Stars disappearing in Subs.

[Deneb]

Hi

Has anyone had this problem before or know what it's cause would be -

Stars gradually disapper from my subs.

It sounds weird does it not. A couple of samples from last night, both are 2min subs, unprocessed:

2min Sub of M81

1 hour later

I took these images with my 200PDS & the Canon 450D w/ the SW Coma Corrector. One thing I do make sure is that everything in the imaging train is nicely secured tight & on the focuser there is no play. The mount was setup as I normally do, making sure nothing is loose and everything is tight also making sure the ota is well balanced on the mount & polar alignment is accurate as possible.

Any ideas ?

Cheers

Nadeem.

[Starflyer]

I regularly get get variation between subs over time, I put it down to changes in transparency, high cloud passing over etc.

[brianb]

DEW!

[Catanonia]

Yup, Dew or high clouds you canna see. Happens a lot.

I use a dew heater and DeepSkyStacker live star count graph to keep an eye out for it.