What’s causing my oval stars?

[RugbyRene]

Hi all,

Was imaging the last 3 nights (I know, 3 clear nights) and I noticed that while my guiding was ok (varied between 1.5-2.5 rms) and my stars were round in the centre of the image, they were slightly ovoid on the edges of the image.

Now you need to really zoom right in to see it but it’s there. And they all flare in the same direction. Bottom left to top right.

My question is what couldn’t be a balance issue, or polar alignment?

The rig is was running was:
SW Esprit 100
AZEQ6 GT mount
IDAS D1 pls filter
Asi294 cooled

Acquisition info
ETA Carina
240sec exposures.

My mount is slightly east heavy and my scope slightly back heavy. Apparently this helps polar alignment by preventing gear chatter.

Cheers in advance

Rene

[glend]

Without actually seeing the image it is hard to be precise, but it maybe a tilt issue.

[Xeteth]

Rene could you post a picture so we can have a look? From what you described it sounds like your backfocus is a little off. Does it look like what you see in the following pic?

[RugbyRene]

It doesn’t look like either of those. The direction of flaring for all the stars on the edge is up and to the right. During guiding I noticed my RA was moving quite a bit above and the below the line. dec was fine.

[Atmos]

If your polar alignment is off, on longer exposures you can have round stars in the centre from shot guiding but suffer from rotation further from the centre.

[Nikolas]

Are you using a field flattener?

[raymo]

From your description of the diagonal direction of the elongation of the stars, I reckon Glen is probably right, tilt.
raymo

[RugbyRene]

I am. The one that came with the scope.