900GTO German Equatorial Mount (900GTO)
with GTOCP3 Control Box

Includes GTO Keypad and CDs with PulseGuide and PEMPro (full version) software

New in 2008

Since it was first introduced 10 years ago in 1998, the 900GTO has been a solid, reliable performer both in the field and permanently mounted in an observatory. These are the only significant changes in this 2008 production run:

Why a 900GTO?

Stability ... accuracy ... ease of setup ... friendly go-to control interface - these were the primary design goals of the 900GTO. Amateurs today demand a precise mount that will allow them to pursue all aspects of astronomy whether it be observing visually, CCD imaging or photographing with 35mm or 6x7 camera. Many of the finest imagers today have been using our GTO mounts as a solid platform for a wide variety of instruments. The largest of the current production mounts is our 1200GTO the next in line is the 900GTO and the most compact is the Mach1GTO. As I write this in early 2008, final preparations are underway for our new very large 3600GTO German Equatorial, also known as El Capitan. However, if your instrument of choice is a medium-sized refractor, Cassegrain, Ritchey-Chretien, Newtonian or astrograph, the 900GTO may be the ideal choice for you.

Since its introduction in 1998, the 900GTO has gained a reputation for both tracking and pointing accuracy, essential to casual visual observation as well as advanced imaging. Quite a few 900GTO and 1200GTO mounts can be found at major star parties throughout the country and the world. It is a favorite of many advanced photographers and imagers. Visit the Cool Sites and Gallery sections of our website to see photos of the mount in action and images taken while using the 900GTO.

These mounts are truly a marvel of engineering - maximum strength and rigidity with minimum weight. Our CNC lathes and mills carve out the excess material in both axes of the 900GTO German Equatorial while retaining a heavily ribbed structure for internal strength and rigidity. A unique dovetail was machined into the mating surfaces of the R.A. and Dec axes. This feature allows quick and easy assembly in the field without any tools.

The 900 equatorial is equally at home in a permanent observatory or as a portable mounting for remote star parties thanks to the ease with which the two axes come apart and the reasonable weight of each component; the heaviest is only 25 lbs! Telescopes commonly used include Astro-Physics 155-180mm refractors, 10-11" Schmidt-Cassegrains, 10" Maksutov-Cassegrains and 10-12.5" Ritchey-Chretiens and other scopes of similar size and weight.

Gilles Cohen, of France, has been using his 900GTO for several years and has documented its impressive performance. He not only prepared a beautiful French page to show off the mount, but he also was inspired to prepare a second page in English for the rest of us. Gilles is a rising star imager who does his work from a major urban site with lots of light pollution and not always the best seeing, but has managed to take some excellent astrophotos - some of which are posted on his Gallery Page.

The 900GTO mount is built from the ground up to be a precision imaging platform while still being totally user friendly. Consider these advantages and features:


Extremely solid, rugged, high payload mount, yet comes apart in two reasonably light-weight components for hassle-free field setup. No tools needed to assemble the mount in the field.

Operate with 12V battery.
You can take it to the darkest skies and power it with a commonly available 12V battery. In the observatory, we suggest a minimum 5 amp filtered, regulated power supply.

Clutches and setting circles.
Allow manual operation if power is not available.

Image past the meridian.
The mount will track and guide well past the meridian in either direction if the object is located such that the telescope will clear the pier. This allows the user to set up the mount for a long series of exposures without stopping in the middle to flip sides. One can start the telescope under the mount while pointing at an object in the eastern part of the sky and track it all the way deep into the western sky. This is very useful for long exposure H-alpha or in cases where a large number of individual exposures are needed for stacking.

Easy alignment for non-critical viewing.
You can align the 900GTO using either of the two clever setup routines in the keypad, or with a polar alignment scope to quickly zero in on the pole for most non-critical observing or to get close before tweaking in for imaging. (Polar scope not included. We recommend the PASILL4). You can even align the mount during the day with our clever Daytime Polar Alignment Routine (outlined in the Keypad Manual) for solar observing, viewing the planets at twilight, or just getting a jump start on a long night.

External computer not needed.
The keypad is a handheld computer with all of the features, functions and databases you need to tour the universe night after night. The unique firmware allows you to precisely polar align your mount in the field, even in broad daylight! The vacuum-fluorescent display with a temperature range of -40 degrees F (and C, they are the same in this instance), allows hardy observers to use this mount on cold winter nights. You can't do that with a PC or PDA! As new firmware versions are released in the future, you can upgrade your keypad directly from the download section of our website - free of charge!

Control with personal computer, if desired.
All functions of the servo drive can be commanded from a laptop or desktop computer using popular planetarium programs or PulseGuide software. Depending on the features of the program, you can position your telescope, center your image and control tracking rate, remote focusing, reticle brightness and park at the end of your observing session. Examples of currently available software: Write your own computer program.
The Astro-Physics GTO protocol for the GTOCP3 Control Box is freely available to those who would like to write their own computer program for controlling the mount.

Precise Mechanical Fabrication

Highly accurate mechanics
Using modern CNC machining techniques, we make all components to a high precision level, which results in a final package that is solid and accurate in all respects. The critical angles are accurately machined so that the mount is orthogonal to a very high degree. This results in pointing accuracies well below 1 arc minute for a properly aligned mount.

Worm gear accuracy
Critical worm gear accuracy is maintained by special machining techniques developed at Astro-Physics after extensive studies and actual field operation. Our worm accuracies are second to none and are guaranteed to be 7 arc seconds (+/- 3.5 arc sec.) or less peak-to-peak for the 900GTO before introducing any periodic error correction. The periodic error of each mount is verified during our extensive testing procedures. If the PE exceeds these specifications, the faulty part is discarded and another one installed. Once we have established that the PE meets our spec, we optimize the PE curve with a special version of Ray Gralak's PEMPro software that was developed specifically for our test equipment. The opimized curve is then downloaded into the control box of the mount. As a result, your mount is ready for a night of critical imaging right out of the box. With good alignment, it is quite practical now to achieve unguided CCD images with today's hi-resolution cameras coupled to a 6" to 12" telescope.

Mechanical Features Specifications of Equatorial Head

R.A. worm wheel

7.2" (18.3cm), 225 tooth aluminum

Declination worm wheel

6" (15.2cm), 225 tooth aluminum

Worm gears


R.A. thrust surface

7.0" (17.8cm)diameter

Declination thrust surface

6.0" (15.2cm) diameter

R.A. shaft

2.2" (5.6cm) diameter

Declination shaft

1.75" (4.4cm) diameter

Latitude range

20 to 68 degrees with or without the polar scope attached.
0 to 20 or 68-88 degrees when using Hi-Lo Latitude Wedge (900WDG or 900WDGA)

Azimuth adjustment

Approximately 15 degrees

Setting circles

Porter Slip Ring design, engraved

Right ascension

4 minute increments, pointer, engraved both Northern/Southern


1 degree increments, pointer


Zero-cogging servo motors

Power consumption

0.4 amps at the sidereal rate

Power requirements

12 VDC, range 11.5 to 16

Weight of equatorial head

54 lbs (24.5 kg), Declination axis is 17 lbs. (7.7 kg) right ascension axis is 26.5 lbs.(12.0 kg), counterweight shaft with washer and knob is 10.5 lbs. (4.8 kg)


Approximately 70 lbs. (31.8kg) scope and accessories (not including counterweights), depending on length. Will accommodate Astro-Physics and similar refractors up to 180mm f9, 12" Cassegrains, 12-14" Ritchey-Chretiens
These are only guidelines. Some telescopes are long for their weight or very heavy for their size and will require a larger mount.

GTOCP3 Servo Motor Drive System

The servo drive system uses a high-quality Swiss DC servo motor controlled by a microprocessor to an accuracy of 0.05 arc-seconds per step. Tracking is very smooth, noticeably smoother than any stepper motor drive or inexpensive servo motor. The system can be accurately controlled over a speed range of 4800:1, which allows 0.25x sidereal for guiding to 1200x sidereal for 5 degree per second slewing. The circuit draws only 0.4 amps when tracking the stars, 2 amps with both motors slewing and requires only 12 volts to operate. The servo drive will satisfy the requirements of the sophisticated, advanced astrophotographer, yet is easy for the casual, visual observer to use.

The GTOCP3 Control Box contains all of the circuitry to drive the two servo motors and the logic required to navigate the sky. It can be easily removed from the mount, if required.

The GTO Keypad is a self-contained computer that serves as the communication center for the operation of your GTO mount. We have designed the interface and firmware to be as intuitive and easy to use as possible while filling it with terrific features to make your observing session productive and pleasurable.

All of the 900GTO electronics are now lead-free. Astro-Physics believes that we who love the night sky should also love and protect our good earth. Please refer to GTOCP3 Control Box and Keypad for Servo Drive for more detailed information.

PulseGuide™ Software

Please note: Ray Gralak, the author of PulseGuide, is currently working on a new piece of software that will combine the Astro-Physics ASCOM driver, all the features of PulseGuide, plus additional features. The new software will also act as a hub for a number of other applications that can then all communicate with the mount through a single COM port. The new software will have a new name befitting its vastly increased capabilities, and will replace PulseGuide when it becomes available. In the mean time, here's the lowdown on the existing PulseGuide software:

PulseGuide is a stand-alone Windows (98, ME, 2000, NT4, XP only) utility that provides complete remote control of the Astro-Physics Mach1GTO, 400GTO, 600EGTO, 900GTO, 1200GTO  and 3600GTO mounts. It derives its name from its most distinctive feature, pulse guiding, which can improve unguided tracking. Specifically, it can help correct tracking errors caused by polar misalignment and atmospheric refraction. You can also train PulseGuide to track objects moving relative to the stars, such as asteroids, comets, and the moon. In addition to pulse guiding, PulseGuide also has many useful utility features. With just a few exceptions, it supports the entire serial Astro-Physics command protocol allowing full control of your mount through your PC. This program is indispensable for remote control of your mount. More information on our website ....

PulseGuide was written by Ray Gralak of Sirius-Imaging. Please refer to his web site for further developments and enhancements. The software will be provided on a CD-ROM with your 900GTO mount. Previous owners of any Astro-Physics mounts can download the software free-of-charge, courtesy of Ray. Many, many thanks to Ray for this powerful program.

PEMPro Software, Version 2

PEMPro (Periodic Error Management Professional), Version 2, by Sirius Imaging is a very powerful Windows software application that allows the user to correct your mounts periodic error, polar alignment and backlash using your CCD Camera or Webcam to dramatically improve tracking and guiding. As mentioned previously, our extensive testing procedures include several PEMPro runs so that your new mount will perform with optimum PE correction out of the box. Simply use the Keypad or the PulseGuide software to turn PE Correction "on." Although you will not need to use PEMPro when you receive your mount, you may wish to do a PEMPro run at a later date after the worm gear has run in. Depending on usage, re-measuring error and re-loading periodic error correction from one to four times a year is a good practice, especially if you do some unguided images from time to time.

Pier Adapter Options

The 900 Standard Pier Adapter (900SPA) was included with 900 mounts sold in the past. However, in 2005, we decided to offer several choices to meet a variety of needs. We ask that you add one of these options to your mount order. If you plan to have multiple pier installations, perhaps one permanent and another portable, you may wish to purchase an extra adapter to install on the the second pier. This will make your setup and switchover much quicker. Many customers have found this to be advantageous.

If you do not purchase one of our pier adapters described below, for instance, if you purchase the Monolith Pier from Particle Wave Technologies, you will need to purchase the Pier Adapter Knob Kit (part# 9KBKIT) in order to attach your mount to the Monolith.

Complete descriptions and photos are available at the links below.

Dimensions for Observatory Planning

People often ask us what the 900GTO's dimensions are. The answer is: It depends. Different latitude settings can drastically affect the shape and dimensions of the mount. To answer this question we have prepared the following PDF document: 900GTO Dimension Sheet and this Excel spreadsheet Latitude Dimension Calculator. Please note: both this sheet and the calculator show the latest version of the 900GTO with the new improved Polar Fork. To see an example of an older polar fork style, go to the History section of our website. As the dimension sheet points out, all horizontal dimensions are the same for ALL versions of the 900 mount. If you have a different looking polar fork from the one pictured on the 900GTO Dimension Sheet, simply subtract 7/8" from the vertical dimensions.


All Astro-Physics mounts are supported by a dedicated staff who will assist with any issues that may arise. You can talk to a real person in a timely manner. Astro-Physics staff also participate the ap-gto user groups in the Yahoogroups forum. The user group is comprised of GTO owners who are willing to share their wealth of experience using their mounts as well as imaging strategies and other concerns of advanced amateurs.

Price and Delivery Information

With rare exception, the 900GTO is not available on the shelf for immediate purchase. We typically maintain notification lists so that we can notify prospective customers when the mount is available to order. Please refer to our availability page for more information on our notification lists. If you are not already on the notification list for the 900GTO, you can sign up on our online notification form. An automatic confirmation e-mail will be sent to you for your records. Refer to Notification Update periodically to find out the status our mount and telescope production runs - what we are making throughout the year.

900GTO $8,450.00

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Astro-Physics, Inc.
11250 Forest Hills Road, Machesney Park, IL  61115, U.S.A.
Phone: 815-282-1513   Fax: 815-282-9847