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Astro-Physics 3600GTO German Equatorial Mount


Solid as a rock!
El Capitan is the ultimate imaging platform for large instruments like the 20" or 24" Ritchey-Cretiens by RC Optical, PlaneWave CDK17 or CDK20, RC20 or RC24 by Optical Guidance Systems 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 now offer two versions of this mount to suit your application. The version designated as 3600GTO is the standard version and the 3600GTOPE includes the built-in Precision Encoder System. Both versions of the mount include these features:

  • Large, robust, rugged, simple modular design that is easy to fix if something should go wrong
  • The design incorporates the same underlying principles as our smaller mountings and uses proven components that have withstood the test of time under all conditions, even in the harshest environment of Antartica.
  • Extremely smooth and accurate dual servomotor-worm gear drive, designed for all astro-imaging situations
  • Machining craftsmanship and beautiful mechanical design second to none
  • Ability to carry very large and heavy scopes comfortably
  • Simple mechanics make adjustment easy and accurate
  • Feature-packed software is intuitive and powerful. You can also use popular commercially available software or write your own.
  • Optional Limit Switch System - for enhancing the operation of the mount for remote imaging
  • Optional telescope mounting system with the convenience of a dovetail that can be locked down into a massive, solid platform.
  • Extensive support via direct communication with our technicians to fix a problem or help you to get the most out of your mount

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.

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 and, in an innovative twist, puts the Super Heavy-Duty Azimuth Adjuster 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 allow 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 and 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.

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 almost 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. 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. With good alignment and PEM training with the Keypad or PEMPro, it is quite practical now to achieve unguided CCD images with today's hi-resolution cameras coupled to a large telescope.

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. However, in the process of testing the RA axis with a special version of Ray Gralak's PEMPro developed specifically for our test equipment, we generate a unique, optimized PE curve for each individual mount that we program back into that mount. Simply use the Keypad or PulseGuide software to turn PE Correction "on," and you will see results are even better than our published 5 arc second spec!

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

Specifications of Equatorial Head

Worm Wheels-RA & Dec Aluminum - 13" (330mm), 256 tooth
Worm Gears Brass - 1.41" (35.8 mm) diameter
Axis Shafts 4.72" (120mm) diameter with 4.02" (102mm) clear inside diameter
Axis Bearings 7.09 " (180mm) diameter deep groove ball bearings
Worm gear bearings 1.57" (40mm) angular contact ball bearings
Latitude range 15 to 70 degrees
Azimuth adjustment Approximately 14 degrees (+/- 7 deg.)
Motors Swiss DC servo controlled - 2 motors per axis
Instrument Mounting Declination hub has ten 3/8-16 tapped holes on a 12.000" bolt circle. 
Capacity Conservatively rated for 300 lbs (not including counterweights). Please feel free to call and discuss your application with our staff.
Counterweight Shaft 2.5" (6.35 cm) diameter, 31.5" (80 cm) total length, 30" (76 cm) useable, 42 lbs. (19kg). Twelve of the optional 30# counterweights will fit onto this shaft. Includes large, machined safety stop. An optional shaft extension (M3655) is also available.
Diameter of Base 11.480" (29.16 cm), fits into 12 " outside diameter pier with 0.25" wall thickness.
Weight Equatorial head (without counterweight shaft or counterweights): 205 lbs. (93 kg), Dec axis is 84 lbs. (38kg), RA axis and base assembly is 121 lbs. (55kg)
Counterweight shaft: 42 lbs. (19kg)

Servo Motor Drive

The drive system uses TWO high-quality Swiss DC servo motors for 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 dual motor system smoothes out the drives even further by applying an averaged power curve to the drive system. In preliminary testing this has resulted in PE curves that are noticeably smoother than the already phenomenal curves on our 1200GTO mounts. 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 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 our electronics are now lead-free. Astro-Physics believes that we who love the night sky should also love and protect our good earth.

For reliable operation in your observatory, we recommend that you use a minimum 10 amp filtered, regulated power supply like our 15V, 10 amp power supply (PS15V10AC). For lighter, well-balanced loads, you can use a commonly available heavy-duty, deep-cycle marine 12V battery.

Flexible Control with either Keypad or Personal Computer

Keypad Operation
The keypad is a handheld computer with all of the features, functions and databases you need to tour the universe night after night. The vacuum-fluorescent display with a temperature range down to -40 degrees F (and C, they are the same in this instance), allows hardy observers to use this mount on cold winter nights.

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. Serial to USB connectivity has been simplified by the addition of Keyspan Serial-to-USB devices to our product line.  Examples of currently available software:

  • PulseGuide by Ray Gralak (included with the 3600GTO) Note: a new, more robust remote control software is currently under development that will ultimately replace PulseGuide.
  • Software Bisque's suite which includes TheSky Astronomy Software, CCDSoft CCD Astronomy Software, TPoint Telescope Pointing Analysis Software and Orchestrate Scripting Software.
  • ACP Observatory Control Software by DC-3 Dreams - Robert B Denney
  • Starry Night Pro by Simulation Curriculum Corp.
  • Earth Centered Universe (ECU) by David Lane of Nova Astronomics
  • SkyMap Pro by Chris Marriot
  • Any software that is ASCOM compliant.

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.

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).

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.

Preliminary 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.

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.

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.

PulseGuide™ Software

Please note: Ray Gralak, the author of PulseGuide, is currently working on a new software program that will combine the Astro-Physics ASCOM driver, all the features of PulseGuide, plus additional features. The new software will have a new name befitting its vastly increased capabilities (tentatively the Astro-Physics Command Center), 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 and Vista) utility that provides complete remote control of Astro-Physics Mach2GTO, 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.

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 will be 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.


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 in 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.

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