16'' LX200GPS runaway during autofocus

[James McCormac]

Hi,
Tonight when observing with our 16'' LX200GPS, ACP started an autofocus run (one of many tonight). I spotted FocusMax complaining about having any stars. As the night was perfectly clear I ran to the dome and found the telescope pointing at the floor. I looked through the log file but could not find anything conclusive about what exactly had happened.

When untangling the telescope I found that it appeared to take off East and possibly attempted to go the long way round to the focusing star (?). The declination position was correct. HA was out ~180deg. Attached is the full log from tonight. You can see the system was working well until ~03:40 UT.

Has anyone seen this type of runaway problem before? I am currently using older versions of ACP/MaxIm/FocusMax while I continue preparing our new observatory PC with up to date versions of everything.

Cheers
James

[Dick Berg]

Hi James,

Wondering whether the mount you have is the stock Meade fork mount, or an aftermarket GEM.

Your slew took 3-1/2 minutes. Does that seem like a reasonable time to slew 180 degrees around the polar axis? that would be LONG for me.

I have had RA runaway problems in the past with my 12-inch on a stock Meade mount. My problem is/was that the feedback loop for the tracking mechanism fails, creating essentially an open loop feedback that can't tell the drive when to stop. So it would just keep going toward the west in my case until the cables restrained it and it stops. This is evidently a common problem with that electronic feedback mechanism. There are two trimming pots under the hood that can be adjusted to balance the feedback loop, but they're not easy to access.

So for me, it's not an ACP problem, or a particular target position problem - it happens randomly but very infrequently. I don't know if it is trying to send the mount around the polar axis the wrong way.

[James McCormac]

Hi Dick,
Thanks for replying. The telescope is using the standard Meade fork. 3.5 minutes sounds about right, I have slowed the max slew speed down a lot (~1 deg/s I think) which seems to match up with that time. I ran the telescope again 2 nights ago and it tried running away twice. I was in the dome keeping a close eye on it at the time. I seemed to slew, almost reach the position, slow down on final approach, then stop, wait a sec, slew a little, wait some more, then slew again at full speed east. Quite strange. I came to the conclusion that this is not an ACP error.

Are the pots easy to access on the 16'' LX200GPS? Can I get at them from the front plate or will I need to crane out the telescope to access the underside of the base? My telescope is on a custom wedge at 28 deg latitude, moving it is not easy Do you have some notes on how things should look when nominal?

I have access to an oscilloscope, hopefully I can get the pots tuned up again asap. Thanks again for your reply.

Cheers
James

[Dick Berg]

James. Sorry, I just saw this reply. I will try to put together a smart response tomorrow. Too tired tonight. I honestly don't know if the LX200GPS has the same drive configuration and feedback control, so I've got to try to find that out. I can get to my pots by removing the front panel but it's an awful position to work in behind that panel. Plus, you have to somehow leave the power on in order to adjust the pots properly. Yes, an oscilloscope is invaluable, though some people have just used a VTVM or digital multimeter.

More tomorrow. I have pictures and schematics for my mount that may help, if the GPS version is similar. If that's the problem!....

[Dick Berg]

Hi again, James,

I'm way behind on everything in my life these days, and my help to you is in that state, too. At home, we're getting my daughter ready to go off to university for her freshman year to the University of St. Andrews, Scotland, and I am the lucky guy who gets to take here there. We leave this coming Wednesday.

Are you still experiencing this problem? I will try to put something together this weekend for you. Hang in there.

[Bob Denny]

Thanks for trying to help, and best wishes to your daughter!!!

[Dick Berg]

Okay. Here's some hopefully helpful hints and pictures. Read my "Notes on runaway" to start. There's one more document I'm looking for that I haven't been able to find yet. It shows the adjustment process using an oscilloscope.

Let me know if you can't unpack these files...