Uniform image drift while guiding - how does it occur?

[wcblack7]

I've gone from a piggyback arrangement for my guide scope and image scope to an asymmetric side-by-side set up. I'm still working through the transition, but have noticed that I'm getting longer hot pixel trails in my stack that seem to be completely aligned with my RA. Also, I believe I'm getting more movement in the RA on my imaging camera, affecting my image sharpness.

I'm not actually concerned about the hot pixels as I know how to deal with them, but I am concerned about this apparent movement they reveal in my RA. Since I'm using a guide scope, it seems like the hot pixels should stay put on the imager and not form trails in the stack (in a perfect world). Of course, in the real world flexure and other random movement between the two scopes is going to cause the hot pixels to form patterns, but it seems like these should be random patterns -- so not straight lines along the RA axis.

I did some live stack testing the other night with a 0.8"/pixel scale on both the guide scope and the image scope. My guiding looked pretty good in PHD with an RMS of 0.4" on the Dec and 0.6" on the RA. I'm still tuning this in on the new set up, but afraid I may be wasting my time if there is something wrong with the set up causing the RA movement in the image scope to differ from the guide scope.

I know that I still have some cone error (about 4 minutes) and pointing error between the guide scope and image scope (about 5 minutes). Initially, I have not been concerned about these, but now I am wondering if this can cause my two cameras to move at different rates in RA. Generally, I understand these errors have very little effect on imaging, but I am wondering if it is just more pronounced with my asymmetrical side-by-side arrangement. Or maybe I'm just not understanding how hot pixels can make uniform trails in your image stack when you are autoguiding.

Any insight or ideas appreciated.
Thanks!
Bill

[admin]

Hi Bill,

it certainly does look like drift - I think I can see other (fainter) streaks that run roughly parallel to the one you have highlighted and also there is the big black area at the botton that is no longer receiving new data thanks to the drift. You can of course confirm that by looking at the Live Stacking drift graphs in SharpCap which will show exactly how much drift has been observed since the beginning of the stack (https://docs.sharpcap.co.uk/4.0/#Drift%20Graph%20Tab)

Check that the guide scope and guide camera are all firmly fixed with cables secured - if the guide camera shifts then PHD will cheerfully adjust the mount to try to keep the guide star steady, which may actually be causing drift of the main camera.

cheers,

Robin

[wcblack7]

Thanks Robin. Your comments got me hunting for flexure and I think I figured this out. I'll find out tonight, hopefully.
I found a particular flex point only on my imager that happens to align with my RA axis when DEC is pointing due E or W.
So a couple of things I've learned (or remembered):

1 - Flexure does not necessarily show up as random movement, especially if it is around a fixed point (I think I already knew this, but forgot),

2 - A side-by-side set up for your guide scope is a terrible idea; it makes it way to easy to create flex points that only apply to one of your scopes.

[wcblack7]

Success... on the flexure issue anyway.
Still have a little flexing, but much more manageable. Now on to reducing my RMS error on the RA. I can still see it, but much less.
Also, the atmospheric dispersion is having an effect as well.
[https://www.cloudynights.com/gallery/im ... s-4-28-e1/]

[admin]

Hi Bill,

nice image - glad to hear that you are getting things sorted

Robin