Declining viewing quality

[matto113]

Last night was my first light with my new XT8. I definitely had a fun time for the first hour and saw M1 for the first time which was so cool. Unfortunately, after about an hour my telrad finder was persistently fogging up too much to be used. I was able to somewhat make do but it made things way more difficult. After about another hour of fumbling around with the foggy telrad, I started noticing that what I was viewing was becoming very faint. I was having occasional problems with eyepiece fogging but this was different. I tried different eyepieces and it was similar with them all. Any ideas?

I'm thinking this was dew on the mirror? If anyone has any ideas or input I'd appreciate it because I ended up having to cut my night short because of it I want to go back out tonight but it will be a pain to go to all the effort only to have that happen again and conditions will be similar.

Thanks for anyone's input.

Matt

[John]

The secondary mirror is more likely to dew up than the primary - perhaps that was the cause ?. You can make a dew shield that projects 25cm or so beyond the top of the tube. This also stops stray light from getting in the top of the tube and onto the secondary mirror.

You can get dew shields for the Telrad finders or even make one I think.