Subs getting darker as night goes on?

[Bagginsies]

I am currently outside now taking my dark frames for M101. I have just tried to take 60 x 2 min subs @ Iso 3200. At first they seem fine with the histogram at about the 1st line (1/4 across), but on checking as I went through, they slowly get darker and darker until the histogram is on the left side?

What is causing this? Scope pointing upwards and moisture on the mirror? (There does seem to be a lot of moisture on the outside)

Sky is still clear! Camera is a Canon 60D attached to a Celestron 130SLT. I did exactly the same last night and got a great image of M51. 

Can't think what else would cause it.

[Bagginsies]

Just packed up and noticed the secondary was completely fogged up so moisture was the problem!

Couple of questions;

I normally image with the scope on a concrete driveway, but tonight I stayed in the garden and had the scope setup on 3 blocks of wood (to stop it sinking in which worked fine), but with it being over grass, could this cause more moisture? 

I would have thought the main mirror would get more moisture as it was pointing upwards, would a fan at the back help this?

[MarkAR]

Could be the grass or could just be more moisture in the air. Fan should help I would have thought.