Odd shaped stars - help please

[Adreneline]

Hi Everyone,

I've just remounted my Samyang 135mm + ASI1600 combination having modified the Samyang with an M42 end plate.

The image is Polaris - just 4 x 2s frames at f2, calibrated and stacked in APP.

I think I've got the spacing spot-on but the star shapes in the corners of the image are not consistent, as per the screen shot above.

Bottom left looks quite different to the other corners and I am at a loss to know what might be causing the problem. With the M42 adapter I am convinced there is no droop or misalignment between the camera and lens; I am also supporting the lens as well as the camera and have gone to great lengths to ensure everything is level/aligned. My ASIair reported HFD on the target star of 1.1 - 1.4. APP reported FWHM figures of ~1.9 so I think the focus was reasonable.

Does this look like optics or a sensor problem?

Any thoughts or advice would be much appreciated.

Thanks for looking.

Adrian

[symmetal]

It's an optical problem with the lens, and is showing coma in TR and BL corners. Normally it appears in all corners but it depends on how well the lens elements are aligned. At f2 it's not too bad really. Some Samyangs are better than others and it's luck whether you get a good one. At least it is on the wide angle lenses. Stopping down the lens should improve the coma effect but that means longer exposure or higher gain. Spacing isn't critical as long as you can actually go through focus but it's preferable if the lens is indicating near infinity when the stars are in focus as then you get the full focus/distance range if needed.

Alan

[Adreneline]

9 hours ago, symmetal said:

Stopping down the lens should improve the coma effect but that means longer exposure or higher gain.

Thanks Alan. I will give it a go and see how far I have to go to reduce the problem; I usually end up cropping off the corners anyway.

I was getting similar problems on my Canon 200mm at f2.8 but I was putting that down to droop as I was reliant on the EoS adapter holding things tight and aligned. The Canon 200mm does not have an aperture control ring so there was no chance to experiment without removing the lens and reattaching to a Canon camera - definitely too much faffing around!

Thanks again for your help and advice.

Adrian