NexStar Resource Site

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Enhanced Commands for N8/9.25/11GPS, N5i/8i, CPC, LCM, SLT, CGE,
CGE Pro, CGEM and AS-GT Scopes

Celestron has included several new commands in the NexStar 8/9.25/11 GPS (version 1.6 or higher), CPC, LCM, NexStar 5i/8i/8i Special Edition, CGE, CGE Pro, CGEM and AS-GT scopes.  Some of these commands are in the NexStar 5i/8i manual, the others will be included soon.  The commands are:

  • Tracking rate. Allows control of the tracking rate in both axes (independently) ranging from rates of a small fraction of sidereal up to 3 degrees/second. This command can be used to move the scope directly to a set of coordinates (your responsibility to know what rate and how long you need to apply it to get there) or to emulate the hand control's direction buttons.  Click here for an Excel spreadsheet that lists several useful rates.
  • Tracking mode. Allows you to set the tracking mode to 'Off', 'Alt-Az', 'EQ North', 'EQ South'. It is required to set tracking mode to off prior to issuing a tracking rate command otherwise sidereal rate will resume automatically after a few seconds. To return to sidereal tracking you set the correct mode: Alt-Az, EQ North, EQ South. There is no command to query the mode prior to setting it to off, so you must provide a control for the user to set the correct tracking mode in your application. See attached application for example.
  • 32 bit Goto and Get RA-Dec. Allows sub-arcsecond accuracy for goto and get operations (1/12th arcsecond actually). This brings the software precision in line with the encoder precision. Click here for a spreadsheet (EncoderCalculations.xls) that calculates the correct values for goto and get commands - you can use it to check the accuracy of a program's algorithm. Note: the goto and get commands in the attached Visual Basic application only uses the standard 16 bit commands - yielding an accuracy of just 1/3 arcminute. The standard commands are sufficient for most applications as the user will not be able to align the telescope to an accuracy of better than a few arcminutes. However, programs that provide advanced alignment accuracy by utilizing multiple alignment stars (TPoint for example) can make good use of these commands.
  • Version.  Allows you to query the hand control for the firmware version.

Download the actual commands here.  You can download the entire source code for NexStar Control Pad (see the Downloads section for details and the compiled program) here (requires that you have Visual Basic 6 to run, but you can open the files in Notepad to view the code).  This application shows how to implement a program that emulates the hand control's direction buttons, sets tracking mode and more.  All of these resources were created by me and you will not violate any copyrights (nor my feelings) if you use them.

Additional Commands

Andre Paquette has investigated the internal communications protocol of the NexStar 8/9.25/11 GPS and NexStar 5i/8i telescopes. Some of the more useful commands control the motor control and GPS modules. He has published an excellent guide of these commands for the technically inclined, which you will find available at his web site:

Appropriate care must be used with any of these additional commands since a few of them can render your telescope inoperable, requiring a return trip to Celestron (most likely at your expense!) to remedy the situation.

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Michael Swanson
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