[JA]

Hi Mark,

Looking at it there's certainly evidence of elongated stars / transverse star trailing, suggesting tracking issues, BUT you said you had problems with the f4 and f5 and NOT the RC8, which brings that conclusion in to question. A number of things might be different: you mentioned suspecting the flattener, but perhaps there were also differences in how the mount handled these loads, long newtonians versus shorter RC8, a blip on one night using the other scopes, guiding on /off, who knows?

Some more info would help delve in to the issue:
1. Is the first image a FULL field of view? Any cropping and if so how much?
2. If it is FULL field, which scope was used?
3. Is the first image a single sub exposure?
4. What was the sub exposure duration of the first image?
5. Provide a higher resolution image.

You need to do some research to find out about the possibility of it relating to tracking. To potentially rule out tracking, using stellarium (or similar) go to that exact portion of the sky as your first image at the same date and time as the original exposure and set it to the same field of view and orientation. NOW ... index the time forward how ever many seconds you exposed for. Is the change in star position equal to the length of the star trail in your image? Is the direction of your star movement as per your image. If it is you have part of the answer.

The RC8, with its longer focal length not exhibiting this trailing is a conundrum in terms of tracking being at fault and the answer may lie elsewhere, but at least you will have potentially ruled out one possibility.

PS... I take it only one mount was used in all 3 cases: f4, f5 and RC8. ???

Best
JA