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Original 60/80/114/4 GT Model Commands

The new GT hand control uses the same basic commands as the GPS model and the updated manual on Celestron's web site (ftp://downloads.celestron.com/pub/manuals/) now includes these commands.  This of course means that a program compatible with NexStar GPS is compatible with the new GT models.

However, if you are using the original GT hand control you will find no mention of the control commands.   Nonetheless there are commands for RS-232 control of the original GT hand control. They are very similar to the commands for the NexStar 5/8, with a few important differences.   First, there is no need to send an initialize ("?") command.  Second, the original GT hand control will not send back an acknowledgement when a slew operation is complete.  Third, the GT hand control accepts commands from the beginning, there is no RS-232 option to set on the menu.  Fourth, besides sending two bytes for each coordinate, you must also send an ASCII character 0 (NUL) after each of the two coordinate bytes.  Please note that these differences mean that a program written to work only with the NexStar 5/8 will not work with the 60/80/114/4 GT.

It must also be noted that after the PC sends a goto command, the GT hand control usually "locks-up" until you press one of the directional buttons or the Undo button.   If you attempt to send another goto command before this, you will likely be forced to reset the power and redo the alignment.  Also, the Get commands will not report valid values for RA above 12 hours, Azimuth above 180 degrees or negative Declination/Altitude.  These problems were fixed in the new GT hand control.

Following are the basics.  Click here to download an Excel spreadsheet that you can explore to learn about the calculations required for the coordinate bytes (the high byte, low byte referred to below).

GoTo RA-Dec positions:

  • PC sends ASCII 82 ("R").
  • PC sends the RA high byte, RA low byte, ASCII NUL,  Dec high byte, Dec low byte, ASCII NUL.
  • NexStar slews to the coordinates.

GoTo Alt-Az positions:

  • PC sends ASCII 65 ("A").
  • PC sends the Azm high byte, Azm low byte, ASCII NUL, Alt high byte, Alt low byte, ASCII NUL.
  • NexStar slews to the coordinates.

Get RA-Dec positions:

  • PC sends ASCII 69 ("E").
  • NexStar sends the RA high byte, RA low byte, ASCII NUL, Dec high byte, Dec low byte, ASCII NUL.

Get Alt-Az positions:

  • PC sends ASCII 90 ("Z").
  • NexStar sends the Azm high byte, Azm low byte, ASCII NUL, Alt high byte, Alt low byte, ASCII NUL.


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