Moisture in OTA

[D4N]

I have noticed that when I take my scope out to cool down (12" dob) I get frost on the primary mirror.

 

I assume that this is because the air in my house contains a lot more moisture than outside and reaches dewpoint as it cools down.

 

How do others deal with this?

I don't really want to heat the primary as this defeats the point of cooling it down.

I also don't really want to leave the tube open as that would be an open invitation for creatures to go inside.

 

I've considered giving it a quick blast with a hairdryer to change out the air once it is outside or fitting a small fan to draw air through the OTA, this would also reduce cooldown time.

[tooth_dr]

I have a small pc fan fitted to the base of mine. Would this help with your problem?

 

 

[D4N]

I'm not sure if it would, I think the problem is the air inside the tube so the fan would need to change this out and not just blow air onto the back of the primary.

[Alien 13]

Two ideas come to mind, cap the scope and purge with argon or nitrogen or try a sacrificial cold plate run off a peltier device to make the mirror appear warmer.

Alan

[Peter Drew]

Might be a case of "prevention is better than cure", is there not somewhere unheated that you could store the telescope?  I have heard of people locking their scope in the car prior to starting observing.  

[AdeKing]

I keep my 8" Dob in either the summerhouse or an unheated garage.  This means that it is never that much above ambient temperature.

I normally put it outside with the end cap off for about 20 minutes before I want to start observing and I've never suffered from this problem.  

However, looking at your signature i can see you're located a lot further north than me, which might change things a bit.

Ade

[tooth_dr]

2 hours ago, D4N said:

I'm not sure if it would, I think the problem is the air inside the tube so the fan would need to change this out and not just blow air onto the back of the primary.

It can be reversed and draw air out of the OTA, the mirror isn't sealed, air flows around it in and out of the tube.  I have shield - not pictured -which forces air to flow through the fan and in/out of the OTA, it seals round the fan and perimeter of the base of the tube. 

[tooth_dr]

Found a picture:

 

[D4N]

Ah yes that looks like it will work, I'm sure I can knock something up that will draw the air through the tube.

Since I sit it on the primary end when it is cooling down I think I'll make something that either goes into the focuser or into the tube cap.

 

I learnt with my imaging obsy that to leave things setup outside I need the provide constant heat to stop it being destroyed, everything freezes up otherwise.

It isn't a problem heating a little frac' but it's impractical to heat a big dob.

 

The advantage of living so far north is that the skies can get incredibly clear when it gets so cold, which is great provided you eyes don't freeze

[tooth_dr]

This sits flat btw, I store it on the primary.