Old lens focus issues - sensor tilt?

[rnobleeddy]

I've been trying out an old 300mm Russian photosniper lens and a DSLR for imaging. I have no expectations other than a few vague forum posts that it's decent for astrophotography.

Having sorted my processing (thanks for the help @alacant!) I just want to confirm my diagnosis  based on the following images. I guess it's tilt?

Not sure the best way to show this - so I have a single frame with calibration done (flats + bias frame, no darks but dithering).

The top left

center

 

 

and bottom right

 

It looks like a pretty strong gradient in focus from top left to bottom right?

 

I think the camera is OK - I haven't used it for a while, but when I did use it with telescopes, the images were good. 

What I do know:

- I stopped down to f/5.something (it's f/4.something) as I've never had a lens that worked well wide open. But I did take some systematic shots at different apertures a while back so I'll dig those out and see if aperture matters

-the lens is heavy and my mounting approach is best described as DIY - please forgive these crimes against DIY - I intended to make something permanent if it worked well

- the adapter that converts the lens to fit the canon is potentially the weak point

- to focus, I slewed to a bright star in the centre of the field and focused using live view on the Canon 550D. One side of focus travel has a blue halo, one side has red. Obviously this approach isn't ideal but I don't think it explains the above?

- the focuser on the camera has a decent amount of travel, but it shakes the mount, making fine adjustment tricky

 

I guess what I'd like to know is if there's any way to systematically determine if the lens is damaged/out of alignment, or it's just that the camera sensor is titled a little with respect to the lens?

I also wondered if anyone has seen this pattern of focus before? 

A quick crop + process lead to this - which I'd have loved last year - but now underwhelms.

 

No loss if it turns out, like many lens, to have it's issues. It was £50 and came with the most amazing case and array of 72mm filters that I don't understand. If not astro, one day, I could take it out in daylight and look like this!

[alacant]

37 minutes ago, rnobleeddy said:

the adapter that converts the lens to fit the canon is potentially the weak point

+1

Get a proper adjustable eos-m42 adapter. Tighten the inner ring under gravity with the lens pushing down upon it.

On our Tair 3s, take focus until the red halo predominates then back the other way tiny amounts until the red ring JUST disappears. Now tape the focus wheel securely. 

Use the click stop at f5.6.

HTH

 

 

Edited October 1, 2021 by alacant

[rnobleeddy]

13 hours ago, alacant said:

+1

Get a proper adjustable eos-m42 adapter. Tighten the inner ring under gravity with the lens pushing down upon it.

On our Tair 3s, take focus until the red halo predominates then back the other way tiny amounts until the red ring JUST disappears. Now tape the focus wheel securely. 

Use the click stop at f5.6.

HTH

 

 

Thanks for the help again. I actually have a proper adapter that I used with another lens, so I'll dig that out.