Pleiades - anyone seen this artefact before?

[Smashtie]

I was shooting the Pleiades the other night during the Geminids, using my 80mm APO.  Stacking the subs, I started seeing the cloudy wash effect you see before you.  Anyone have any thoughts on what this might be?  I've boosted the contrast to make it more obvious.

I've used the same kit before and not seen this effect.  I'm assuming its something to do with the bright stars in this cluster - normally I shoot DSO, so there tends to be a lot less light.

Technical details:

Ascension 80 ED APO (a Meade 5000 80ED twin)

Ascension field flattener

300 lights, 10 secs.

30 darks, 10 secs.

30 flats, 1/40th sec.

30 flat dark, 1/40th sec.

Nikon D7000, ISO 400.

HEQ5 pro, unguided

The moon hadn't risen when I took the shots.

There is some light pollution (I'm in a small village), but nothing too bad.  Perhaps a bit of light from a neighbour's upstairs window, maybe 30-40 degrees off the scope bore.  

The scope had its generous dew shield extended.

It was slightly frosty, and I didn't think to check for anything forming on the optics.

I'd be very glad of any suggestions.

Rupert.

[Stargazer33]

Hi Rupert, does the artifact appear on any of your subs? either lights or flats.

[Smashtie]

Ah, good question.  No, it doesn't seem to be in just one (or two) images.

[Smashtie]

But such a good question, I'm going to go back and double check them all.

[Smashtie]

Ah ah!  Found it - it's on the dark frames.  I must not have completely screwed in the lens cap, or maybe some light leaked under the dew shield, or through the view finder.  Thanks for the suggestion, Bryan!