Odd Star Shapes with different filters

[barx1963]

I have noticed some odd star shapes in quite a few of my subs lately. One interesting factor is that the effect seems to vary from filter to filter. Attached images are NB images of the South Pillars Area of Eta Car. Taken with an Edge HD 8" scope, ZWO ASI1600MM cam and Astrodon 36mm filters in a ZWO filter wheel. These are only crops of single subs that I have stretched so plenty of stars are visible. In order they are

SII Top Left (nice round stars)
Ha Top Left (really bad stars!!)
SII Centre (nice round stars)
Ha Centre (Not bad stars but still not good)

Any idea what may cause this? Only thing I can think of is that the filters are not sitting flush in the carousel. If wit was tilt or collimation, it would surely show up in all images?

Malcolm

[multiweb]

Definitely a tilt of some kind. Just swap the Sii and Ha to be sure if it's the caroussel or the Ha cell.

[barx1963]

Thanks Marc
That's exactly my plan. Am going to wait for the current run to finish and then I will pull the train apart and try and figure it out. want to at least get some useable data! Just got my first OIII sub of this run and it's fine so may try a bi-colour image just using SII and OIII for something a little different.
I just wanted to make sure my thinking was correct. This filter wheel uses little plastic masks that hold down the unmounted filters so I am thinking that may be where the issue lies.

Malcolm

[that_guy]

Do you refocus with each filter? It could be that the filters are not parfocal (although, not sure if astrodons are advertised as parfocal or not)

[Atmos]

Potentially silly questions but have they all been taken on the same side of meridian?

[barx1963]

Yes, I always refocus everytime the filter changes and try not to stay on one filter for longer than 1 hour.

Malcolm

[barx1963]

Yes, I believe so. Why would that be an issue?

Malcolm

[Joshua Bunn]

You want to minimise the things that change to be a fair comparison. And having all images taken in the same spot is ideal. This will help minimise tube flexure between different filter exposures. What do the exposures look like through no filter?