3600GTO German Equatorial Mount (3600GTO)
3600GTO German Equatorial Mount with Precision Encoder (3600GTOPE)
" el Capitan "
Includes GTOCP3 Control Box, GTO Keypad and CD with PulseGuide and PEMPro v.2 software (full version)
3600GTO with GTOCP3 Control Box is no longer available

Solid as a rock!

El Capitan is the ultimate imaging platform for large instruments:
RC Optical Systems - 20" or 24" Ritchey-Cretiens
PlaneWave Instruments - CDK17, CDK20, or CDK24
Optical Guidance Systems - RC20 or RC24 or instruments of similar size.
Setups that employ smaller multiple instruments would also be ideal.

Why a 3600GTO ?

The 3600GTO is the culmination of over 30 years of experience designing and building mounts at Astro-Physics for the amateur and professional astrophotographer. We take great pride in the soundness of our designs, the quality of our workmanship and unparalleled support of our customers.

"I am in the process of planning a trip to the remote observatory I own at Arizona Sky Village. I realized that the last time I was there was to install the AP3600GTO in April of last year, and that since that time it has been operating every clear night and has not missed a beat ...The mount has been running for over one year and surpassed my expectations in every way". David Churchill. Read more of his comments here.

At the Advanced Imaging Conference (AIC) in late October 2010, Roland Christen demonstrated how "El Capitan" can easily hold (and track) with a 24" PlaneWave Corrected Dall-Kirkham (CDK) in an inverted position (scope underneath the counterweights). The scope weighs 240 lbs. and the counterweights are an additional 240 lbs. The mount never broke a sweat.

We offer two versions of this mount: the 3600GTO is the standard version and the 3600GTOPE includes the built-in Precision Encoder System. Both are available with the optional Limit/Homing Switch System. If you have a large instrument or several smaller instruments, we invite you to review the information below.

Mechanical Features

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.

. . . 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... (commenting on his 1200GTO that uses the same control system. All AP mounts have this feature!)

Full 360 degree rotation of both axes.
The full 360 degree rotation of both of the 3600GTO's axes also allows this mounting to be placed onto an astrographic pier for complete freedom from movement restrictions.

Through-the-mount cabling capability.
We designed this mount to allow cabling to be run through the axis shafts for tangle and clutter-free operation. Cables from cameras, guiders, dew heaters and the mount's own servo drive cables can all be run through the mount's cavernous 4" hollow shafts where they are out of the way and where they won't ever get caught or tangled during your imaging session. Ports have been included for the mount's servo cables. Additional ports can be added by the user, as needed.

Components are modular and easy to maintain.
The design of the 3600GTO is straightforward with modular construction and no complicated internal wiring. 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. The mechanical design of the mount is extremely simple and straightforward, allowing easy adjustment by anyone - no need to have a mechanical engineering degree to service the mount. We will provide information in the Technical Support section of our website pertaining to typical adjustment issues.

Integrated Pier Adapter
The pier adapter for the 3600GTO has been integrated into the mount's design and incorporates the design principles of the Precision Adjust Rotating Pier Adapters available for our smaller mounts. In an innovative twist, the Super Heavy-Duty Azimuth Adjuster is UNDER the BACK of the mount where it is both conveniently located for fine polar alignment and remains out of the way. Polar alignment is smooth and easy, and the system locks down tight with no unintended movement.

For most installations, we recommend using our 3600 Flat Surface Adapter (3600FSA). Many of the photos on this page are of a prototype mount and prototype ATS Portable Pier. The photos do not include this recommended adapter.

Easy to Adjust and Polar Align.

Precise polar alignment can be achieved easily using the precision adjusters for both altitude and azimuth. The system locks down with zero alignment shift for an unprecedented ability to achieve that "perfect dead-on" polar alignment that is so critical to the best imaging and the most accurate go-to slews.

The PEMPRo Software program (included with the mount) includes the "Polar Alignment Wizard," a powerful feature that allows you to dial in on the pole quickly.

Extremely solid, rugged, high-payload mount, yet the equatorial head comes apart in two components for transport and field setup. Because of this mount's size and weight, we recommend that field assembly be done by at least two people working together. The RA axis/base assembly alone weighs just over 120 lbs. You wouldn't want to transport it as your regular mode of operation, but it can certainly be done for special occasions.

The 3600GTO is shown in the photo on the right with a 12" x 30" portable pier developed by ATS (Advanced Telescope Systems). You can review more information regarding ATS pier options at As noted above, the photo at right does not include the recommended 3600 Flat Surface Adapter (3600FSA). ATS 12" piers can now be purchased with a flat top plate which has been drilled and tapped to accept this adapter.

ATS also makes permanent piers for the 3600GTO that can be fitted with the same flat top plate. Please see the ATS website for details on their permanent piers.

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.

Our machinists are among the most skilled available anywhere, attending to the smallest detail to ensure a precision instrument. The finishing department assures that all parts are deburred, sanded, polished and painted to provide an exceptional aesthetic appearance. Our mount assembly is directed by a life-long astronomer with more than 20 years of experience here at Astro-Physics! All of our production personnel take great pride in their work and are true craftsmen and women.

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. The periodic error of each mount is verified during our intensive testing procedures prior to optimization as described below.

Pre-programmed with PEMPro
Periodic error corrections for each individual mount are optimized during extensive testing, which results in excellent performance right out of the box. 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. We optimize performance further by characterizing the error using a special version of PEMProAP and downloading the appropriate correction into the mount's GTOCP3 before it leaves Astro-Physics. Simply use the Keypad or PulseGuide software to turn PE Correction "on," and you will see results that are even better than our published 5 arc second spec! With additional PEMPro runs, you can achieve even higher levels of accuracy.

If your application requires the highest possible levels of tracking accuracy, you may want to consider the Precision Encoder System described below.

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./Dec. 13.0" (330 mm), 256 tooth aluminum
Worm gear - R.A./Dec. Brass, 1.41" (35.8 mm) diameter
Axis shaft - R.A./Dec. 4.72" (120 mm) diameter with 4.02" (102 mm) clear inside diameter
Shaft axis bearings - R.A./Dec. 7.09" (180 mm) diameter deep groove ball bearings
Worm gear bearings 1.57" (40 mm) angular contact ball bearings
Latitude range 15-70 degrees
Azimuth adjustment Approximately 14 degrees (+/- 7 degrees from center)
Counterweight shaft 2.5" (64 mm) diameter x 31.5" (800 mm) long [30" (760 mm) useable length], incl. large machined safety stop knob. Will fit 12 optional 30 lb. (13.6 kg) counterweights. Optional 10" (254 mm) shaft extension available.
Weight of mount Total: 247 lb. (112.3 kg); R.A. axis/polar fork: 121 lb. (55 kg), Dec. axis: 84 lb. (38.2 kg),
Counterweight shaft: 42 lb. (19.1 kg)
Capacity Conservatively rated for 300 lb. (136.4 kg) scope and accessories (not including counterweights). Recommended for: 20" and 24" Ritchey-Chretiens and CDKs.
Please feel free to call and discuss your application with our staff.
Instrument mounting interface

Declination hub has ten 3/8-16 tapped holes on a 12.000" bolt circle. Pdf available

Diameter of base 11.480" (291.6 mm) diameter. Use of the Flat Surface Adapter (3600FSA) is highly recommended.

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 0.044 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, removable
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 ~ 0.4 to 0.8 amps at 15 volts - tracking
~ 1.5 to 3.0 amps at 15 volts - both motors slewing at top speed of 600x
Power requirements Nominal 15 volt DC supply at 10 amps recommended
Periodic error

5 arc seconds peak-to-peak (+/- 2.5 arc seconds) or less in one worm cycle (5.6 minutes); native, prior to any error correction

Maximum slew speed 2.5 degrees / second

Servo Motor Drive

The drive system uses a high-torque precision Swiss DC servo motor in each axis controlled by a microprocessor in the GTOCP3 control box.   The encoders on these motors provide a resolution of 0.05 arc seconds per step for incredible pointing accuracy.  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 manual guiding to roughly 720x sidereal for 3 degrees per second maximum slew rate. The circuit draws well under an amp when tracking the stars, 3 to 5 amps with both axes slewing and requires only 12 volts to operate (15-18 volts is recommended). This 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 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 our electronics are now lead-free. Astro-Physics believes that we who love the night sky should also love and protect our good earth.

Flexible Control with Your Keypad, Computer or even your Mobile Smart Device!

As this chart shows, there are many options available for controlling your mount. Take a few minutes to link to descriptions of the varied software and devices that are available.

You will be able to further link to a full size view of this chart and discover more about each method.

Precision Encoder System for the 3600GTOPE Version

Please note that the Precision Encoder System is only included when you order the 3600GTOPE version of the mount. It is not included in the standard version (3600GTO) and cannot be retrofit at a later date.

A precision encoder installed in the RA axis provides feedback to the servo to eliminate periodic error to a level below 1/2 arc second for long-period exposures.   This feedback system eliminates periodic error by monitoring the actual output speed of the RA axis using this encoder ring to continuously adjust the speed of the servo motor, which keeps the axis rotating at a very precise and constant rate. The operation of the encoder does not interfere with any external guiding, centering or slewing commands sent by the keypad, CCD camera or external software. Its only function is to keep the axis tracking precisely at the sidereal rate, and will also follow any variable rate that is programmed into the servo for purposes of countering atmospheric refraction and other sources of drift.

Test results on the new 3600 mount with optional feedback encoder have exceeded all our expectations. The measured tracking error was well below 1/2 arc second for a 20 minute time period. With this option, it is now practical to do long period unguided imaging with a fixed setup where other sources of drift (due to polar misalignment, tube flexure, etc) have been minimized or eliminated.

Astro-Physics pioneered the use of precision encoders, normally used in the machine tool industry, to help you achieve optimal results during unguided or guided astrophotography. The 9" stainless steel Renishaw encoder that we have chosen is dimensionally stable and has guaranteed subarc second resolution over a 24 hour cycle. It is precisely aligned with the corresponding Renishaw Read-Head and tested. This is not a simple tape encoder, which cannot guarantee precise alignment and can shrink with temperature.

To further refine the unguided error of the mount, we are adding an all-sky tracking rate to the software, which will minimize RA drift everywhere in the sky. This will allow significant unguided exposure times with a well polar-aligned mount.

PEMPro v. 2.0 Software - FULL Version!

PEMPro (Periodic Error Management Professional) is a Windows software application developed by Ray Gralak that makes it easy to characterize and reduce periodic error. While the periodic error of your 3600GTO has already been optimized with a special version of PEMProAP to be well under 5 arc seconds (+-2.5 arc sec.) or less, you can reduce it even further to maximize performance without auto-guiding. PEMPro v. 2.0 is the latest version of PEMPro and includes exciting new features like the "Polar Alignment Wizard" to quickly, accurately and easily dial in your polar alignment. The software will be included with all new 3600GTO mounts.

3600GTO - What's Included?

3600GTOPE (with the Precision Encoder) - What's Included?


We have prepared some pertinent support documents relating to this mount and many of our other products on our Technical Support page. Please check there first to find the information that you need.

Upgrades and Spare Parts

Check out our website to find accessories, replacement parts or learn about possible upgrades for your mount.

Copyright © 1999-2019, Astro-Physics, Inc. - All Rights Reserved
This page was last modified: March 11, 2019

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