America Nebula ASI2600mc compared with 450D

[Ouroboros]

I recently acquired an ASI2600mc Pro and thought it might be interesting to compare its performance with my unmodded Canon 450D on my SW ED80 refractor.  The processed pictures below  were taken with my 450D (left) and ASI2600 (right).  This is not a very fair comparison because the 450D data was processed with 15 x 300s sub frames whilst the ASI2600 image was obtained from 38 x 300s sub frames.  

A fairer comparison is between two typical frames.  The 450D 300s subframe (left) was obtained at 400ISO and the ASI2600 subframe (right) was obtained with gain at 100.  

I haven't yet given much attention to choosing the best ASI2600 gain setting and the duration of exposures.  Because I'm using an ASIair Pro I am limited to a gain of 0 or 100 with this camera.  So I'd be interested in comments on appropriate exposure settings. 

I notice that there is evidence of star distortion at the corners of the ASI2600 image. This distortion is symmetrical and more-or-less points at the centre, where stars are round.   So I assume this may be due to not having quite the right back focus rather than sensor tilt.  Nominally the sensor should be  at the required 55mm back focus from the 0.85 flattener/reducer using the ZWO supplied extension tubes.   Has anyone else noticed this? And what can I do about it? Actually the distortion is quite small and you have to really zoom in to see it.  Below is a 30x zoomed-in image of the top left corner. 

[Padraic M]

This should be an unfair comparison and very hard on your poor 450D! The 2600 is a superb camera. I've seen lots of really good images from it on here in the last few months.

That star distortion isn't bad, but could possibly be improved a little alright. You may need to move the sensor very slightly outwards, according to this helpful chart:

[Ouroboros]

@Padraic M Thanks for that. Better out than in as they say since adding distance is presumably easier than taking it away. I’m guessing I’m looking at sub-millimetre extensions to do this?  
Yes, the ASI2600 is a good camera. I was amazed at how little noise there is on the images and how there’s no obvious amp glow. 

[Padraic M]

To be honest, I'm not the best person to ask as I'm struggling to fix a similar problem myself with a long focal-length SCT. I think I have it sorted if only I could get a clear night to test it out.

[Ouroboros]

@Padraic M Yes, you do seem to have the same problem. What have you done so you think you have solved it?