow system (used with the DayStar solar filters) and how does it work?

THIS NEEDS TO BE REWORKED-from Astro-Physics-up Aug 8,2000- Roland wants to add drawings, etc

Does that mean that at the H-alpha filter there is no divergence from aparallel beam, thus the filter is operating at its parallel beam specedbandpass?

I don’t think you understand the geometry. An F30 beam means that there is 1" of convergence for every 30 inches of longitudinal dimension. Draw an f30 beam on a piece of paper.What is the included angle? Is it not NON-zero? In fact, an f30 beam has an included beam of 1.9 degrees calculated thus: angle = arctan (F#). This is for ONLY the center most beam of the optical system. The other, off-axis beams, have not only this angle, but added to that are the divergent angles caused by the Barlow magnification. You can draw this on a piece of paper also. Let’s assume the sun is ½ “ in diameter at the focal plane without any barlow. Draw a straight line at 90 degrees to the central beam, +- ¼ “ high to represent this solar diameter. Now place a “Barlow" element (also +- ¼" high) 3 inches inside (before) the focus point and connect the lines from the top and bottom of the “sun" to the top and bottom of the “Barlow". What you should have are two parallel lines, 3 inches long, above and below the central axis (in other words, a rectangle ½" high, 3" long). These lines represent the central portion of the off-axis rays only, to make it easier to see. So far so good? Have I lost anyone yet? Now comes the really interesting part (the part that shows how a Barlow works). Suppose we want the Barlow to amplify the image of the “sun" 2x, that means that the image is now 6" from the “Barlow", rather than the original 3", AND it is now 1 inch high (+- ½ “ from the optical axis). Now connect the lines from the Barlow to the new “sun" image, which is 6" behind the Barlow. Are those lines still parallel? Of course NOT. They have DIVERGED, and this divergent angle is depended on the amount of amplification and inversely proportional to the focal length of the Barlow. Long focus Barlows diverge less, Shorties diverge more. ALL BARLOWS DIVERGE unless they are TELECENTRIC.In order to straighten out these divergent lines, a second positive, lens is added (telecentric lens) just before the focal point, which will render these off axis rays parallel again. However, this represents only the axial portion of the light beam coming from the lens. You must add to this the original F30 convergent rays to get a true picture of what is happening.

PHOTOGRAPHY

I have noticed uneven field illumination in the corners on 35mm film. What causes this?

Edge illumination (light drop off in the corners) is not a function of the lens. It is a result of the small size of the t-ring or a typical camera adapter. The smaller the opening, the more light falloff at the edges and in the corners. In order to fully illuminate the corners of a 35mm format, the t-ring opening needs to be 50mm at f8, and 54mm at f6. Since typical t-rings are between 36mm to 40mm opening size, there will be vignetting or light fall off at the edges and corners of the format. The prime focus camera adapters made prior to mid 1998 (part # began with "PFC") and 35mm camera adapter to use with 2.7" field flattener made prior to late 1998 (part #  began with 67R) allowed for the maximum possible opening and greater film coverage. To use these adapters, the three set screws along the outside edge of the t-ring were loosened and the middle ring was removed. This modified t-ring was then attached to the camera adapter with the 3 set screws. However, we had to discontinue this design since the t-rings became more complicated, each different from the other and many would no longer fit on our camera adapters. In our current design, the t-rings thread onto the camera adapter using the t-threads.More technical info:Vignetting (light falloff) is always due to a mechanical obstruction somewhere in the light path. Optics, whether F9, F6 or any F ratio do not have any inherent reduction in light transmission at different field angles, not until the field angles become large compared to the aperture. In other words, the 4"f6 lens would show no significant light falloff until the field exceeds 4" in size or more. However, near the film plane there exists all kinds of restrictions, starting with the focuser inside diameter and proceeding to the t-ring and internal opening of the camera itself. It is well known that t-rings are smaller in inside diameter than the diagonal size of the 35mm film format. As this t-ring is located further towards the lens, it increases the vignetting effect which comes in from the edges. Pushed to extremes, if the t-ring were to be located at the front of the scope, then the vignetting effect is fully realized, and the effective diameter of the scope is reduced over the entire film format from a 4" aperture lens to about 1.4" aperture. To see the cause of the vignetting, simply do a "ray trace" with your eye. With the film removed, open the back of the camera, place your eye at the center of the film plane and note how much of the full lens aperture you see. Now move your eye to the edge and finally to the corner of the 35mm film plane and note how much lens you see, and where the cutoff occurs. You will see the t-ring intruding rather quickly as you move off center. that's because at F6, the lens looks quite large and is easily vignetted. Substitute the 7"f9 lens, and it will look 50% smaller, and will be affected less by the t-ring. It is a well known optical, geometrical and physical principle that faster F ratios are more affected by mechanical field stops than longer F ratio lenses. If you are still puzzled, draw it out on paper and place the various mechanical restrictions where they are in your camera setup. You will see immediately that there is no way to fully illuminate the corners of a standard 35mm camera with a simple 4" F6 lens. We are working on a special field flattener for the Traveler that will more evenly illuminate a 6x7 format right into the corners. It will become available this spring. This same flattener can also be used with 35mm formats with an adapter. Unfortunately, it will still need a standard t-ring, so the edge illumination will never be 100%.

Eric Roel #1I have the mounting very well polar aligned, I have it permanently mounted on a pier at the observatory. Also the OTA is orthogonal with mounting axes. Several months ago, I wrote Marjorie explaining herwhat my problem is, but gladly I will explain it again.

1.- When you try to track a Star using sidereal speed, the star keeps wandering my reticule center (My scope has a 5000 focal length), so with the 12.5 mm. reticule we are taking about 400X, it shows periodic error plus the Star drifts slowly away.

2.- When I try to train manually the Pec at this power, the trained tracking is worse than with the PEC off, I have retrained it many times but with no correction, I have even tried an St-4 guider for training, I have trained my 12 "LX200 with it and it tracks very good.

3.- When I image planets at the prime focus (5000mm), I use an HX516ccd camera. I have used the solar tracking speed, though the sidereal speed works better. When you center the image, the planet starts drifting right and left (periodic error) until finally the planet slowly drifts away after 15 to 20 minutes. I have tried the PEC but it tracks worse, so I am sticking to the sidereal tracking speed.I also have the Digital Voice and works very good, but not the tracking.

4.- When I use the Go-to function, I put a 45mm or 50mm eyepiece to get around 100X to 111X and a wider field of view. I put the clock onetime and because the mount is permanently fixed, I just go to a Star (not Planet or the Moon) and make a recalibration, just check that the telescope was not moved since parked, I do another goto to a near object, usually is in the field, then I center it and hit there calibrate button, then the object begins to move very slowly if Keep the button on, so what I do is just hit rapidly the recalibratefunction and then the Menu.Then if I go to another object, most of the time I will not find it in the field. Marjorie told me thatt he new chip would fix my problem, so that is why I found this e-group web and wrote you. My mounting has the electronics on top of the R.A. axis with the plugs facing south, I think the new model looks up.
So as you see I have some problems with my mount, hope you can help me fix them.

Eric Roel #2

Roland: I forgot to tell you that in #4.-, when I do the first or subsequent recalibrations, if I keep the button on the object it moves step-like and very slowly. When imaging the Moon, the tracking error is much worse than with Planets and Stars, even using Lunar tracking speed, actually it looks like there is no difference using any of the available tracking speeds. I use 600 for slewing, that way the motors do not wear too much, aside of that the mount is free, the dampers are well tightened as not to stop the axes but just enough to have nice soft feel. I always do a drift align every month or so, so I cantell you that the mount is well polar aligned. In the same observatory I have a 12"LX200 and one of your 6"f/12 "Superplanetary" triplet refractors, mounted on an HGM-200 equipped with the "Gemini" goto system, both telescopes can track incredibly well, even that the LX200 has toy electric motors (though I did make the axes orthogonal using a laser, changed the declination teflon bearings for needle bearings, took out the image shift using bearings and taking the mirror flop out), the Gemini tracks like anything I have ever seen, it keeps the object for hours without drifting at all, before the Gemini installation it step-tracked. That is why I expect much more from your flagship mount, now I just use it for Moon and Planetary imaging using sub second integrations due to the poor tracking. Hope this helps to see what is wrong. Thanks, Eric.Response to above two e-mailsOk, after reading your other two emails, I believe you have trouble with the tracking, pec and goto. But you give me no information to make an accurate diagnosis of your problems. Obviously, if all our mounts had this trouble, we would not sell any. On one hand, I can probably explain everything you see on improper setup, without blaming the mount at all, which would maybe not be correct. I am not there, so I cannot check things out for you. You will have to be my eyes and ears, and tell me what you see based on what I tell you in this Email and following ones.1. It is easy and tempting to explain all faults by saying something is wrong with the chip. That may or may not be the case. As an example, in an automobile, there is a chip that controls a lot of functions. Let's say the car runs rough and also won't go straight down the road, instead keeps heading for the ditch. A good mechanic would check out the spark plugs and wires, and also the wheel alignment, tire pressure, and perhaps ask if the driver really had his hands on the wheel while driving. So, for now, lets  forget about changing the chip, and try to weed out the herrings from the mackerel. 2. The mount won't rack the Moon. Well, I would expect that. The Moon does not have one tracking rate, neither does it track parallel to the earth. I have done a lot of lunar photography, and can tell you that you cannot track the Moon accurately with any kind of mount without changing the altitude and azimuth alignment. The Moon is a special case, that is not adequately covered in any book that I know of. It would seem that one could dial in a lunar rate, and the Moon would stay still in the eyepiece. At low power, yes. At high power, not at all. The Moon's distance from the earth varies from day to day, so the rate at which it moves from east to west varies also. There is no one lunar rate. The Moon also moves up and down in its orbit, sometimes begin high in the sky, other times being very low. The Moon's orbit is not parallel to the earth, so simple polar alignment will not follow the Moon's motion. There will always be declination drift, as the Moon moves between its northern and southern nodes. If you want the Moon to stay still at high powers, you will have to radically change both the elevation of your polar axis, and the azimuth angle. Otherwise, it will slowly drift away in your CCD camera field. I do this whenever I want to precision high resolution  photography of the Lunar surface. One would think that the dec drift could be counteracted by following it with the dec motor. This is difficult to do smoothly because there is a limit to how slowly a servo motor can be run before it becomes almost stepper like in its action. The drift in Dec and RA is many times slower than the sidereal rate ( varying between 3 to 20 times slower), that no single motor circuit can be built to smoothly and accurately follow this motion, and also slew at 1200 times sidereal. The mount would have to have 2 sets of motors, one for slewing, and one for very slow motions. This means 4 gears etc. So, at this point, I say that the Moon is a special case, a red herring, so to speak, that is not the fault of the mount, no matter how much you like it to be. 3. Slow RA drift. In 99% of the cases where a mount will not follow the stars for a period of time, the person did not polar align it properly. Do you understand that this can be the case? In fact, one way TO polar align a mount is to monitor the RA drift over time, and correct the altitude angle of the polar axis. Did you do this to the 1200 mount? The mount cannot do this by itself, it is something the user must do. Please do not rely on the Polar  routine in the keypad, it is only approximate. At the powers you are using your telescope, you must drift align. Did you drift align the Polar axis? 4. Slow Dec drift. Same thing applies here. A slow dec drift is always caused by the azimuth axis being aimed wrong. The dec axis will not "drift" due to faulty motors or chips, the motors are not turning in the dec axis, unless you want to go somewhere else.5. The GOTO does not accurately place the object in the field of view. If the mount is not polar aligned, I would expect exactly that. Also, what affects GOTO accuracy on a German mount is the degree to which your telescope is orthogonal to the polar axis.6. The PEC will not "fix" the star drift. If the mount is not properly polar aligned, I would expect that too. PEC is not intended to fix drift due to  polar misalignment. It is ONLY intended to counter the slow periodic back and forth motion due to worm gear error. Do you understand why this is so, and why you cannot use PEC to "fix" polar misalignment drift?If we can agree on the above, an it makes sense to you, we can proceed to what could perhaps be possible problems with the mounting.7. Let us assume that you have polar aligned the mount properly, but the mount will not track a star for more than a few minutes. Now we need to see whether the motor is actually turning. Using the crosshairs of a reticle eyepiece, can you determine the drift rate? Is it 15 arc seconds per second? If so, the RA motor is totally stopped. You can see for yourself very quickly by removing the motor gear cover (the one with 6 small screws) and see if the motor shaft is turning. Is the motor shaft turning? Perhaps the motor is turning, but doing so in an erratic manner. This can be caused by the worm gear being jammed into the worm wheel. Although we do everything possible to adjust the worm mesh, do to shipping mishandling we cannot always guarantee that the worm will not be forced into tight mesh because of some heavy blow the package received while being dropped out of the airplane onto the tarmack below. How do you check this? It is exceeasy. There is one gear in the motor gear train that is attached by a screw. Remove the screw, and pull the gear out. Now the motor runs free without any load. Now you can also check the worm pressure with your own fingers. Simply turn the little spur gear that is attached to the end of the worm. Does it move smoothly, or do you need an Olympic wrestler or long crow bar to turn it? If it does not turn freely and smoothly, there you have your problem of the erratic motion. To fix it, you simply have to adjust the worm tension. But before you do that, you need to answer all the above questions so I can get a better idea of where your mount might be at fault. 8. There is a very slight chance that the drive rate of the mount is not correct due to a faulty motor drive circuit, but this is highly unlikely. If it is, there are a few things you can do in the control box to trace this down, but I would rather rule out all mechanical things first. Film coverage In search for an autograph that will fully cover and fully illuminate a6x7 negative, I haven't found one scope yet that can do this.Does the 155 EDF fully cover and 100% illuminate (all the way to the corners) a 6x7 neg.?Nothing can illuminate 100% unless the opening to the film is equal in diameter to the diagonal distance of the 6x7 film format. This diameter is 3.5".  Measure your camera body opening. This will determine the maximum coverage at 100% illumination. Our 155EDF field flattener will cover a 4" circle, but when coupled to a Pentx 6x7, the opening is only 3", so coverage at 100% drops. The corners will still receive light, but not 100%. The light falloff is not really noticeable. Hints for CCD imaging I have no experience with the STV as a tracker, but I have heard that others are having problems also with all different mounts. It might be a good idea to check out the SBIG user's group to see what others have found. Here's what you need to do first of all to make sure your system is sound. Get yourself a cross hair eyepiece and monitor the motion of a guide star on the cross hairs with no inputs to the servo controller, other than your keypad. Do you see the star dancing all over the place? If so, you might have an atmospheric seeing problem. If not, do you see any rapid drift in declination that needs to be guided out? If so, then your polar alignment is off, most probably the azimuth axis. Remember, any drift in declination is NOT the fault of the mount, since that axis is not driven. Then check drift in RA. This should be a slow back and forth movement to describe a smooth sinusoid with a total time period of about 7 minutes for one full cycle. You can characterize your periodic error by simply offsetting your azimuth a few degrees and taking a 7 to 8 minute unguided CCD exposure (or film if you have no CCD camera). The resultant squiggle can then be analyzed to give you all kinds of vital information that you will need to set up your guiding later on. If you don't do these things (and many beginners fail to do them), you will not have the info needed to do a first class imaging job. When I set up my SBIG CCD capera for guiding, I calibrate (always at 1x speed) with the guiding chip on the side of the mount that I will be imaging. If I then go to the opposite side of the sky, I have to either re-calibrate, or go into the guiding parameters in CCDOPS and swap motion. Otherwise, the system will go ape - i.e. all errors get worse with time rather than better. I also use an aggressiveness of 7, which essentially means that the camera will correct any error only 70% so as to avoid error overshoot and oscillations back and forth. I normally guide at 1x, or .5x with my 146 inch focal length scope. In your case, you have a much shorter focal length, so there is no reason that the star should be hunting all over the place, unless the sky is very unsteady. Then you should not even try to chase the guide star, rather turn the aggressiveness down even further. If your Dec axis is jumping all over the place, there is definitely something wrong with your guiding parameters, because normally the guide star motion in Dec is close to zero in a well polar aligned mount. There is no reason the star should be moving at all. Finally, there is another test you can do with SBIG cameras. You can go into their Track & Accumulate mode, set it for 20 - 30 or so exposures at 10 - 15 seconds each, and then watch the little chart on the page. It will plot for you the actual motion of your chosen guide star without the mount getting any external commands. This is the purest way to see whether your mount is tracking well all by itself. You will see a gradual motion in the Dec axis that reflects quite accurately how well you are polar aligned. You will also see the periodic error in your RA axis presented as well as any constant drift due to polar misalignment. In fact, I used this chart recently to super tune my observatory 1200 mount. I was able to get the dec axis flat for 5 minutes with not 1 pixel of motion by adjusting my polar alignment.If you know what your maximum periodic error and its shape is, then you can plan your maximum guider exposure time. For instance, my 1200 mount with its well worn-in RA worm has a 5 arc sec P-P error maximum. I could reduce this by training the PEM, but this is not really necessary even with my 146 inch focal length. The 5 second error occurs twice over a 6.4 minute time period (picture a sine wave from bottom to top back down to bottom). The error has two points at top and bottom where the star does not appear to move at all for a long time, and then a connecting period between the upper and lower peaks when the star is moving at the maximum rate. This time is approximately 3.2 minutes (194 seconds) where the star is moving 5 arc seconds total in RA. So the rate is approximately 38 seconds of time per arc second of movement in RA. If I want to limit my maximum error to 0.5 arc seconds, then I can take up to 14 second exposures with my guider (minus the download time etc) and still have perfectly round stars. I can also take 14 second Track & Accumulate images all night long and have a very nice result.Some things that can screw up your guiding that you should watch out for: 1. wires dangling off the back of CCD cameras that can subtly "pull" the camera position in the focuser as the mount slowly tracks across the sky. Place the wires along the tube assembly and tie securely to the mounting rings before dropping them to the ground below. 2. the fan on the ST7/8/10 adds a lot of vibration to the system. You cannot disconnect it, but what i have done is to isolate it completely from the camera body by glueing it to two layers of carpet backing with a hole cut into the backing to allow airflow. Carpet backing can be purchased in large sheets at KMart for a song. It is gummy soft rubber, about 1/8 inch thick and is full of holes. The mounting screws cannot be used, so make sure the glue holds well. I use Pliobond. Without doing this, my stars are not round, but measure sometimes 1x3 arc sec, depending on the orientation of the mount. Worst orientation is looking due south. 3. People walking near the scope will shift the ground enough to cause 1 - 4 arc second jumps in image position on the CCD chip.4. Not fully cooled down scope will cause all kinds of tube currents which can drive a guider nuts.5. Put the Dec backlash setting to zero. There is no need for a guider to get instant reversals. You cannot follow random motions of guide stars, only the slow steady drift caused by slight polar misalignment and the natural periodic error of the worm.6. DO NOT have "The Sky" planetarium program up and linked to your mount. This program will interfere with your tracking in a big way. I found out the hard way after wasting many hours imaging, only to have erratic guide star motion during every exposure. The Sky, when linked to the mount can (and will) send random motion commands via the Com port, and this is NOT what you want during your exposure. There are others which I can't think of right now. I hope I have given you some info. Good luck. It can be done. I know because a complete Doofus like myself has figured it out (just took me a month of sundays).

Roland Christen

I got about 16 arc seconds peak to valley but this was near meridian and I'm fairly new at this. 16 arc seconds seems high for that gear.

The best way to understand periodic error is to imagine a constant rate mover pushing on the periphery of a radius bar. The further out the mover pushes, the slower the rotation, and the further in it pushes, the faster will be the rotation. Worms are held at the ends in precision bearings, but even the best bearing available today has a runout of several tenths, so a perfect worm will wobble in and out by that amount. Worms are never perfect either even though the utmost care is taken to ensure zero runout. The total wobble of worm plus two bearings can reach 5 tenths (.0005"). This translates to an in-out motion and the driving point on the radius of the worm gear will vary by this amount. Normally, this amount of motion would translate to less than an arc second movement over one worm rotation, assuming the worm is properly meshed with the wheel. If the mesh becomes too loose, the effects of this motion become amplified and the periodic error can have strange peaks in it where mesh is close to being lost.To minimize the effects of this runout, the entire worm assembly sits on a platform that has a spring machined into it. By properly meshing the RA gear, the spring will keep the worm in contact with the worm wheel teeth during the whole rotation, even if the runout were twice that. Unfortunately, the spring has a very short working length, only about .002", beyond which it looses its ability to keep the worm in contact. Longer range springs could have been used (and have been used in the past by other mount makers), but the effect of this is to make the mount mushy, and vulnerable to wind load. When a scope is on a mount and a gust of wind comes along, the energy is applied to the worm wheel, which pushes against the worm, which then pushes against the spring, and the whole shebang will move the entire distance of the spring length. So, what is an amateur to do? The idea is to keep your mount in top tune by simply adjusting your worm mesh once in a while. It's not hard to do. It consists of loosening a few screws, gently pressing the entire worm assembly into the worm wheel and retightening the screws. The built-in spring will do the rest - you are basically preloading the spring. How much pressure? Too little of course will show up as a small amount of backlash in the axis. Too much and your motors will begin sounding strained during high speed slewing. I use the pressure of my index finger with the other hand tightening the screws underneath the assembly. I wish there were a more automatic way to do this. For old hands at this, its no big deal. For new guys just entering this hobby and wanting to take dynamite images right out of the box, well it means a bit of practice. In the future, I have some ideas to make this process unnecessary, but it requires a whole new way of driving and mounting the worm assemblies. Having said all that, it may very well be that there is something wrong with the mount. The best way to see that is for you to take the "pulse" of the mount, i.e. let me see a trailed image complete with arc sec per pixel data. The image should show at least two worm revolutions, around 15 minutes will be enough. To trail your image, simply offset your azimuth by a couple of degrees and shoot. The resultant wiggles will show a sinusoidal pattern along with some minor dips. If there is any damage to the worm, there will be tell-tale effects in the sinusoid. Without this sort of data, you are always just shooting in the dark. This is advise for everyone. Learn as much about your mount as possible. Don't treat it as a dark box which will automatically always do what you want. The more you know about its ins and outs, the easier it will be for you to spot trouble and be able to describe what is really wrong with the system. A log of your periodic error is nice to have just to see how it changes over time (it should get better and smoother).

Roland Christen

Recommended Auto Guiding controller speeds.

1. Calibrating at 1x sidereal, setting the software aggressiveness to 0.5 (or 5 in MaxImDL) and guiding at 1x sidereal.
2. Calibrating at 1x sidereal, setting the software aggressiveness to 1.0 (or 10 in MaxImDL) and guiding at 0.5x sidereal.

The reason that I recommend calibrating at 1x is that this minimizes the Dec >reversal delay so that you end up with the most accurate
parameters for subsequent guiding. You can then guide at 1x and adjust the agressiveness depending on how much the stars are jumping around, or you can guide at .5x and even .25x which would correspond to .5 and .25 agressiveness.

Roland Christen

I bought a 4X powermate, which gives me the recommended f/30. My problem is that at that high an F/ratio I loose much of my image quality due to over magnification, (even with a 40mm eyepiece at only 78X). When I replace the powermate with an AP convertible barlow I get much nicer views at 39X with the 40mm

If you are placing the Daystar filter after a 2" diagonal which is after the Barlow, then the TV system is working at much more than 4x. This is why you cannot see the whole sun. The thing to do is to add a telecentric lens to the back of the AP Barlow, one extension tube between and you will get your F30 beam. You will see the whole sun easily. At 78x, you will get almost twice the field needed to show the entire disc. In this arrangement the normally 2x Barcon will be operating at around 4x and there is no need to worry about overheating the filter. Another secret I will tell you that Del might not be aware of, if you use our Maxbright diagonal before the Daystar, no IR heating energy will be passed through to the filter. The Maxbright cuts off all light down to very minute level past the Ha wavelength. It's transmission dies to under 1% above 700 nm. Thus, no worries about heating of the sensitive filter elements or premature ageing of the cover filter. This same thing is true of white light viewing. if you are in the least worried about a filter like the Baader foil passing dangerous levels of invisible IR energy, well nothing gets through. It is completely eliminated after the diagonal. One astute customer noted to me some months back that he can actually notice the difference in the amount of heat transmitted by an unfiltered refractor when placing his hand straight back without the diagonal and then over the diagonal. He said the bright light did not feel anywhere near as warm behind the diagonal. Still WARNING: DO NOT EVER PLACE YOUR EYE INTO THE DIAGONAL WHEN POINTED AT THE SUN IN AN UNFILTERED TELESCOPE!!!. Please be careful, there is still a lot of white light and UV energy with an unfiltered scope, even when the IR is missing.

Safe viewing

Roland Christen