[JohnH]

I have been attempting NB synthetic colour imaging

http://www.iceinspace.com.au/forum/showthread.php?t=58758

for the last week or so and I have hit a bit of a snag, well more than one as you can see, but I am looking for some ideas to help me track down why my Oiii frames show distorted stars.

Last night was clear again so I went back to NGC3372 see if I could get better Oiii data, before I did so I took my filter wheel apart, removed and cleaned the filters and re-assembled. As far as I can tell all the filters are screwed fully in (they were anyway), I also removed the filter holder and re-seated it.

However this did not improve things - my Ha frames show round stars but my Oiii images show a distortion in the star shapes - I am at a loss as to the cause of this? Is my filter faulty? I am beginning to think so as I have not seen this in LRGB frames.

Or could it be a focus issue - if I am not quite critically focussed the stars are better shapes but then I loose details, I can't really think it could be anything else as the only things changing are the filter and therefore the wavelength in the CCD light path.

Attached are a couple of raw light frames to show the issue, these are 4 min subs taken in sequence - and I have 32 of each showing the same issue...

Filters are the Astronomics ones
Filter wheel as an ATIK USB unit
Camera is an Opticstar 145M
Scope is a 127mm North Group APO (F7.5)
Image Scale is 1.4" / pixel