Functions of the GTO Servo Control Box and GTO Keypad

We have separated the functions of the servo controller and hand held controller to allow for maximum flexibility to the user. The servo is a stand-alone unit that can be controlled entirely by the keypad, by a desktop computer with a suitable planetarium program without the keypad, or with both keypad and desktop attached.

Functions of the GTO Servo Control Box

The main function of the servo controller is to drive the RA and DEC motors. The motors are DC servos with built-in shaft encoders. The rotation rate of the motors 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 servo is digitally controlled, and the shaft position is updated at a rate of 2000 times per second.

The central computer in the servo is a microprocessor that converts input commands to electrical signals to move the motor. The input commands are in the form of the AP protocol, eg. :Sr 02:45:32.5# which would define a move position of 2hr 45min 32.5sec in right ascension. The microprocessor converts those numbers to specific shaft angles of the two axes, and determines whether the telescope should access that position from the east side or west side of the mount. The servo responds to commands to move in RA or DEC when any of the 4 buttons are pushed, according to the rate set on the hand controller (from 0.25x to 1200x). The servo will also initiate movement in the 4 directions from inputs sent to the CCD guider port at rates set by the hand controller, typically 1x, 0.5x or 0.25x the sidereal rate.

The servo microprocessor stores vital date, time and location information sent to it from the hand controller, or a planetarium program via one of the input ports. It stores worm periodic error information (PEM) so that it can be played back when required for high accuracy tracking.

The servo controller's main function is to convert input data from the hand controller or a planetarium program (in the form of the AP protocol) to electrical signals to the motor to produce the desired motor motion and to result in the telescope pointing at the desired object in the sky.

Functions of the Keypad

The keypad is really a small hand held computer with planetarium software. It has a database for tens of thousands of objects that can be accessed by keypad entry or scrolling through a menu.

To orient the system properly with respect to the sky requires date, time and location information. The hand controller contains an accurate date and time clock powered by a lithium battery. Together with the location data that is entered by the user, the hand controller uses this information to calculate what part of the sky is up at any given time. The keypad also sends this information to the servo controller to calculate where the horizon limits are, and where the meridian line is located. This prevents the scope from pointing into the ground and to allows the servo to do a meridian swap so the tube assembly does not dive under the mount and hit the pier.

The main function of the keypad is to send RA and DEC numbers to the mount for the objects that the user enters into the keypad. This is exactly the same as the functions of a typical planetarium program. In fact, most of the keypad functions can be replaced with those contained in PulseGuide by Ray Gralak of Sirius-Imaging or an advanced planetarium programs, like TheSky Level 5 and above from Software Bisque, or Starry Night Pro Plus from Simulation Curriculum Corp. . You can therefore disconnect the hand controller entirely from the mount without losing any of the functions of the mount including parking, jogging, centering or moving to various parts of the sky. You can also control things like focus motors and reticle brightness from your desktop computer. Not all planetarium programs support all of these functions, please check the features of the program you are considering.

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This page was last modified: October 20, 2008

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