For Celestron German equatorial mount users, this may be important for setting pier flip safety zones in ACP's flipconfig.txt and preferences panel. I've only tested a CGE and CGEM, but I suspect that other models, such as the CGE Pro may be vulnerable.

I wanted to take advantage of the ability of my mounts to track past the meridian...get rid of that delay/wait at the meridian. Well, until Celestron fixes what I think is a firmware problem...I have to keep using a safety/wait zone around the meridian...especially just east of it.

Details below.

Tom Krajci
Cloudcroft, NM
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I can create a runaway slew on my CGE when the slew destination is just east of the meridian. Same for my CGEM.

You might want to see if yours is also vulnerable.

Below is what I sent to Celestron tech support.

If you have other brands of Celestron German equatorial mounts, please test them too. My hunch is that other brands are also vulnerable. (The test can be done in daytime, no OTA on the mount, in the garage.)

Please feel free to cross-post this to other Celestron forums that can benefit from this information.

Thanks in advance.

Tom Krajci
Cloudcroft, NM
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Attn: Engineering Dept. Slew just east of meridian can cause runaway slew

It is possible to have a runaway slew when the slew destination is just slightly east of the meridian (and a few degrees south of zenith).

I have repeatedly created this situation with CGE and CGEM mounts and their handpads. (I've also created this problem using NexRemote/TheSky, or with ACP controlling the mount.)

This is what you must do: power up/align mount, and slew to a point east of the meridian that is close to the meridian. This point must also be slightly south of the zenith. (The OTA will now be west of the polar axis, and pointing slightly south of zenith.) Call up the current mount coord's, and use them in the Goto RA/Dec command. Do this repeatedly - command a slew to the same RA/Dec coordinates each time the mount comes to a stop at the destination coords.

Eventually the slew destination will be very, very close to the meridian, but still east of it. The mount has not yet performed a meridian flip. Continue slewing to the same RA/Dec coords.

Eventually the mount will start a slow runaway slew. This slew can be aborted - the handpad still responds to commands.

What exactly is happening here? I am not sure, but my guess is that at the time the slew command is issued, the destination coordinates are still slightly east of the meridian. But, Celestron German equatorial mounts have the 'goto approach' feature in firmware for the purpose of consistent backlash takeup. This makes any slew take an extra 10 - 15 seconds to complete. By the time the mount is approaching the destination coordinates....they have crossed the meridian, and this confuses the firmware. The (slow) runaway slew is at the speed the mount uses for the goto approach feature...and it continues without end because the firmware is confused as to the pier-side status of the mount. It thinks it's on one side of the pier, but actually it's on the other side. Therefore the goto approach slew will never reach the destination coordinates.

Again, this is a guess as to the inner workings, but I can repeatedly create this slow runaway slew on two different brands of mounts.

Note that the OTA must be pointed slightly south of local zenith.

Note that if you waste too much time between repeated slew commands to the same RA/Dec coords....mount tracking will take it past the meridian, and the next slew you command will force a proper pier flip. In other words, there is a very narrow zone, just east of the meridian, that the mount must be in, for the slew command to the same RA/Dec coords to force this runaway slew.

Such a narrow zone means that this problem will not crop up very often, but it will happen once in awhile.

I only have two mounts at my disposal to test. Does this problem exist for other Celestron German equatorial mounts?

Feel free to email me at tom_krajci@tularosa.net or call me at 575-682-2028.

Thanks for reading this far,
Tom Krajci

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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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