M52 and The Bubble Nebula

COMMENTS
The bubble nebula is an object I like a lot. Two problems with it. First, it's not particularly bright (depnding how you measure it, of course). Second, it's rather small, so at a focal length of 540mm it barely takes up more than 5% of the entire field of view.

For that reason this is a target I'll probably try in the future with the Powermate x4 barlow, though I'm worried the increase in focal ratio might make it a hard target to catch as it'd need too much exposure in order to capture details (update: it's done and my fears came true).

An odd problem with this image is with the stars at the bottom, right of M52. Notice in the original size image how the red and blue tones are not properly aligned in that area. This wasn't an optical problem but a stacking issue (one more reason to stop using DeepSkyStacker?), as the original raw images did not show this problem. Another issue is with the corners. Notice how the stars at the corners are stretched paralell to the center (that is, not pointing at or away from the center). This is certainly NOT field rotation, and my guess is that it's due to the camera not being right on axis with the scope, perhaps due to one of the adapters not being inserted exactly in line with the rest.