Improving Star Shape with Lens/ZWO OSC Combo

[Stuf1978]

I've just bought an Altair Astro dslr converter so I can use my ZWO ASI 294MC Pro with a number of camera lens' that I already own. I tested this out for the first time at the weekend using my Sigma 150mm F2.8 and the results weren't the best as the star shapes were pretty poor. Now this lens has always shown slight comma (but manageable) when I'd previously used it on a APSC dslr but the field of view was always pretty flat. This is the image I took at the weekend, ignore the vignetting and amp glow as it's uncalibrated and largely unprocessed and I also think focus is slightly out. However, the stars are elongated on one side of the image and relatively OK on the other side. I've also included a picture of the camera/lens set up to show how it's all connected. 

There is some rotational slop in the bayonet fitting for the lens (maybe a 1mm or so) so I'm wondering if this could be the cause of the issue? If so is there anything I can do to improve the situation? Maybe shim it out slightly or add a spacer to take up the slop? Is there anything commercially available that will help or am I going to have to improvise?

Thanks,

Stu

 

Edited December 13, 2022 by Stuf1978

[Elp]

If it's on one side it's more like tilt, a tilt adjuster may fix it, or try relieving some of the weight off, either on the lens or camera as a test to see if it goes away. Is there any flex between the two when they're connected together?

[Stuf1978]

7 minutes ago, Elp said:

If it's on one side it's more like tilt, a tilt adjuster may fix it, or try relieving some of the weight off, either on the lens or camera as a test to see if it goes away. Is there any flex between the two when they're connected together?

Yeah I did suspect that too as the lens is pretty heavy, maybe I can try placing something between the lens foot and the dovetail to ease some of the weight. There doesn't appear to be any real lateral flex but I'm guessing it doesn't take much to impact star shape. 

[Ratlet]

Not sure if the orientation of the sensor relative to the dovetail, but the image looks to my untrained eye like it is in focus on the right and slowly drifts out of focus on the left.

I'd be inclined to say tilt is possibly your biggest issue.  It may even cause an apparent elongation of the stars due to it being defocused.

[Stuf1978]

2 hours ago, Ratlet said:

Not sure if the orientation of the sensor relative to the dovetail, but the image looks to my untrained eye like it is in focus on the right and slowly drifts out of focus on the left.

I'd be inclined to say tilt is possibly your biggest issue.  It may even cause an apparent elongation of the stars due to it being defocused.

The short side of the sensor is facing the dovetail in this configuration which suggests that the weight of the lens may be the issue. I'll try bracing the lens against the dovetail so the bayonet isn't taking all the weight and see what impact that has. 🤞