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Upgrading the Electronics of NexStar 8/9.25/11 GPS Telescopes

The NexStar GPS series of telescopes was sold from late 2001 through early 2005.  Over the course of those three and a half years, it underwent many changes in the electronics.  The changes we care about most are changes to the hand control and changes to the motor control board.

NexStar GPS scopes have two sets of processors.  There are two processors in the base of the scope on a board known as the motor control (MC) and a single processor in the hand control (HC).  The MC has two processors as there is one for each axis (altitude and azimuth).  Each of these three processors has its own version of firmware - the program that the processor executes to do its job.  As noted in my version history of the NexStar GPS series:
the original NexStar GPS telescopes shipped with hand control version 1.2 and motor control 1.0.  The final versions released for the NexStar GPS series were hand control version 4.22 and motor control 4.06.  Caveat - the very last production run had MC version SA 4.00 which is functionally equivalent to 4.06 but prevents hand controls 1.6 and 2.2 from working on the mount.

After reading through the version history, you should next determine what versions are installed on your NexStar GPS.  With hand control version 1.6 or higher, you will find a Version option on the Utility menu.  The HC is the first version number displayed.  The MC version number is displayed twice, once for each axis.  With a version 2.2 hand control two more version numbers will be displayed - the serial bus board and the GPS interface module.  Neither of these is upgradeable so we won't worry about those here.  If you don't find a Version option on the Utility menu, you have hand control version 1.2 and motor control version 1.0 or 2.0. 

Comparing your HC and MC versions to the version history, you can determine if any new features or fixes are available that you determine are worth the effort to upgrade.

Other reasons you may want to upgrade:

  • Your original hand control may be having issues.  Perhaps it will not power up, or the buttons or display are giving you problems and none of these suggestions helped.  It is unlikely you will find an original equipment version 1.6/2.2 hand control so you will need to replace it with a newer version.
  • Your mount is afflicted with the GPS rollover issue discussed here.  The fix requires a version 4 or NexStar+ hand control.
  • Some external software (for example, SkySafari) is only compatible with hand control version 4 or higher.

Upgrading the Hand Control
As you can see, most of the upgrade scenarios feature a newer hand control.  The best version of hand control that addresses all these issues is the version 4 hand control.  The version 4 hand control went out of production in early 2012 when it was replaced by the NexStar+ hand control.  That said, the version 4 hand control is widely available on the used market and is the best choice for the NexStar GPS mount.  One common source is a search on for "nexstar* hand controller".  Regardless the mount in the description of the eBay advertisement (CPC/SLT/LCM/etc.), any version 4 HC works with the NexStar GPS once it is loaded with the correct firmware.

It is possible to use the newer NexStar+ hand control on a NexStar GPS mount but the oldest mounts may have problems with the NexStar+ HC when running the current versions of firmware.  Common symptoms are intermittent "No Response" errors and sluggish arrow button response.  If you must use a NexStar+ HC and experience these problems, this article provides instructions for downgrading to a known good version.  The only drawback is the older version does not correct for the GPS rollover issue so if your mount has that problem, you will need to turn off GPS in the hand control menu and lose the use of the GPS receiver.  For this reason, a version 4 hand control is highly recommended.

Quick note - if you do pick up a version 4 or NexStar+ HC for your mount, you may find my hand control version 4 user's guide useful to aid in learning the newest features.

Once you have the new hand control, you will want to upgrade to the latest firmware.  For a version 4 hand control, that is currently NXS 4.22.  Use these instructions to upgrade:

For a NexStar+ hand control, new versions are released quite regularly.  Use these instructions to upgrade:
Again though, if you experience "no response" errors or sluggish arrow button response, you will need to downgrade via that link above.

One final note - it is possible to load the latest version 4 firmware (NXS 4.22 as this time) on a version 1.2, 1.6 or 2.2 hand control by using a PIC programmer.  If you need to update a version 1.0 or 2.0 motor control with a PIC programmer (see the next section) this could make sense.  Otherwise, it is cheaper and much less complicated to purchase a version 4 hand control.  Instructions for upgrading a 1.2/1.6/2.2 hand control with a PIC programmer are found in this discussion:

Upgrading the Motor Control
Here is where things can get tricky.  The version 4 and NexStar+ hand control will only work with motor control version 4.06 (or the functionally equivalent SA 4.00).  If the existing motor control is version 3.0 or higher, use these instructions to upgrade to version 4.06:

If your existing motor control is version 1.0 or 2.0, Celestron's MCupdate program is not able to perform the upgrade.  Instead, you will need to purchase a PIC programmer and follow the discussion here:
The MC board is in the fork arm with the vertical handle.  Remove that handle and then the plastic cover on the outside of the arm and it is easily visible.  At the time of this writing, there is no one offering this update service for the older MC boards.  If you feel this is beyond your comfort level, check around for an electronics hobbyists in your area - many of them have experience with PIC programmers and might even have the equipment.

It is the opinion of many that the NexStar GPS series was the best fork mount telescope Celestron ever produced.  The upgrades outlined here can help resurrect a faulty scope or bring it new life with Celestron's current feature set.

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Michael Swanson
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