Trails when guiding

[JSeaman]

I just had a really strange anomaly and I'm wondering if anyone has seen it before

The effect is shown in the images below, I suddenly got significant trailing for 20 minutes on 4 different subs:

 

Not unusual in itself but the guide log was at a steady 0.6" the whole time and showed no deviation in RA or Dec. 

Has anyone seen a clean guide log and trails this bad before?

James

 

[Stargazer33]

Was a cable snagged?

[JSeaman]

No but that would have showed in PHD2, the thing I can't explain is it thinking everything was good

[almcl]

Looks as if it could be differential flexure between guidescope and main scope?

And, yes, until I switched to an OAG, I used to get something similar.

[JSeaman]

I knew someone would say differential flexure. I sort of agree in that the guide scope thought everything was fine and the imaging scope disagrees but this was on the same target I did last night over the same time period and there was no issue. The guide scope is bolted to a 5mm metal plate I welded so there's not much room for flexing

[JSeaman]

I'm interested how you found the switch to an OAG though

[almcl]

To answer the first post point, if you are imaging with a reflector (of any sort) the guidescope mounting isn't the only source of flex.  The mirror mounting can and does move as the mount and scope's orientation change and a heavy camera load hanging off of a Skywatcher focuser can also shift position.

The shift to an OAG improved my imaging quite a bit.  Stars were much rounder and I no longer got the image drifting steadily across the frame during the session.

There's a thread here which describes a similar issue:

 

and Oddsocks answer halfway down the page in this one was revealing:

 

Edited February 27, 2021 by almcl

[JSeaman]

Yep it's a 300PDS. The mirror wouldn't move at a consistent speed over 20 minutes so I can rule that out. The focuser was on my hit list but it can't be because it went back to normal after about 5 pics. 

I do have the drift you show in your thread and always have, I'm considering OAG at the mo

Thanks for the links, I will continue to look at moving to an OAG but that will inevitably lead to buying a CR2 focuser and it would be rude not to get a sceond lakeside at that point ... Here we go again!