900GTO German Equatorial Mount (900GTO)
with GTOCP3 Control Box

Includes GTO Keypad and CDs with PulseGuide and PEMPro (full version) software

The 900GTO is not available to purchase. This information is historical and is presented for present owners and those who are curious about its history.

The 900GTO German Equatorial Mount with the GTOCP3 Control Box is no longer available to purchase. We are excited to introduce our new mid-size German Equatorial Mount. This mount will be debuted at the Northeast Imaging Conference (NEAIC) and the Northeast Astronomy Forum (NEAF) in a few weeks. It incorporates the design features of the 1600GTO in a smaller, more portable package. For your first peek at this full-featured mount and its specifications, please check out this link: 1100GTO German Equatorial Mount . You can sign up to be notified when this mount is available for you to order using our Online Notification Request Form.

Why a 900GTO?

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. Astro-Physics now offers four German Equatorial mounts listed in order of size from largest to smallest: 3600GTO German Equatorial, also known as El Capitan, 1200GTO, 900GTO and the most compact is the Mach1GTO. If your instrument of choice is a medium-sized refractor, Cassegrain, Ritchey-Chretien, Newtonian or astrograph, the 900GTO may be the ideal choice for you.

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 major star parties throughout the country and the world. It is a favorite 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.

Over the years, a number of improvements have been incorporated into the design of the 900GTO, among these are smoother azimuth adjusters; increased height of the polar forks to allow more room for your hand when making altitude adjustments; 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 7 arc seconds (+-3.5 arc sec.) or less; internal changes to improve overall accuracy; additional 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 900GTO. As a result, the 900GTO has evolved over the years to meet the needs of the advanced imager or visual observer.

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: Versatility

Portable or Permanent Observatory Setup.
Extremely solid, rugged, high payload mount, yet comes apart in two reasonably light-weight components for hassle-free field setup. The heaviest component is 26.5 lbs (12 kg). 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.

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 You can align the 900GTO using setup routines in the keypad, or 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 imaging (polar scope not included). We recommend either the PASILL4L or the prior PASILL4 or PASILL3 models since they can rotate in the axis. You can even align the mount during the day with our clever Daytime Polar Alignment Routine (outlined in the Keypad Manual) for solar observing, viewing the planets at twilight, or just getting a jump start on a long night.useful for long exposure H-alpha or in cases where a large number of individual exposures are needed for stacking.

Easy alignment for non-critical viewing.

Multiple Options for Controlling your Mount in the Field or Your Observatory
You have your choice of the GTO Keypad (included), 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). The Astro-Physics command language is available to those who need to write their own software. GTO control options are discussed in detail at this link.

Mechanical Features

Precise Mechanical Fabrication
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. Massive thrust bearings and preloaded shaft bearings provide excellent stability under all imaging situations.

Easy to Adjust and Maintain
The design of the Mach1GTO 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. All electronics including the GTOCP3 control box, motors, gearbox or cables can be removed for servicing. The R.A. and Dec. axes can be shipped separately, if needed.

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. 7.2" (183 mm), 225 tooth aluminum
Worm wheel - Dec. 6.0" (152 mm), 225 tooth aluminum
Worm gear - R.A. Brass, 0.710" (18 mm) diameter
Worm gear - Dec. Brass, 0.705" (17.9 mm) diameter
Thrust Bearing - R.A. 7.0" (178 mm) diameter
Thrust bearing - Dec. 6.0" (152 mm) diameter
Axis shaft - R.A. 2.20" (56 mm) diameter
Axis shaft - Dec. 1.75" (44 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 20 to 68 degrees with or without the polar scope attached.
0-20 or 68-88 degrees when using Hi-Lo Latitude Wedge (900WDG or 900WDGA)
Azimuth adjustment Approximately 15 degrees (+/- 7.5 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 14" (356 mm) long [12.875" (327 mm) useable length], incl. large machined safety stop knob. Optional 9" (229 mm) shaft extension available.
Weight of mount Total: 54 lb. (24.5 kg)
R.A. axis/polar fork: 26.5 lb. (12.0 kg)
Dec. axis: 17 lb. (7.7 kg)
Counterweight shaft: 10.5 lb. (4.8 kg)

Approximately 70 lb. (31.8 kg) scope and accessories (not including counterweights), depending on length. Recommended for: Astro-Physics and similar fast refractors up to
180 mm f9, 12" Cassegrains and 12-14" Ritchey-Chretiens, 12.5" 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.

Pier adapter base 7.730" (196.34 mm) diameter. The base is part of the chosen pier adapter - either the Rotating Pier Adapter (900RPA), the Standard Pier Adapter (900SPA), or the Hi-Lo Latitude Wedge (900WDGA or 900WDG). 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 0.4 amps when tracking the stars and 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.

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 7 arc seconds (+/- 3.5 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 7 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 7 arc seconds (+/- 3.5 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 7 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 900GTO uses the same GTOCP3 Control Box as the 1200GTO 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.

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.

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 0.050 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 0.5 amps at 13.8 volts - tracking; ~ 1.0 to 2.3 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

7 arc seconds peak-to-peak (+/- 3.5 arc seconds) or less in one worm cycle (6.4 minutes); native, prior to any error correction

Maximum slew speed 5 degrees / second

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.
PEMPro Software, Version 2

PEMPro (Periodic Error Management Professional), Version 2, by Sirius Imaging is a very powerful Windows software application that allows the you 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.

Dimensions for Observatory Planning

People often ask us what the 900GTO's dimensions are. The answer is: It depends. Different latitude settings can drastically affect the shape and dimensions of the mount. To answer this question we have prepared the following PDF document: 900GTO Dimension Sheet and this Excel spreadsheet Latitude Dimension Calculator.

Please note: both this sheet and the calculator show the latest version of the 900GTO with the new improved Polar Fork. To see an example of an older polar fork style, go to the History section of our website. As the dimension sheet points out, all horizontal dimensions are the same for ALL versions of the 900 mount. If you have a different looking polar fork from the one pictured on the 900GTO Dimension Sheet, simply subtract 7/8" from the vertical dimensions.

900GTO - 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 websiteto find accessories, replacement parts or learn about possible upgrades for your mount.


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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