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Runaway, Unsolicited and Missed Slews on AS-GT Scopes

Several new owners of Celestron's Advanced Series with GoTo (AS-GT) telescopes have reported runaway slews (a slew that does not appear as if it will ever stop) and missed objects during GoTo operations.  In many cases there was no problem, but rather the owner was not yet familiar with the normal operation of a computerized German equatorial mount (GEM).  In some cases a GEM will seem to head off in the wrong direction when directed to GoTo an object. 

The hand control is programmed to prevent the scope from making contact with the tripod.  This can lead to some interesting maneuvers!  As part of that protection, the scope will generally not cross the local meridian (the line running from directly south, straight above, then directly north).  This can cause the scope to take the long way around even though the target is very near to the current  location at the time you issue the GoTo command.  You should also note that this "no-cross" line is related to the position of the scope when you line up the index marks at the beginning of the alignment process.  Reportedly many scope come with these alignment marks in the wrong location.  Read here for adjusting the index mark locations.

That said, runaway slews do occur on the AS-GT scopes.  Normally you can stop any slew in progress by pressing any of the arrow keys, but several owners have reported that the only way to stop a runaway was to turn off the power on the mount.  Additionally, some scopes have been know to slew several degrees on their own when the mount is first powered on.  Following are some suggestions to prevent runaway and unsolicited slews from occurring.

  • First, are you sure the scope isn't just taking the long way to the object due to the "no-cross" line?  Allow the slew to continue while keeping a finger on the power switch and an eye on the declination motor cable (if the scope is a runaway, the Dec cable can wrap around the mount, damaging the cable).
  • Are you running on good power?  Low power or faulty power cables often induce runaway slews.  Read here for power source suggestions.
  • Are you only having problems with runaway on the third star of an auto alignment after a polar align?  Read here.
  • The cable going between the RA and Dec motor housings could be the problem.  Bill on the Celestron_AS Yahoo Group suffered from runaway slews and noted that even though pulling and wiggling the cable didn't cause runaway slews, a replacement cable solved his problems.  Cleaning the connectors and jacks with alcohol might also fix the problem on some scopes.  Also, look carefully at the pins in the jacks to insure none of them are bent.  Some folks have even gone so far as to crimp new RJ connectors onto their cables, though cleaning would be the first thing to try.
  • A poor connection between the hand control and the mount can cause runaways.  Clean the hand control cable connector and the pins in the jack with alcohol.  Also, look carefully at the pins in the jack to insure none of them are bent.
  • There might be a loose connection inside the mount.  Joe Jaramillo reported to me that this was indeed the problem with his scope.  In fact, he found it necessary to re-route the cable leading to the hand control connector to prevent it from pulling loose when he reassembled the mount.  See the following suggestion for an article discussing disassembly of the mount.
  • Several members of the Celestron_AS Yahoo Group, including Joe Jaramillo, Peter Bruce and Ian Phillips noted that problems with insulation on a circuit board in the RA motor housing can lead to this and other misbehavior.  Peter Bruce has written an excellent article describing the problem and the fix.  Click here to download the article in Adobe Acrobat format (reader available from

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Michael Swanson
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