Is this a Focus or Dew problem?

[gnomus]

As my second attempt at astrophotography, I decided to have a go at imaging the Andromeda Galaxy last night.  I thought I would try piggy-backing my Nikon on top of my scope (shooting through my C8 would not give me a wide enough field of view, even with the focal reducer).  I used my Nikon 70-200 mm lens (quite a decent lens, albeit a zoom).  I coupled this with a x1.7 Nikon Teleconverter to give me 340 mm at f/4.8.  

First problem was getting the galaxy in the image since the camera.  But I eventually got the thing roughly centred.  I focussed by using live-view to zoom in on a nearby star.  I then adjusted focus to get the "point" of light as small as possible.  My test image (30 sec exposure at 6400 ISO) seemed just about OK.  I then set off my intervalometer to capture 25 x 2 minute exposures (at 800 ISO).

When all was through I checked my images.  To my disappointment the 2 minute exposures did not seem to be well focussed at all.  Indeed, on some of the images the stars seemed to exhibit a central doughnut.  I attach examples of the high ISO "test" exposure and the 2 minute exposure showing the doughnut-ting.  

My question is:  can this doughnut effect be produced by the effect of dew on the lens alone or does it indicate a focussing problem.  I would not expect the lens to shift focus as the hour or so wore on (it is a high-end expensive lens).  On the other hand, I guess that it would not normally be used pointing straight up at the sky - perhaps it wasn't designed for this.

So .... could this be dewing alone or has the focus shifted?  The first smaller image is a 100% crop of the test image at 6400 ISO.  The second image is a 100% crop of the doughnut effect.

Thanks in anticipation

More questions will no doubt follow.

[D4N]

The focus point can have shifted due to changing temperature as your kit cools down. It certainly looks more like a focus issue than dew.

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[Leveye]

LensCreep after your focusing. Lock focus somehow. I use large elastic bands on telezoom lenses.

[gnomus]

Thank you for your replies.  I will investigate how I might lock focus.  I did, however, take the camera straight out from a warm house to the scope.  The first thing I did, after attaching everything was focus.  Perhaps the focus-shift was, therefore, related to the camera/lens cooling down in the night air.  How long do folks generally leave their cameras out before attempting to image with them?

For your amusement, I attach the processed image of my "out-of-focus Andromeda".  

It's such a poor picture I'm thinking of entering it for the Turner prize next year.

[D4N]

It's still immediately recognisable as m31 though so not all bad

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