GTOCP3 Control Box for Servo Drive

No longer available for purchase - Information for reference only

The GTOCP4 Control Box, is now available as an upgrade for all GTO mounts that we have produced since 1998. It is also included with all new mount orders.

Information regarding the GTOCP3 in this document is provide for reference.

Mounts that can use the GTOCP3 control box: All GTO mounts - All Mach1GTO, 900GTO, 1100GTO, 1200GTO, 1600GTO, 3600GTO and prior models 400GTO and 600EGTO. The parameters of the GTO control box depend on the gear ratio and motor of the mount. Therefore, the control boxes are not necessarily interchangeable from one model to another. For instance, the control box for the 400 mount cannot be used with the 1200 mount. However, the control box for the Mach1GTO, 900GTO, 1100GTO, 1200GTO and 1600GTO mounts are interchangeable. Please check with Astro-Physics before substituting control boxes. Additional information is provided at the bottom of this page.

Please note the following additional considerations if you are upgrading from a GTOCP1 control box:

  • 400GTO and 600EGTO - If you are upgrading from the GTOCP1 control box, you will also need to purchase a new carrying pouch, our part #C0024 or our Control Box Adapter (CBAPT).
  • 900GTO - If you are upgrading from the GTOCP1 control box, you will need a the Quick Release Adapter for Control Box Kit (9CBKIT) to attach it to the RA axis. This dovetail-style adapter has the added advantage of allowing you to easily remove the control box from the mount, if you wish.You will also need a new RA/Dec Y-Cable (S900GYCR) to replace the two separate RA and Dec cables that you now have.
  • 1200GTO - If you are upgrading from the GTOCP1 control box, you will need the 1200GTO Quick Release Adapter for Control Box Kit (12CBKIT) to attach it to the RA axis. This dovetail-style adapter has the added advantage of allowing you to easily remove the control box from the mount, if you wish. You will also need a new RA/Dec Y-Cable (S1200GYCR) to replace the two separate RA and Dec cables that you now have.

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). 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 latest versions of the GTOCP3 Control Box:

  • Allows Automatic "Deadman Switch" Safety Timer function. This feature confirms that mount is communicating with the PC and COMs are functioning properly. If not, the mount will park at the end of a user-defined countdown preventing pier crash. This important safety feature is implemented in both APCC Standard and Pro Versions.
  • Reads capability of PEM state, guide, center and slew rates and of drift (both axes) and trim (both axes). This feature is implemented through polled status command in the ASCOM V2 driver and APCC Standard and Pro Versions..
  • Allows Auto-Park. This will cause the mount to enter a parked state whenever power is removed and this state is remembered through power cycles. When the command is invoked, default sidereal rotation will NOT start up the moment that power is applied. This will be the recommended setting for any remote observer/imagers. This does NOT try to move the mount into a pre-defined park position.It simply puts it into a parked state when power is removed. This feature is the default in APCC Standard and Pro Versions..
  • Improved Meridian Swap logic. The new logic is more streamlined and robust. It is applied to all go-to slew input - Keypad and computer programs.
  • 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 through at least February 2016. 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.
  • 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

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)

Check our website for upgrade procedures and information to obtain the most recent ROM chip.

Purchasing a Used GTOCP2 or GTOCP3 Control Box via the Second-Hand Market
There will be a number of used CP2 and CP3 control boxes becoming available through second-hand markets, such as Astromart, once the CP4 is released. The buyer will need to be aware of a couple things to ensure that it will work with his or her mount.

There are no concerns if the seller's control box is from a Mach1, 900, 1100, 1200, or 1600 mount and being used on the buyer's 900 or 1200 mount as the mounts' "personalities" are the same.

However, if the control box is either coming from a 400, 600 or 3600 mount or going to those mounts, then it will be necessary to change the "personality" setting of the control box. Please call Astro-Physics to learn more.

It is recommended that a CP2 or CP3 box have the latest chip installed in order to maximize its performance and safety. Please call Astro-Physics to learn which chip is appropriate for your box, or check the technical support section of our website.

Although a CP3 will perform well with a "Q" and above chip, a "V" or later chip is recommended to maximize performance, as well as being required to use the APCC software.

Note that a CP2 box cannot utilize the APCC software.

Finally, it is very important that PEM is turned off in the Keypad and disabled in the AP V2 ASCOM driver and APCC until a new curve is run. If this is not done, then you will experience large and incorrect surges in the PE of the mount, as you will be entering false PE curves that are from the previous owner's mount into yours. Once you run a new curve, you can turn the PE back on.

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This page was last modified: March 10, 2019

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