This particular shot is (I think) a 100% crop.
I wanted some new images to practice stacking and I had a small window of opportunity before the heavy clouds moved in last night to shoot a few frames. I'm not happy at all with the results, though.
Either I missed the focus (very well could have) or something is amiss with the lens. These shots were taken with a Canon EF-S 17-55 f/2.8. Lens was set as follows:
Manual focus
IS turned off
Exposure time: 16-17 seconds
Aperture: f/2.8
ISO: 1600
I focused on a bright star via Liveview zoomed to 10x. Once focus was attained I stuck a piece of tape over the focus ring and lens mount ring to keep the focus from being moved if bumped.
This was basically a shot of Orion rising from the easy and in my more light polluted direction. The temperature was around 45*F with an approaching rain system...water vapor could easily be seen when I exhaled.
The problem (that you've probably already noted) is the triangular, blue-tinted stars. This is much similar to the symptoms of a "pinched mirror" in a reflector telescope...but we're talking camera here.
I'm thinking that it is probably CA caused from a slightly out-of-focus image. This was the first in a series of images so the focus wasn't bumped (but maybe not focused correctly to begin with).
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