DSS Stars and Sky blackness decrease over session

[Shaun_Astro]

I've just noticed with my 130pds that the images at the beginning of the session show 90-100 stars, and by the end 45. Also sky blackness goes from 0.9% to  0.3%. I use a 130pds. There didn't seem to be any dew on the primary at the end, but something has to have caused this as my target was approaching zenith at this time, and the nearby street lights turned off near the end.

Has anyone else had this issue, and what can one do about a primary dewing up?

[dph1nm]

Probably the secondary dewing up. I have had that problem (and those exact symptoms) with both Newtonians I have owned. Solution - a hairdryer, but I do have mains power available.

NIgelM

[Les Ewan]

2 minutes ago, dph1nm said:

Probably the secondary dewing up. I have had that problem (and those exact symptoms) with both Newtonians I have owned. Solution - a hairdryer, but I do have mains power available.

NIgelM

A hair dryer is actually a indispensable piece of equipment because otherwise a observing session can be ruined by dewing in less than a hour sometimes,however its amazing how much racket they seem to make in the middle of the night.