Runaway, Unsolicited and Missed Slews on
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
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
- 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