Astro-Physics GTO-3600 Mount „El Capitan“ (various versions available)

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Astro-Physics GTO-3600 Mount „El Capitan“ (various versions available)

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Price: please inquire

  • German Equatorial Mount with Servo Motor Drive
  • approx. 300lb. capacity (not including counterweights)
  • the R.A. and Dec. axes split easily for a more comfortable assembly and / or transport
  • optional with Precision Encoder in RA axis (eliminates periodic error to a level below 1/2 arc second) - various versions available at the dropdown below

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Astro Physics GTO 3600
Comparsion between the 1200GTO and "el Capitano".

Astro-Physics 3600 GTO German Equatorial Mount "el Capitan"

with GTOCP4 Controll Box and optional with Precision Encoder

The 3600GTO is the culmination of over 30 years of experience designing and building mounts at Astro-Physics for the amateur and professional astrophotographer.

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

Astro-Physics offers two versions of this mount:

  • 3600GTO, German Equatorial Mount with Servo Motor Drive (standard version #1401440) and
  • 3600GTO, German Equatorial Mount with Servo Motor Drive AND Precision Encoder in the RA axis, #1401441 (eliminates periodic error to a level below 1/2 arc second)

Please note that the Precision Encoder System is only included when you order the 3600GTOPE (#1401441) version of the mount. It is not included in the standard version (3600GTO) and cannot be retrofitted 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.

3600GTO Mount "el Capitan":

  • the drive system uses a high-torque precision Swiss DC servo motor in each axis. The encoders on these motors provide a resolution of 0.05 arc seconds per step for incredible pointing accuracy.
  • completely redesigned CTOCP4 Control Box with modern chip technology resulting in faster processing speed and increased memory capacity for future feature additions. More information about the GTOCP4 Controll Box are available here
  • 300lb. capacity (not including counterweights). The declination axis can also be separated, but the 3600GTO is transportable only to a limited extent and better used as a stationary mount.
  • through-the-mount cabling capability
  • full 360 degree rotation of both axes
  • components are modular and easy to maintain
  • integrated Pier Adapter
  • the 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.
  • 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 to adjust and polar align. 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.
  • using modern CNC machining techniques. 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.
  • multiple options for controlling your mount in the field or in your observatory: You have your choice of the GTO Keypad (optional), which is a full-featured handheld computer, PC computers using a wide variety of software with or without our ASCOM driver, Mac computers or even your mobile smart device (like your iPhone or iPad)

For detailed information about the 3600GTO Mount please visit the original website of Astro Physics. If you have any further questions, please send us an email to kontakt (at) or call us.

Astro Physics GTO 3600 Antarktis
ASTRO-PHYSICS 3600GTO German Equatorial Mount at Dome C in Antarctica

The reliability of Astro-Physics mounts is legendary. Two "el Capitan" are working in the Antarctic for several years now, even at temperatures of -75° Celsius. Many of our customers use the same mounts for 20 years or longer without any problem and without failures.

GoTo in perfection

The 3600GTO is compatible with PulseGuide and PemPro. With PulseGuide and PEMPro, you can achieve even higher levels of accuracy.

CP4 Control

For detailed information please visit the original website of Astro-Physics

GTO Servo Drive System Specifications

Electronic components Rated for industrial and automotive applications
Motors 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 GTOCP4 Control Box, removable
Hand-held computer (optional) 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

Please visit for further information and useful accessories about the 360GTO Mount the original website of Astro Physics. If you have any further questions, please send us an email to kontakt (at) or call us.

Detailed information about the Astro-Physics German Equatorial Mount are available here.

What's included: (standard version without the Precision Encoder)

  • Polar Axis (Right Ascension-R.A.) Assembly with Integrated Pier Adapter and Y-Cable installed
  • Declination Axis (Dec.) Assembly
  • Azimuth Adjuster Assembly
  • GTOCP4 Servo Control Box
  • 31.5" Counterweight Shaft, 2.5" diameter
  • Counterweight Shaft Safety Stop
  • 22" Extension Cable
  • 8' Power Cable
  • 15' Straight-through Serial Cable
  • Allenhead wrench set and 3/8" long arm hex key
  • Astro-Physics Keystrap and Velcro Cable Ties
  • Various fasteners to assemble everything together
  • CD containing: PEMPro v2 Software and PulseGuide Software, both written by Ray Gralak of Sirius Imaging. Also, PDF of 3600GTO and GTO Keypad instruction manuals
  • Manual: 3600GTO German Equatorial and GTOCP4 Controll Box
  • Registration Card

Please visit for further information and useful accessories about the 360GTO Mount on the original website of Astro Physics. If you have any further questions, please send us an email to kontakt (at) or call us.

Detailed information about the German Equatorial Mounts and GTO Servo Drive System of Astro-Physics are available here. Verlinkung auf Wordpress-Artikel..


Applies to all mounts which are controlled through plug connections of hand controllers and / or through plug connections with electronic accessories.

Again and again, we receive mounts for repair work, which have malfunctions due to corroded connections. Therefore, the following note:

The plug-in connections at all contact points in astronomical telescopes are generally exposed to humidity and dirt. For the current transmission on such plug-in connections there is often a higher risk of damage due to humidity or corrosion as is ussually the with plug-in connections in the household.

It is advisable to spray the connectors of all cable connections generously with contact spray and then plug in the respective plug several times. In this way, the contact is cleaned and at the same time corrosion and the accumulation of condensation water are prevented. Please do not spray into the socket! In this way, short circuits can be produced if contact liquid is used too generously and runs through electronic components inside the device.

In any case, if there are undefined malfunctions during the electricity or signal transmission, it is first of all advisable to clean the plug-in connections by means of contact spray.

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