Diagnosing differing star shapes across the frame

[Brent]

I've just installed a low profile Crayford focuser and started using a Baader MPCC.

The MPCC seems to be doing its job but from about the centre of the frame, and worsening as they go across to the right, I'm getting stars that have the appearance of star trails but rotating around the centre of the frame.

Is this the focuser not being perpendicular to the focal plane do you think?

This is a 5 minute sub:

[Brent]

Thanks to daz for posting this thread about a free alternative to CCDInspector called MaxSelector.

Below are some screenshots of the results for curve, tilt and collimation, FWHM, etc, of the sub in the post above. They appear really bad

I also don't understand why things like the tilt and collimation values are so different between subsequent subs! (first screenshot)

I've read through the program documentation but I still don't really know how to approach sorting this.

Can anybody help please?

[Thalestris24]

Hi

I've had similar problems though in my case I'm fairly sure it's tilt. These things can be tricky to diagnose and you might have to follow a process of elimination. Yours could be a tilt issue too, though with the appearance of rotation I think I'd suspect a PA error first. Does it vary with sub length or is it a 'fixed' problem? How far away was your guide star? If it varies over time then I'd check PA. If it's fixed then maybe it's tilt. I'm not certain about the above software but I think CCD Inspector likes to have just a star field without a galaxy in the middle. Maybe you have some other subs you could use?

Perhaps someone more knowledgeable will chime in.

I'm about to strip out my 130pds, collimate/centre the focusser and re-collimate the secondary and primary mirrors. First time I've attempted a strip out... 

Hope you can sort your problem.

Louise

[Brent]

I'm not certain about the above software but I think CCD Inspector likes to have just a star field without a galaxy in the middle. Maybe you have some other subs you could use?

That's a VERY good point Louise, thanks. Not sure why I didn't think about it but the galaxy possibly heavily skewed the results. I deleted all other subs apart from this set but tonight looks good enough to be able to just get some star fields for testing.

PA should have been very good - Alignmaster was reporting sub arc-minute accuracy (I haven't a hope of drift aligning here, unfortunately). The guide star was very close to M31 as well and actual rotation reported by DSS was 0.02 degrees over the one and a half hours of subs (I have more subs than those in the MaxSelector screenshot above).

[Brent]

Seeing was unfortunately not good enough Saturday night but I've just downloaded the demo of CCD Inspector.

Results of the above M31 image are below whilst I await the sky to hopefully clear tonight.