quick one..

[all-on-black]

Im in the garden imaging the leo triplet. Everything went perfect and the dew hasnt troubled me yet.

My images are nice and bright (lp is a killer) but they are gradually getting darker so initially i thought dew was getting on the primary mirror. But no.. its clear.

Any ideas why theyve got darker? They seem to have reached a level of darkness now and not getting worse.

'Elp!

Thanks!

[all-on-black]

Ok, you can ignore this now. I decided I\was wasting my time and that my hands were just about as numb as they were gunna get.

I brought my gear inside and then noticed my primary mirror was completely fogged up. Woops! That explains it then!

I've ended up with 18 lights that are usable.

Hey ho, lesson learnt! Must get a dew controller!

[cuivenion]

I was under the impression the tube on reflectors acted as a natural dew shield, or does that only work up to a point?

[Russe]

It does, up to a point I guess. If the humidity is really high, then even though your mirror is far down the tube in your Newtonian, you will get some fogging. Have not had much problems, imaging throughout the night.

Different with catadioptric scopes: my Mak's front glas fogged up all the time.

[all-on-black]

Well its only a 750mm tube and the leo triplet was pretty high in the sky so the dew goes straight down it.

You can get away with it if you have the tube slightly more horizontal or if theres enough of a breeze to keep the dew from settling as much.

[RikM]

I find the secondary dews up quite readily on the 150P. I do use a dew shield, which helps but on a heavy dew night it will still fog up. I normally use a little hair drier blowing cool if it is too bad but some sort of heater would be a better option. Not quite sure how to fit them on a Newt though.

[Bizibilder]

If your images were "getting darker" it may simply be that , as the evening progressed, the amount of light pollution was getting less as lights were turned off!  Additionally it was not "astro dark" until about 19.30 - so If you started imaging before this you would have found yet more light around to affect your subs.  If the subs were dark but sharp then these are the most likely cause.  Fogging usually shows up as a loss of sharpness / complete blurring of the image.

Astrodark:  http://www.syn-ack.com/astrosigs/astrolocations.html#kelling_heath   (Just find your nearest town from the list).

[sidelight]

I put a dew strap around the base of the dew shield on my newts (where the dew shield connects to the scope), that seems enough to stop dew on both mirrors and the MPCC , although here at least it seems unusual for the primary to fog up. 

Cheers

Ross

[all-on-black]

I had a look at each sub, and they lost all contrast gradually. I originally thought that as the target moved across the sky it was just moving away from the light pollution.

But as I said, it was just the primary mirror fogging up.

I didn't go out until around 10.30pm and came indoors around 1am. There were tiny ice crystals beginning to form anyway so I knew I'd had enough!