GTO Servo Drive System
GTOCP3 Control Box and Keypad

Astro-Physics Mach1GTO, 900GTO, 1100GTO, 1200GTO, 1600GTO, 3600GTO and the older 400GTO, 600EGTO mounts feature high-quality DC servo motor drive electronics, the rugged GTOCP3 Control Box and our GTO handheld computer Keypad for GoTo operation. These sophisticated, versatile mounts can be operated with the GTO handheld computer keypad alone or in conjunction with various popular software packages on your Windows-based or Mac computer or mobile smart device.

New and experienced observers will appreciate the many advantages of the GTO computer system. It allows you to locate objects easily in light-polluted areas where star hopping is very difficult. Even if you only recognize a few of the major stars in the night sky, you can explore the beauty of the universe. You can even polar align and find bright stars and planets in the daytime! CCD imagers and astrophotographers will set up their shots quickly to maximize the number of images that can be captured in a single night.

GTOCP3 Control Box - the "Brains" of Your GTO Mount

gtocp3 photo

gtocp3 drawing

The GTOCP3 Control Box contains all of the circuitry to drive the two servo motors and the logic required to circumnavigate the sky. It functions as "the brains" of your mount translating the commands that are received from external software (Keypad, planetarium program or other software) into electrical signals to the motors to produce movement and other critical functions. It also makes all the required calculations and determines the safe and proper path for GoTO slews.

The main function of the GTO Control Box is to drive the R.A. and Dec. motors, which are DC servos with built-in shaft encoders. The rotation rate and precise number of rotations of each motor is controlled with a servo feedback loop. The servo looks at the return pulses from the shaft encoders and adjusts the current to the motor so that the rate is identical to the commanded rate (i.e. from .25x sidereal to 1200x sidereal). The precise number of motor rotations is translated into incredibly accurate R.A. and/or Dec. distances.

The servo is digitally controlled, and the shaft position is updated at a rate of 2000 times per second.

This electronics box is mounted directly on the R.A. axis of the 1200 and 900 mounts (as shown in the photo), on the side plate of the 1600 and 3600 and is a stand-alone unit for the Mach1, 1100, 400 and 600E mounts. The freestanding unit attaches to your pier or tripod with the handle on the fabric pouch or the Control Box Adapter (CBAPT) both included with the Mach1GTO.

The GTOCP3 shipped with the Mach1GTO mount is completely black-anodized to match the mount. The control boxes shipped with the 900GTO, 1100GTO, 1200GTO, 1600GTO and 3600GTO mounts have a black-anodized panel where the connectors are located and the remainder of the box is painted the same cream-white as the mounts.

Control Features

The command language protocol that is programmed into the microcontroller of the GTOCP3 Control box allows the sophisticated control functions of your mount. Please keep in mind that the software that communicates with the mount will determine which commands are utilized. For instance, although the control box is programmed to include a command that can implement a timed guide move, if the software does not issue the command, this feature cannot be used.

For optimum utilization of the advanced functions in the command language protocol, we recommend that you control your mount with either the GTO Keypad, software that utilizes the ASCOM V2 driver or the Astro-Physics Command Center (APCC) when it is released. All of these control options were written by Astro-Physics. For more information, see: Control Your GTO Mounts with Your Keypad, Computer or Mobile Smart Devices.

Here are some of the key features and functions available in the GTOCP3 Control Box:

  • Accepts timed guide move command. This allows control programs to introduce precise motion without the complication of providing accurate time separation between the move and quit commands.
  • Allows variable tracking rate commands in both RA and Dec axes. These are useful for tracking objects that are not fixed relative to background stars, like comets, asteroids, satellites and the Moon.
  • Accepts variable centering rate commands in addition to the commanded rates of 0.25x, 0.5x, 1x, 12x and 64x.
  • PEM - Periodic Error Memory correction - make your correction just once, remains in memory permanently.
  • PEM memory command. PEM memory is accessible to external software programs in order to smooth out and modify the curve. PEMPro by Ray Gralak and distributed by CCDWare supports directly reading from/writing to the PE storage locations. This allows more accurate programming of the mount's PEM (that is, the mount does not have to interpret move or autoguider commands). Also, programming is significantly quicker than waiting for one worm cycle's worth of playback (5.6 to 7.5 min, depending on the mount model) as is the case for the GTOCP2. Also, PEMPro reads parameters and calculates the mount's worm period (useful if you have a non-Astro-Physics mount with the GTO Control System.
  • Query to mount regarding the position of telescope on east or west side of the pier/tripod. This command is useful for dome control software and remote control.
  • High-precision mode assures accuracy of R.A. format to tenths of seconds in R.A. (1.5 arc seconds)
  • Servo control can be commanded using Alt-Azimuth coordinates.
  • Park command de-energizes motor and remembers R.A./Dec. coordinates when power is removed. Restores saved R.A./Dec. coordinates on next power up and calibrates to them when commanded. When the power and keypad are restored, the mount will stay parked and not resume tracking until the Resume from Park function has been entered on the keypad.
  • Allows recalibration even when scope has crossed the meridian and is on the wrong side of the mount.
  • If motor stalls more than 1 second, the drive will shut off. This is a safety feature to prevent motor burnout in case of severe load on the motor due to an extreme imbalance condition or if the power to the mount has been accidentally left on and the scope has hit a hard stop.
  • Automatic meridian swapping in GoTo slewing mode.
  • Continues to track at sidereal rate, even while slewing so that your position is always accurate.
  • Allows E-W reversal in R.A. and N-S reversal in declination for correct object orientation and movement in eyepiece.
  • Allows R.A. and declination backlash control.
  • Functionality to support GTOELS and GTOAE control boxes, which are used with the Precision and Absolute Encoder options.
  • Provides full functionality of the AP V2 ASCOM Driver and APCC. Provides all the commands required for all functions of the AP V2 ASCOM driver (available free of charge) and our Astro-Physics Command Center (APCC) software (optional purchase).

Mechanical Features
  • Machined aluminum housing provides robust protection for electronics.
  • All input lines are protected against heavy static discharge with transorbs. Larger internal chip to allow additional functions. FCC and CE certified. Lead-free electronics.
  • Two threaded drainage holes are drilled into the lower part of the control box housing so that excess moisture can drain easily. This may prove useful if you live in an area with high humidity.
  • Dovetail construction allows quick removal from Mach1GTO, 900, 1100, 1200, 1600 and 3600 mounts.
  • Removeable microprocessor chip allows upgrades in future.
  • 12V connector for locking power cable.
  • Autoguider connector for any CCD with modular RJ-11-6 connector.
  • Two 9-pin RS-232 serial ports (DB9 female connectors, a.k.a. DE9) so that user can use two programs, for instance PulseGuide and a planetarium program, simultaneously, if desired. The user must have two COM ports available on their personal computer to use the software together. Serial to USB connectivity products are available here to simplify connection to your computer.
  • Focus connector - for 3.5mm phono plug (JMI, Meade or other DC synchronous electric focusing motors only).
  • Reticle connector - for guiding eyepiece with an illuminated reticle that has a 3.5mm phono plug or the cable for our own PASILL3, PASILL4 and PASILL4L (versions of Polar Alignment Scopes).
  • 6-V 200mA output accepts 3.5mm phono plugs to power Pentax 6x7 camera directly from the mount.
  • Northern and Southern Hemisphere switch.
  • GTO Keypad Controller locking connector.

Dimensions of servo box 4.9" x 4.9" x 1.8" (12.5 x 12.5 x 3.4cm)
Weight 1.5 lbs. (0.7 kg)
Suggested power sources Portable 12V battery pack, auto or marine battery
110 to 12V DC Power Supply (5 amp minimum, regulated and well-filtered to prevent spikes), like our 13.8-volt, 5-amp power supply (PS138V5AC) . For the 1200GTO mount with a heavy load in colder climates and for the 3600GTO, we recommend the 15-volt, 12-amp power supply (PS15V12AC)
Standards Meets FCC and CE standards

GTO Manual
Link to GTO Manual Astro-Physics GTO Servo Motor Drive System - Model GTOCP3 - Manual

Control Box Upgrade and Purchase Information
The GTOCP3 can be purchased as an upgrade for your GTOCP1 and GTOCP2. This link provides important information for people who want to know how the GTOCP3 differs from the earlier control boxes and the specific items that they need to purchase. If you currently own a GTOCP3, you may also purchase a second one as a backup, if you wish.
Chip Upgrade for GTOCP3 Control Box
If you already own the GTOCP3 control box, you may wish to consider upgrading the ROM chip to take advantage of the newest features and bug fixes which can impact your mount's performance.

Here is list of the Revisions to the ROM chip. (PDF document)

Upgrade procedures and information to obtain the most recent ROM chip.

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GTO Keypad with Keypad Protector

The Keypad is a self-contained handheld computer that allows you to control all essential telescope functions and includes an extensive database of objects. We have designed this unit to be as intuitive and easy to use as possible, while filling it with terrific features to make your observing session productive and pleasurable.

Most buttons are assigned to one function only. You do not have to remember shift key patterns or complex keystroke sequences. The directional buttons are shaped like arrows to help you find them in the dark without looking at your keypad. The stop button is readily accessible if you need to cancel any slewing request immediately.

The menu system is very easy to navigate. Each 4-line screen will display several choices which correspond to a number button. Simply press the number button to initiate the command, e.g. "1=M, 2=NGC." When you press "1" the Messier entry screen will display and you can enter the number of the object that you wish to see. Press GOTO, which will cause the mount to slew. The <PREV and NEXT> buttons will allow you to scroll quickly through the lists of common names or from one menu to another.

You will be very pleased with the intuitive operation of this controller. There are no complicated sequences of keystrokes to remember. It is so easy to use that even if you don't use it for a few months, you will feel at home with the Keypad very quickly.

  • Handheld keypad with red backlit alphanumeric keypad and red 4-line by 20-character data display.
  • Vacumn-fluorescent display rated to -40 degrees F or C (F and C are are equivalent at this temperature) allows hardy observers to view or image on cold winter nights (please contact Astro-Physics if you wish to operate the mount in severe temperatures below -20F). Normal computers cannot be used at these temperatures.
  • Directional N-S-E-W keys provide immediate response when you wish to center an object or cruise the surface of the moon.
  • Set tracking, guiding, centering and slewing rates.
  • Sets R.A. and declination reversal (E-W reversal in R.A. and N-S reversal in Dec.) for correct object orientation and movement in eyepiece.
  • PEM - Periodic Error Memory ( make your correction just once, remains in memory permanently) - turns this feature on and off.
  • Set R.A. and declination backlash control.
  • Stores 9 observing site locations.
  • Quick and easy polar alignment routines, even during daylight!
  • Adjust brightness of the reticle output for your illuminated guiding eyepiece or polar alignment scope.
  • More than 17,000 objects in extensive databases: Messier (110), NGC (7840), IC (5,386), ADS double stars (215), Abell galaxy clusters (2,712), calibration stars by common name (200), Greek star names by constellation (1053), common objects (100), planets (8) , Moon and Sun.
  • Enter catalog or common star number, solar system object, R.A./Dec. (or Alt/Azcoordinates or common name of object; press GO TO and your telescope will automatically slew to the object.
  • Search and tour functions assist you to plan observing session.
  • Display will indicate if object is below horizon and cancel slewing request.
  • STOP button to cancel your slewing request when the telescope is already moving.
  • Display information about objects (object type and magnitude) before slewing.
  • Timer will let you know when the exposure time of your astrophoto is complete.
  • Continuous readout of current position.
  • Select one of three preprogrammed park positions.
  • Activate meridian swap delay function to allow astrophographers to take a series of images for a mosaic near the meridian without the mount swapping sides.
  • SmartGuide control provides variable speed capability to both axes.
  • Altitude/Azimuth display allows you to position the mount for fixed targets to any Alt/Az point. This is great for taking dark frames.
  • What's Up Now function will suggest objects to view based on what is actually above the horizon at the time.
  • External Auto Connect mode allows primary mount control from an external computer and places the GTO Keypad into a secondary role for seamless operation in computer dominated applications like remote imaging.
  • Reference Park allows you to start the mount in the field by placing the telescope in Park1 position for quick daytime polar alignment.
  • Retractable hanger.
  • Future firmware upgrades available through internet downloads.

Specifications of the Keypad
Dimensions 7.2 x 4.1 x 1" (181.6x104.1x24.6mm)
Weight 1 pound 8 ounces, including cable and KEYPRO (680 grams)


Red LED fiber optic backlit, alphanumeric
Display screen 4x20 character alphanumeric
Pre-defined Tracking Rates Sidereal, solar, lunar and stop (Note: lunar rate is R.A. only, and does not include the moon's movement in declination.)
Rate selections for Auto-guider port 0.25x, 0.5x, 1x
Pre-defined Direction Button Rates

0.25x, 0.5x, 1x, 12x, 64x for centering and manual guiding.

600x, 1200x for rapid button movement. (Actual rates will be slower for 3600GTO mounts and for older 400GTO / 600EGTO mounts with the 32:1 gearboxes. Keypad display will still read 600 or 1200 - see below for actual rates)

Pre-defined Rapid GoTo Slew Rates - All 900GTO, 1200GTO and Mach1GTO 600x, 900x and 1200x ( top speed - 5 degrees per second)
Pre-defined Rapid GoTo Slew Rates - 3600GTO and 3600GTOPE 300x, 450x and 600x ( top speed - 2.5 degrees per second)
Pre-defined Rapid GoTo Slew Rates - 400GTO and 600EGTO with 32:1 gearbox 590x, 860x and 980x (Top speed dependent on available voltage from power supply. Note that these rates differ from those in the GTOCP2 - 460x, 580x, and 675x)
Pre-defined Rapid GoTo Slew Rate - 400GTO and 600EGTO with 19.8:1 gearbox 600x, 900x and 1200x ( top speed - 5 degrees per second)

V.4.12 Manual GTO Keypad Controller, Version 4.12 (03-07-08) - PDF document
V.4.17 Addendum GTO Keypad Controller, Version 4.17 Addendum to the v.4.12 Manual - Includes the complete Revised GTO Quick Star Drift Method for Polar Alignment Instructions and a handy one-page Quick Reference Sheet (06-01-10) - PDF document
Purchase Information
The Keypad is included with all GTO mount purchases. If you wish to purchase an extra Keypad as a backup, click here.
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Keypad Protector (KEYPRO)

The KEYPRO Keypad protector (shown in the keypad photo above) is a heavy-duty molded rubber casing with a full 1/4" thickness on all impact surfaces. If you accidentally drop your Keypad, the rubber casing will absorb much of the blow. Your display, Keypad, electronics and the case itself will be protected from damage. In addition, the rubber prevents the keypad from sliding when placed on a table or other flat surface and the Keypad will feel secure in your hands as you observe. The KEYPRO also offers protection during transport and storage.

The black case is designed with cutouts for the cable and retractable hanger.

Purchase Information
The Keypad Protector is now included with all mounts and Keypads, however it can be purchased separately here, if you need one.

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This page was last modified: October 23, 2014

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