900GTO German Equatorial Mount (900GTO)
with GTOCP2 Control Box

Includes Go-to Keypad Controller and DigitalSky Voice™ software

The 900GTO German Equatorial Mount with the GTOCP2 Control Box is not available to purchase. This information is provided for present owners and those who are curious about its history. It is based on original information with some updates. If you are interested in some of the other models, check out this Brief History of the 900 German Equatorial.

Additional photo to view:
900GTO with 155mm f7 StarFire EDF(4" focuser model), 15" Ribbed Plate, 80mm Guidescope and other accessories
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. If your instrument of choice is a medium-sized refractor, Cassegrain, Ritchey-Chretien, Newtonian or astrograph, we invite you to read on.
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 Mount Pinos in California, favorite observing site of many advanced photographers and imagers.

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.

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:

Flexibility Write your own computer program.
The Astro-Physics GTO protocol for the GTOCP2 Control Box is freely available to those who would like to write their own computer program for controlling the mount. While based on the Meade protocol, it has many more features and enhanced precision level which allows pointing and tracking to the 1 arc second level.

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. With good alignment, PEM training with the Keypad or by using PEMPro software, 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
0 to 20 degrees with latitude wedge Azimuth adjustment

Azimuth adjustment

Approximately 14 degrees

Setting circles

Porter Slip Ring design, engraved

Right ascension

4 minute increments, pointer, engraved both Northern/Southern


1 degree increments, pointer


Approximately 70 lbs. (31.8kg) scope and accessories, depending on length. Will accommodate Astro-Physics and similar refractors up to 180mm f9, 12" Cassegrains, 12-14" Ritchey-Chretiens
Some telescopes are very heavy for their size and will require a larger mount.

Weight of equatorial head

50 lbs (22.2kg), Declination axis is 15 lbs. (6.8kg) right ascension axis is 25 lbs.(11.4kg), counterweight shaft is 10 lbs. (4.6kg)

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 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 diagram shows several dimensions that are commonly requested. In addition, if the scope is horizontal and pointing west 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.


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