1200GTO German Equatorial Mount (1200GTO)
with GTOCP3 Control Box
Includes GTO Keypad and CDs with PulseGuide and PEMPro (full version) software

The 1200GTO German Equatorial Mount with the GTOCP3 Control Box is no longer available to purchase. Please refer to information regarding our 1600GTO model if you are interested in a new mount in this size class. The information below is provided for present owners and those who are curious about its history. If you are one of the lucky owners, you may want to consider some of these accessories. If you are interested in some of the earlier models, check out this Brief History of the 1200 German Equatorial.

Why a 1200GTO?

With the advent of the CCD camera, amateurs are exploring the skies to an ever increasing level of precision, easily exceeding the image quality of large observatories using film techniques decades ago. This new level puts a higher demand on the precision of the equatorial mounting. Many of the finest imagers today have been using our GTO mounts as a solid platform for a wide variety of instruments. Until the introduction of the 3600GTO at the Advanced Imaging Conferrence in 2007, the largest of these was our 1200GTO.

Since its introduction in 1998, the 1200GTO has gained a reputation for both tracking and pointing accuracy, essential to casual visual observation as well as advanced imaging. Quite a few 1200GTO and 900GTO (smaller cousin) mounts can be found at Mount Pinos in California, favorite observing site of many advanced photographers and imagers. You will find them at star parties and in observatories throughout the world.

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 1200 and 900 German Equatorials 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.

Over the years, a number of improvements have been incorporated into the design of the 1200GTO, among these are smoother azimuth adjusters; increased height of the polar forks to allow more room for your hand when making altitude adjustments; rounded profile of the base to prevent cords from catching; improved gear profiles in the motor gearbox for quieter operation and reduced backlash; motor bracket with built-in spring-action to keep the worms loaded at the proper setting; verifiable worm gear accuracy of 5 arc seconds or less; internal changes to improve overall accuracy; several pier adapter options; updated GTOCP3 control box with additional capability; updated keypad with larger memory capacity and LED fiber optic backlit panel; included PulseGuide and PEMPro software as described below. Each time we begin a production run, we ask ourselves what improvements we can make to improve the accuracy and functionality of the 1200GTO. As a result, the 1200GTO has evolved over the years to meet the needs of the advanced imager or visual observer.

The 1200 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. You CAN have it all. This is the perfect mount for a large refractor, Newtonian, Cassegrain or astrograph. Telescopes commonly used include Astro-Physics 155-206 mm refractors, 12-14" Schmidt-Cassegrains and 10-16" Ritchey-Chretiens and other instruments of similar size.

Our customers have been our greatest promoters through the years. Look for them at your next star party, read a few comments we have gathered and check out photos of various scopes on the 1200GTO. You are also welcome to ask any questions or check the archives of the ap-gto users group.

The 1200GTO mount is built from the ground up to be a precision imaging platform while still remaining totally user friendly. Consider these advantages and features:


Extremely solid, rugged, high payload mount, yet the equatorial head 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 like our 13.8V, 5-amp Power Supply (PS13V5A). For a heavy payload in cold conditions, we offer a 15V, 12-amp power supply (PS15V12AC).

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.

The 1200GTO’s ability to continue tracking when past the meridian is an absolute winner for me. It avoids meridian flips (=imaging time) and reduces the need for expensive rotators. Jan Rek. More…

. . . I love the ability to place the OTA on the "wrong side" of the mount when starting an imaging session ("premature meridian flip") and which I have used extensively. This feature is pure INGENUITY! Anthony Ayiomamitis., More…
Easy alignment for non-critical viewing.
Can align 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 CCD. You can even align in the daytime for solar observing or viewing planets at twilight.

Components are modular.
All electronics including the GTOCP3 control box, motors, gearbox or cables can be removed for servicing. The RA and Dec axes can be shipped separately, if needed.

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!

Free Keypad Firmware Updates
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!

Remote Control with personal computer, if desired.
All functions of the servo drive can be commanded from a laptop or desktop computer using PulseGuide or popular planetarium 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. The GTOCP3 control box provides two RS-232 inputs so that you can operate the mount with two programs simultaneously. For instance, you can control most mount functions with PulseGuide, which takes full advantage of our firmware protocol, and use TheSky as your planetarium program. Examples of currently available software:

If your computer does not have serial port inputs, there are many USB to serial port adapters available on the market. Serial to USB connectivity has been simplified by the addition of Keyspan Serial-to-USB devices to our product line. Please review the products on this page to determine which one will work best for your set-up.

I love my mount! And, it works just great in an automated setup! In fact, it's just waiting for ACP to start imaging once it gets a bit darker! James McMillan. More…
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, actual field operation and many years of experience making precision mounts. Our worm accuracies are second to none and are guaranteed to be 5 arc seconds (+-2.5 arc sec.) or less, peak-to-peak. Since late March 2002, all 1200 mounts have been tested on our specialized periodic error tester. Only RA worm gears that provide an accuracy of 5 arc seconds or better will pass this inspection. Any worm gears with greater errors are removed from the mount and are scrapped. Another worm gear is installed and the mount is retested.
… did some autoguide testing and the results were very good - what strikes me was now smooth the PE was. James Providakes. More…
Pre-programmed with PEMPro.
Beginning in 2007, all 1200GTOs have been pre-programmed with a special version of PEMPro to reduce the periodic error even further before it leaves Astro-Physics. This information is stored in the control box so that the mount performs optimally out-of-the-box without further PEMPro runs. We will continue to provide a CD with the PEMPro program so that you can tweak the performance further after your worm gear has run in over time.

Easy to Adjust and Maintain.
The design of the 1200GTO is straightforward with modular construction and no complicated internal wiring. We provide information in the Technical Support section of our website pertaining to typical adjustment issues. If the electronics ever need servicing, the GTOCP3 control box can be easily removed and returned.

Mechanical Features

Mechanical Specifications

Construction All CNC machined aluminum bar stock, stainless steel, brass; stainless steel fasteners
Finishing and Assembly Every part is hand-finished and inspected. All assembly is done by hand, by highly skilled mount assembly staff.
Worm wheel - R.A. 10.3" (262 mm), 225 tooth aluminum
Worm wheel - Dec. 7.2" (183 mm), 225 tooth aluminum
Worm gear - R.A. Brass, 1.210" (30.7 mm) diameter
Worm gear - Dec. Brass, 0.710" (18 mm) diameter
Thrust Bearing - R.A. 9.5" (241 mm) diameter
Thrust bearing - Dec. 6.5" (165 mm) diameter
Axis shaft - R.A. 3.35" (85 mm) diameter
Axis shaft - Dec. 2.36" (60 mm) diameter
Shaft axis bearings - R.A./Dec. Each shaft is supported by 5 bearing elements, two preloaded ball bearings and 3 sets of massive thrust bearings
Latitude range 22 to 69 degrees with or without the polar scope attached.
0-22 or 69-90 degrees when using Hi-Lo Latitude Wedge (1200WDG or 1200WDGA)
Azimuth adjustment Approximately 14 degrees (+/- 7 degrees from center)
Setting circle - R.A. 4-minute increments, pointer, engraved both Northern/Southern, Porter Slip Ring
Setting circle - Dec. 1 degree increments, pointer, engraved, Porter Slip Ring
Counterweight shaft 1.875" (48 mm) diameter x 19.5" (495 mm) long [18.5" (470 mm) useable length], incl. large machined safety stop knob. Optional 9" (229 mm) shaft extension available.
Weight of mount Total: 95 lb. (43.2 kg)
R.A. axis/polar fork: 50 lb. (22.7 kg)
Dec. axis: 31 lb. (14.1 kg)
Counterweight shaft: 14 lb. (6.4 kg)
Capacity Approximately 140 lb. (63.6 kg) scope and accessories (not including counterweights), depending on length. Recommended for: Astro-Physics and similar fast refractors up to our 206 mm f8 Starfire EDF, 16" Cassegrains and Ritchey-Chretiens, 17" CDKs.
These are only guidelines. Some telescopes are very long for their weight or heavy for their size and will require a larger mount. Remember also that imaging requirements are more rigid than visual observation.
Instrument mounting interface Refer to list of suitable mounting plates below
Pier adapter base diameter 9.775" (248.3 mm) Base. The base is part of the chosen pier adapter, either the Rotating Pier Adapter (1200RPA), Standard Pier Adapter (1200SPA), or the Hi-Lo Latitude Wedge (1200WDGA or 1200WDG). The desired pier adapter must be purchased separately.

GTO Servo Drive System

The 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 manual guiding to 1200x sidereal for 5 degree per second slewing. The circuit draws approximately 0.5 amps when tracking the stars, 2.5 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.

Gear Accuracy and PE Performance
The heart of this mount is a precision gearbox using custom high-quality fine-pitch gears, coupled to a highly accurate worm and worm wheel. These parts are manufactured and assembled at our facility and individually tested to meet or exceed our periodic error specification of 5arc seconds (+/- 2.5 arc sec.) before introducing any periodic error correction. The PE is smoothly variant with extremely low ripple or moment to moment error. In contrast, mounts that use integral gearboxes have very small spur gears with tiny teeth and too few of them. This generally causes ripples in the tracking rate which are difficult to guide out.

Each mount is rigorously tested on our specialized periodic error tester. Only mounts that provide an accuracy of 5 arc seconds or better will pass this inspection. If the mount exhibits greater error, the substandard part is identified, replaced and the mount is retested.

Pre-loaded PEMPro Curve
As mentioned above, the spec of 5 arc seconds (+/- 2.5 arc sec.) is the maximum native mechanical error inherent in the mount's RA axis WITHOUT any software correction. In order optimize performance even further, we use a special version of Ray Gralak's PEMPro (described below) developed specifically for our test equipment. We generate a unique PE curve for each mount that we program into each GTOCP3 Control Box. Simply use the Keypad or your Astro-Physics software (ASCOM V2 driver, PulseGuide or APCC) to turn the PE Correction "on" and you will see results significantly better than our published 5 arc second spec without additional PEMPro runs! However, you may use PEMPro to further refine your PE, if you chose to do so.

GTOCP3 Control Box
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 1200GTO uses the same GTOCP3 Control Box as the 900GTO and Mach1GTO (the control box is black-anodized for the latter mount). In fact, the control boxes for all three mounts are interchangeable with one another in an emergency. Extensive information regarding the GTOCP3 can be found here.

GTO Keypad
The Keypad is a self-contained computer that allows you to control all essential telescope functions and includes an extensive database of objects. 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. Click here for detailed information regarding features and functions of the Keypad.

GTO Servo Drive System Specifications

Electronic components Rated for industrial and automotive applications
Motors High-torque, zero-cogging Swiss DC servo motors, enclosed in machined aluminum housing
Motor encoder 2000 tic quadrature yielding an effective resolution of 0.050 arcseconds per tic
Motor reduction gear train 57.6:1 gear reduction through a custom built set of large diameter, fine-toothed, precision, spur gears for vastly superior performance
Servo Motor Control Box GTOCP3 Control Box, removeable
Hand-held computer GTO Keypad to control all mount functions. Includes extensive databases and tour features in a simple, intuitive interface. Firmware updates via the internet
Power consumption ~ 0.3 to 0.6 amps at 13.8 volts - tracking
~ 1.3 to 2.5 amps at 13.8 volts - both motors slewing at 1200x
Power requirements Nominal 12 volt DC supply at minimum of 5 amps continuous
Periodic error

5 arc seconds peak-to-peak (+/- 2.5 arc seconds) or less in one worm cycle (6.4 minutes); native, prior to any error correction

Maximum slew speed 5 degrees / second

Servo Motor Drive

The 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 drive. 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 degrees per second. The circuit draws only 0.4 amps when tracking the stars, 2 amps with both motors slewing and requires only 12 volts to operate. This servo drive will satisfy the requirements of the sophisticated, advanced astrophotographer, yet is easy for the casual, visual observer to use. Please refer to GTOCP3 Control Box and Keypad for Servo Drive for additional information.

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 1200GTO 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

PulseGuide is a stand-alone Windows (98, ME, 2000, NT4, XP, Vista and Windows 7) utility that provides complete remote control of 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. More ....

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 1200GTO 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

Prior to 2004, the Standard Pier Adapter was included with the mount, however there are now several options available for your consideration. 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# 12KBKIT) in order to attach your mount to the Monolith.

Standard Pier Adapter (1200SPA)
This 1200 Standard Pier Adapter is similar to those that we have included with mounts in the past, however the azimuth adjustment block is slightly taller to accommodate the azimuth adjustor assembly. If you have a permanent installation, this base is a good choice since you will not have to set up every session. Please read the full product description for more information.

The adapter plate will either fit into your Astro-Physics Portable Pier and be bolted from the side or you can bolt it to the top of a flat surface. Refer to the diagram for bolt pattern information. For a more finished look, consider using our Flat Surface Adapter.

Precision-Adjust Rotating Pier Adapter with Azimuth Bearing (1200RPA)
The Rotating Pier Adapter was designed for very accurate and smooth adjustment of the azimuth angle without loosening the lock-down knobs on the base of the mount. Upgrade your previous model 1200 mount (any version) and enjoy the ease of use.

The Precision-Adjust Rotating Pier Adapter must fit inside another part, i.e. our Astro-Physics Portable Pier, and be bolted from the side. It cannot be bolted through the top as you can with the Standard Pier Adapter, however, you can use a Flat Surface Adapter.

Hi-Lo Latitude Wedge (1200WDG) and Wedge Assembly (1200WDGA)
If you live in the 0-22 or 69-90 degree latitude ranges, you will need the Latitude Wedge Assembly (1200WDGA) in order to align on the pole. Under certain circumstances you may only need the Latitude Wedge (1200WDG) without the the assembled parts .

The Latitude Wedge will fit into your Astro-Physics Portable Pier or if you wish to attach it to a flat surface, you will need our Flat Surface Adapter. It is also possible to use in conjunction with the Rotating Pier Adapter.


The diagram shows several dimensions that are commonly requested. In addition, if the scope is horizontal and pointing north or south, the measurement from the top of the pier to the middle of the cradle plate is 11.25." Please note that these dimensions will vary somewhat depending on your latitude. We quote them for 42 degrees because that is the latitude of Astro-Physics and all of our equipment is set up that way. It is also a good average value.

To determine the appropriate pier height for your system and to calculate many of the dimensions that vary with your latitude setting, please try this Excel spreadsheet Latitude Dimension Calculator.

You can also refer to this 1200GTO Latitude PDF document if you are planning an observatory and need to know the variations in the horizontal distance from the center of the mount base plate to the center of the declination axis that are caused by different latitude settings, but the Excel spreadsheet linked above will probably serve you better.


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.

We have prepared some pertinent support documents relating to this mount and many of our other products on our Technical Support page. You may find the information that you need simply by clicking here.

Price and Delivery Information

The 1200GTO German Equatorial Mount with the GTOCP3 Control Box is no longer available to purchase. Please refer to information regarding our 1600GTO model if you are interested in the next-generation mount in this size class.

1200GTO - What's Included?

Items that You will Need for Your Mount

Additional Accessories to Consider

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This page was last modified: October 11, 2017

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