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CC knows the score.. At my widest zoom on my Sony, I can only go for 15 seconds before I get startrails, so when you're zoomed in more you'll only be able to do it for a much shorter time.
The halo could be a processing artifact, but I assume they're present in the original uncompressed version as well? How did you take the shot? Using the shutter or with a remote? It could be the camera wobbling if you used your finger, especially if your tripod isn't very sturdy..
I try and use the self-timer when I press the shutter, so that the shaking/wobbling has stopped by the time the picture gets taken.. unfortunately my camera only has a 10s self-timer, so there's a lot of waiting around between shots, especially because it has an auto-NR feature that can't be turned off, so I have to wait an additional 15s after the shot while it takes a dark-frame.
The halos are also only around the brightest stars - is it perhaps something to do with the quality of the lens? Like Chromatic Abberation or something like that?
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