Misting up

[dave34]

Can someone tell me whats just happened, and does this happen often?

i was just looking at the moon checking the finderscope alignment when all of a sudden my views became misty, there was a sudden drop in temperature and it seemed like my DOB had misted up, i looked at my 25mm ep, that seemed fine, i think the problem was in the scope itself, the scope had been outside for about 2hrs before this happened, can sudden temperature changes cause this, and is it something to worry about, advice needed please. Dave

[LukeSkywatcher]

This is normal, nothing to worry about. It happens to everyone who has no means of dew prevention on their reflector scope. You need to buy or construct a dew shield of some kind.

More answers will quickly follow and set you right.

[brantuk]

No need to worry Dave - it's a common thing. You need a dew shield to protect the secondary mirror from dew - it's very exposed at the top end of the tube. Make a cheap one from matt black camping mat for a couple of quids - very effective.

[dave34]

Thanks guys, a dew shield it is then, cheers Dave

[JamesM]

Hi Dave, the length of the dew shield should be between 1.5 to 2 times the aperture of the scope. If you want to know what it could look like have a look here and scroll down to post no.7.

james

[LukeSkywatcher]

No need to worry Dave - it's a common thing. You need a dew shield to protect the secondary mirror from dew - it's very exposed at the top end of the tube. Make a cheap one from matt black camping mat for a couple of quids - very effective.

Camping mat is great and cheap. The colour doesnt really matter though. Black ones are hard to find.

[LukeSkywatcher]

Hi Dave, the length of the dew shield should be between 1.5 to 2 times the aperture of the scope. If you want to know what it could look like have a look here and scroll down to post no.7.

james

Sound advice. So for a 200mm scope, the shield should extend about 12-16" infront of the front of the scope. It wont lag and cover the front of the scope and it wont block incoming light. It WILL keep the optics of the scope nice and snug and dew/mist free.

[dave34]

Outstanding advice, cant believe how big its going to be, the wife is in charge of finding a foam mat that is suitable, thanks again folks.

clear skies, Dave

[dave34]

I went to my local factoryshop today, they are selling camping mats slashed down in price to £3, they are lightish blue 6mm foam, on one side they have some sort of thermal foil, would this be ok, foam on the inside, foil side on the outside to help regulate the temperature, would light blue foam be ok. clear skies Dave

[LukeSkywatcher]

The colour doesnt matter. I'm not sure if the thermal inlay will be a good thing or a bad thing. You really want the optics of the scope to be the same temp as the outside temp.............that what cooling the scope down before use does.

Maybe the thermal layer will create an even warmer temp infront of the optics and keep dew at bay better, or it may create a bit of heat thermals infront of the optics?

I really am not sure.

You can try it both ways. Heat sheet inwards and heat sheet outwards (if you know what i mean). See which works best.

£3 is a good price.

[king-jaffethehutt]

No probs at all Dave, I use a blue one it's dark so no one will see it ;-O

[LukeSkywatcher]

Mine is khaki green.