Dew prevention, where do I start?

[Sepharus]

I finally have a clear night and got a good few hours in notching up 3 new M objects but had to come in before I really wanted to as my primary mirror and just about everything else was dripping wet. Anyone got any advice for a noob to help with this? I am planning some extended sessions soon and need it to work all night! I have seen lots of dew prevention devices on various websites but I have no idea whats right for me.

Thanks in advance

[brantuk]

I'm surprised your primary had dew on it because it's deep inside the scope and reasonably well protected. Primarys only tend to dew up when the dew is particularly heavy - so much so you can't see the sky anyway. The secondary is usually the first to go because it's the most exposed to moist air.

The dew can be held back with a dew shield. Commercially made dew shields tend to be £silly money for what they are - a lot of folks make one with a thin camping mat for under a fiver - very effective for prolonging your sessions. But you can get dew heaters for the secondary which usually stick on the back of it (out of the light path) with a power lead taped along one of the vanes. Of course you'd need a dew controller and power source to go with.

To clear warm air currents from inside the tube a fan can be attached to the back of the primary - this also helps prevent dew - not quite sure of the temperature physics for that but a lot of newtonian owners do this to keep the mirror temperature at a steady level. Hth

[NGC 1502]

Good advice in post #2. If that is followed, there is not a lot else you can do. (There are other solutions for refractors SCTs & Maks - a dew heater band can be fitted around the objective or corrector plate cell)

Regards, Ed.

[Velvet]

Also surprised there was dew on primary. I have the same set up and have never had that problem. I use dew sheild on OTA, then dew heaters on EP, finder and telrad. Not had any problems with this set up.

Use kendrick dew heater controller and all seems good.

Velvet

[ronin]

I just wish it wasn't called dew prevention, you may minimise it but you will not always prevent it. In other words no matter what you do you may still get some.

[bish]

Likewise only had the secondary dew up. Dew shield and hair dryer always worked for me , although I only use the hair dryer at star parties because of the noise.