Misshapen Stars

[Grierson]

I wonder if there are any wise heads out there who could comment on a issue I have with star shape. I have an ZWO OSC camera attached to an Esprit 120 which is on an EQ6R-Pro controlled with a ZWO ASIAIR. The rig has been giving good results however over the last few nights I have noticed that the stars are no longer round and appear to have a slight 'bulge' to the circumference, Hopefully this is obvious in the attached image. This occurs with  and without a filter in the light train. I recently fitted the ZWO EAF which seems to work well but I can't see that fitting the EAF would contribute to the issue. I have not yet had the opportunity to uncouple it to see if there is any difference but I thought I might get some sage thought from the forum in the mean time! All the stars exhibit the same 'bulging' no matter what position they appear in the image. Images are processed in PI. Any observations would be appreciated. Thanks.   

[inFINNity Deck]

Hi Grierson,

it is difficult to tell from this image as it is heavily clipped on the dark side, but it appears to me as pinched optics.

Can you upload a raw image file?

Nicolàs

 

[Grierson]

Thanks for the response. The image is clipped to try and accentuate what I call a ‘bulge’. I’m not sure the system would cope with a large raw file

[inFINNity Deck]

Then perhaps a crop of a raw file as a tiff?

Nicolàs

[Gary Clayton]

I agree it does look like pinched optics, try turning up your dew heater as it's very could overnight at the moment.

[Peter Drew]

I also agree that it looks like a classic case of pinched optics.  No doubt a consequence of the current low temperatures.  I seem to recall similar issues with Esprits in the past.   🙂 

[Stuart1971]

It looks astigmatism caused by field curvature at the edges…but you say it’s all over the image, in which case it’s probably still astigmatism, but due to collimation being out, somehow, so has the cell come loose or something on the scope…or something shifted…??

i have had this issue with my TAK FSQ85, this is what I was dealing with, but only in the outer 25% of my images…

 

[Grierson]

Thanks again.

Post was deleted when I uploaded a TIF example. Not sure what the max file size is allowed.

With regard to temperature the first time I noticed it was during a windy night but the temperature was well above freezing. I naively put the problem down to the wind!. I will however recheck the position of the dew heater.

[ollypenrice]

Pinched optics would align with a recent onset of the problem if the temperature has been dropping.

Olly

[Grierson]

Thanks Olly. The ‘scope is 18 months old and I have never noticed the problem before the last three 

weeks. However I will recheck on a clear warmer evening if we get one!!

[Peter Drew]

Pinched optics can also produce astigmatism.  If the cell is oval, pinching due to cold shrinking the cell will pinch across the minor diameter.     🙂

[windjammer]

Are you sure its not a mount glitch or vibration? To me eye it looks a though part of the exposure took place whilst scope momentarily off target.  All the bulges are suspiciously spherical.

[Grierson]

Again thanks for the contributions. Windjammer’s comment takes me back to my original comment about noticing the issue after a particularly windy session. I am one of the many who have to set up each evening. It went through my mind that something might have been loose but when it occurred in calm conditions I start to worry!! Hopefully, and in the light of members comments, I will be able to look again at the problem in the coming nights.