Bizarre star issue

So I have no idea what happened here, but I had a very very odd star issue recently when shooting M31. I was using the Nikon D5300 and 70-300MM lens. Got everything dialed in and shot 60 subs of 60 seconds each.

Initially I was planning to take the bias frames and call it a night, but everything was working so well (compared to my “1:1” Zoom disaster the previous time out) that I wanted to collect another 30 mins of data.

That was where things went a little bit South. As far as I recall I didn’t change any settings on camera or the tracker (of course, everyone says that). However, all the stars in subsequent frames ended up with a ghostly look.

Previously they had been fine, and I had (and still have) no idea what this might be. It affected ALL the stars (not just edge stars, or stars in a certain part of the frame) and in every frame.

As an example, here are the stars (from the final image) in the “Good” frames

Not perfect, but pretty round (this is a 200% zoom)

And the bad stars

It’s as if the stars are hollow in the middle or something. Bizarre.

At first I thought it might be dew on the lens, but then I realized I had no idea what that might look like. I tried and tried to refocus but nothing.

After taking these, I did zoom to 300MM and the issue went away so maybe it was something physical with the lens (zoom slipped ?)

Will definitely keep an eye on this (although I am hopefully moving to the 300MM prime lens so hopefully that will give me better quality overall and avoid this entirely)

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