odd shaped stars?

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Name
Tim
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PS3A6567 by Good Lux Photos, on Flickr

Here is a shot of Orion's belt from the weekend. The stars of the belts look crescent shaped - any idea what is wrong?
 
it could be...just looking that up. Is this fixable do you know?
 
might be that focus was a bit off too, here is a crop from one with slightly better focus

PS3A6570.jpg
 
Wider angle lenses are better for Astro. Less movement captured. I'd agree there is either focus issues or long exposure movement. Judging by the other stars it may not be coma at all.
 
I would guess at a combination of focus and flirting with the 500 rule. 5 seconds is just OK at 85mm but obviously if you look closely you will see evidence of the Earth's rotation!
 
It looks like a combination of several things. Either a bit of haze or over processing is probably responsible for the fuzz around the brighter stars. Then there is some trailing which is more obvious in the brighter stars but is there is the fainter ones too. If you're shooting stars off a fixed tripod you only have a certain amount of time before they trail. The time depends on the focal length of lens (longer lens = more magnification = less time before unacceptable trailing) and and where you're pointed (stars near the celestial equator move a lot faster than stars near the celestial poles). Then you have to factor in what size are you going to view the image at? The larger you intend viewing the more star trailing matters. Focus could be an issue too. A bahtinov mask is extremely accurate but the lens might not be long enough for it to work properly. Live view is a possibility if you can get enough magnification, though my lcd screen has spent so long against my nose everything looks fuzzy on it.......... Autofocusing on a very distance object (by day or by night if there's a distant light or the Moon is around) and locking the focus works pretty well for my 17-50mm lens.
 
Thank you for all the responses, very helpful. There was cloud in the sky that night. Other nights where it's been completely clear I've not noticed the problem. Fingers crossed for more clear nights to practice.
 
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