WO GT81 first light..star shapes weird!

[StargeezerTim]

Not sure if this is the right forum for this. No doubt the moderator will move it to somewhere more appropriate if required!

Tried my WO 81 and 0.8 flattener/reducer for the first time last night. The first set of lights (15 of M101) all showed strange heart shaped stars. Sub length was 5 minutes and guided with a Lacerta MGEN and a LP filter was used. This is a sub, just adjusted with levels and colour balance to make it easier to see.

Here is another sub from later when I went galaxy hunting before the clouds rolled in. The only change in setup, as far as as I can remember, was that I just adjusted the focus ever so slightly. This one looks much better, there is some star elongation in the corners but that could be due to polar misalignment possibly? Here is that sub.

Anyone any idea what could have caused the strange shapes in the first set of subs? I'm guessing its not an issue with optics as the second is much better, but would appreciate any thoughts you may have.

Cheers, Tim. 

[gnomus]

It looks to these inexpert eyes like coma.  Have you paid close attention to the requirement for spacing between the reducer/flattener and the camera sensor.  This needs to be quite precise.

[Stu]

I don't think it's coma, as that would radiate out from the centre and this doesn't. Plus it is improved in the second sub. I think either a minor guiding glitch or focus related.

[StargeezerTim]

26 minutes ago, Stu said:

I don't think it's coma, as that would radiate out from the centre and this doesn't. Plus it is improved in the second sub. I think either a minor guiding glitch or focus related.

Perhaps it is down to focus... I got 15 or so subs of M101, all with the same issue. 

[Barry-Wilson]

I find that for the fast FR scopes, the corner star shapes really snap into focus and circular shape only within the CFZ as it is so relatively narrow.  This is very noticeable with the very fast Baby Q that I have.

With reducer you're at F4.7, is that correct?  This would explain the better shape in the second frame you posted which you had tweaked focus.  You may still have some spacing or focuser tilt too, but focus is also critical for star shapes.