What causes misshaped stars on left edge only?

[MilwaukeeLion]

Stars are misshaped only on left side, then works itself out just before M31.  The center, top, bottom & right sides seem fine.  Is this mount issue or something else?  Was doing 5 min subs using auto guider on Atlas EQ-G mount, field flattener & DSLR.  

Thanks for any help.

ML

[spaceman_spiff]

You are probably seeing the affects of field curvature. This happens when your telescope focuses the light not on a flat plane but on a slightly curved one. Therefore when the stars in the center of the field are in focus the stars on the outside are out of focus. Hope that helps. 

[spaceman_spiff]

Great pic BTW!

[ollypenrice]

It's best to post corner crops in large scale if you can.

The usual suspects are;

Tilt. This can come from the focuser, the chip in the camera, the attachment method of the camera (screwed on beats push-fit) or some other aspect of scope construction. Turn the camera through 90 degrees to see if the tilt follows the parts you turned.

Polar misalignment. This gives a rotation-like effect around the edges if the guide star is central to the image. If the guide star is not central to the image then the effects can be highly asymmetrical.

Coma. (Field curvature). This, all things being equal, should be symmetrical but life does like to play tricks so it isn't always!

Chip distance. Coma correctors and flatteners have a correct lens-to-chip distance which can be very intolerant in some cases. Check the manufacturer's spec.

Collimation.

Olly

[Astrosurf]

Good advice from Olly and spaceman.

Great image!

Alexxx

[ollypenrice]

It's best to post corner crops in large scale if you can.

The usual suspects are;

Polar misalignment. This gives a rotation-like effect around the edges if the guide star is central to the image. If the guide star is not central to the image then the effects can be highly asymmetrical.

Olly

Sorry, I should have added that the test for any kind of mount motion related issue is to take a 10 second sub on a bright starfield. If the error is not there then it points to PA or guiding since these need time to show a problem, though in your case it can't be guiding because it isn't uniform across the image.

Olly

[opticalpath]

Olly's advice is spot-on and I would suggest starting with his suggestion to take a series of quite short exposures with the camera oriented one way, then at 90 degrees, in order to narrow down the possible causes. 

It can sometimes be difficult to find a star field that has bright enough stars in all four corners with a short exposure.  If that's the case, take a few images of the same star, but positioned in each of the four corners of the frame in turn.  Then rotate the camera 90 degrees and repeat.

Adrian

[Owmuchonomy]

Great and clear advice above.  So for example in my situation subs of a few seconds (or of a few minutes) both show oval stars in two corners.  Rotating my chip 90 degrees moves the anomaly around the image by 90 degrees (so its not following the chip).  Thus there is likely to be an issue with my scope.  I convinced the retailer by sending images who in turn convinced the manufacturer and thus my scope is back with them.  By the way I use a manufacturer specific flattener which excludes spacing issues to a point.  The manufacturer also sent me a duplicate flattener, to no avail.  Great after sales service but I am without imaging scope for a while.

[Thalestris24]

Rotating my chip 90 degrees moves the anomaly around the image by 90 degrees (so its not following the chip). ?? If the anomaly moves as you move the chip, doesn't that point to the orthogonality (or lack of) of the sensor chip?

Louise

[MilwaukeeLion]

Thanks for the tips, will eliminate one by one, see what happens.   I know for a fact that my finder scope is not spot on where my refractor points.  Just in the general vicinity, didn't even consider guide star being potential issue, figured once locked on any star and spot on polar aligned it didn't matter. 

ML

[Owmuchonomy]

Rotating my chip 90 degrees moves the anomaly around the image by 90 degrees (so its not following the chip). ?? If the anomaly moves as you move the chip, doesn't that point to the orthogonality (or lack of) of the sensor chip?

Louise

If the chip was not square in the camera the image would have the same anomaly no matter where I rotated it. For example, my eggy stars sit in the top left corner when all is at the home position. If I then rotate the camera body and thus chip 90 degrees my eggy stars appear 90 rotated for example at the bottom left. If the chip was at fault the eggy stars would remain in the top left corner. In addition the issue is repeatable with my other camera. Therefore the issue is not the chip. Let's see what comes back from the manufacturer.

[Thalestris24]

If the chip was not square in the camera the image would have the same anomaly no matter where I rotated it. For example, my eggy stars sit in the top left corner when all is at the home position. If I then rotate the camera body and thus chip 90 degrees my eggy stars appear 90 rotated for example at the bottom left. If the chip was at fault the eggy stars would remain in the top left corner. In addition the issue is repeatable with my other camera. Therefore the issue is not the chip. Let's see what comes back from the manufacturer.

Hiya

"If the chip was not square in the camera the image would have the same anomaly no matter where I rotated it" - that's what I thought you said! Never mind

Louise

[Owmuchonomy]

No problem  .  I hope my scope is back soon!