East to West drift while guiding?

Well, I have been doing some testing, the weather has not been good enough to image. So I decided to work on some issues.

I did a series of 15 x 10 minute exposures of M81. When I finished, I checked the frames and found that there was a steady drift from East to West. This amounted to about .9 arc minutes in 2.5 hours or about .0004 error ( or 3% error over 2.5 hours). But this did make a 5mm drift on a 160mm scale, so it is significant.

Now this would normally mean that the mount was running slow and should have the RA Divisor number reduced by about 22-23 to get the speed correct. However, the real question for me is, WHY is there any drift when guiding? I mean, the guiding should speed up or slow down the RA as necessary as it keeps the star in place, right? So, how does the mount drift in RA during guiding?

I have seen some elongation in the East/West direction for a while in exposures. I figured it was due to flexure, but have done a lot of mods to eliminate flexure, which I really did not think was the issue.

Now, if it were flexure, the mount was on the West side of the Meridian and the flexure would have caused the mount to lead (speed up) not lag (slow down), so the drift would be from West to East as the scope leaned toward the West which would cause the star to drift to the East, right?

So, what do you all think about this issue? Flexure just does not seem to be a possibility with this kind of drift. But, if it is the mount moving too slow, then wouldn't the guiding compensate? So, would this mean that the guiding program must be the cause? I am a bit confused to say the least.

I plan on going out and checking the speed with no guiding and see if there is indeed an East to West drift in the mount.
Floyd

This has been mystifying me for a while.....

Possible polar misalignment is showing up as east-west drift.

Possible that your guider is not actually guiding between exposures while the main camera is downloading.

Flexure.

If you get a real answer, I'd like to know.

Alex

Not sure what you setup is.... You are guiding with
one scope and shooting with another?
Or off-axis guiding? Is it just stuff flexing between
the guider and camera? Where did the track start and
stop -- on meridian and 2.5 hours past, etc.?

You know, an awful lot of the things we are mentioning (besides flexure) are darn good explanations for things moving around. The key to the issue is that the autoguider is not correcting them!

If it were the wrong divisor, the autoguider would have simply sent signals to "catch up" or "Slow down" and it would not be noticeable.

Same thing if it were polar alignment. (I still think there is something that makes sense--a mount out of alignment will show N-S we know--but an AUTOGUIDED mount may also drift a bit---but that is for another thread.) The autoguider would have corrected for it by speeding/slowing the drive.

It must be flexure.

I know I cured my problem (of the same type) when I went OAG with my 9.25.


Alex

Floyd, Are you dithering with the guider between frames? In theory, that should introduce random errors (at least over the long term) but it could add up to some drift in the short term. Its just a thought. Otherwise, if the guider is working correctly and sending corrections, the only solution is flexure. You could try a very long exposure to see if flexure shows in the image. It is often most pronounced when crossing over the zenith with a GEM. That is my 2-cents worth.
Cheers, and good luck,
Tyler