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Thanks for your NGC 253 shot Alex as it has given me a good idea for a target for my Canon F2.8 200mm prime lens. Its a nice composition with the Globular Cluster in the field.
I am also having issues with my lens adapter in that it is not a snug fit. I am attaching the 200mm lens to my QSI 683 CCD using the stock QSI adapter but it has rotational shift as well as it drops slightly under the weight of the lens. I have made up a very secure mounting plate but it is a bit of a guessing game as to exactly where to position the lens. It probably only has about .5mm (or less) of play but this is enough to throw off the image plane which is causing elongated stars in the corner of the image, not too dissimilar to what you are experiencing. I will be interested to see how you go making up a shim to suit your adapter. It may also be an issue with the distance between the lens and the focal plane of the CCD so a very thin shim may fix this. I will watch the results of your experiments with interest.
I don't see any much in the way of diffraction spikes which is interesting if you were capturing these without an aperture mask. For what it is worth, I purchased a step down adapter from ebay to use as an aperture mask and this works quite well on the 200mm lens. It steps down the front of the lens from 72mm to 49mm. Something to think about if this suits your requirements.
Thanks again for the idea on the NGC 253 image. My resulting FOV with the 200mm is about 5 degrees by 4 degrees so will suit this composition nicely.
Good luck with your set up Alex.
R
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