Dew and Refractor Care

[Profiler (Profiler)]

I was asked to-day about caring for refractor optics with respect to dew.

Interestingly, the question wasn't the usual concern about dew control whilst in the field but rather care of your refractor once you have finished and, for example, the classic back-yard observer of what to do when the scope is brought inside a house and "then" the lens fogs up.

My advice was to not straight-away pack the scope up but instead leave it in a cool room with the lens cap off for the dew to evaporate naturally and thereafter store the scope with some silica satchels. However, I was just wondering whether anyone else had a few extra pearls of wisdom to share for long-term refractor care.

[Larryp (Laurie)]

I use a 12volt hair dryer on mine when I bring it indoors-clears it up beautifully! I don't know about a 240 volt one-maybe too hot.

[Logieberra (Logan)]

I worry about 'shocking' the glass after a sub-zero night of imaging. The telescope tube is basically frozen, and the glass isn't far behind. Get a hair dryer too close to cold glass and I think a crack is possible...

[Profiler (Profiler)]

My initial thought was a dew-heater like a Kendrike or Astrozap but I think what was being sought was basic advice for simple observing on a cold night and then coming into a warm house when the dew (aka condensation really hits) and what to do with subsequent care.

I thought about the dew heater option which stops dew forming in the first place but what if you are going the low tech portable options of scope and tripod and nothing else.

[mercedes_sl1970]

Hi Profiler- your approach which you mention in your first post is very much what I have always done. Famous last words, but I have never had a problem. I use a silica pack in the focuser as well.

Andrew

[jamespierce (James)]

Bring your gear inside, leave it open, caps off and let it dry off before sealing everything up in cases etc.

[multiweb (Marc)]

If you keep the glass dew free with proper heating it's best. The whole glass will dew up if you put the cap on and take it in inside. As previously said it's best to cover it back only when it's back to ambient.

[jamespierce (James)]

It goes without saying that you should use a dew heater to stop the glass getting wet in the first place if possible.

[LewisM]

I bring it in, put it back on the mount, and turn the Dec head so that the objective is pointing back along the polar axis. I leave the caps OFF, but do drape a plastic shopping bag loosely over both ends to stop dust settling on the glass (dust + moisture = MUD!). Next morning, I fresh zap a silica dessicant bag in the miscrowave, and put it in the objective cap and between focal reducer and draw tube. Also one in the zip lock bag with the CCD AND the guider.

I am NOT going to risk damaging my precious FL102s - SHE'S MY BABY!

[dannat (Daniel)]

Richard I do as you suggest, esp with my large binos which can get quite wet, they dry sitting atop their case

[Larryp (Laurie)]

Originally Posted by Logieberra

I worry about 'shocking' the glass after a sub-zero night of imaging. The telescope tube is basically frozen, and the glass isn't far behind. Get a hair dryer too close to cold glass and I think a crack is possible...

The 12volt ones are only just warm-no chance of cracking glass. We are only talking about condensation, which is nowhere freezing anyway.

[Sarge (Rod)]

Like most I leave it uncovered till dry, then pack it away with one of these in the focuser with silica gel:

http://www.bintel.com.au/createimage...ht=85&width=85

Clear skies

Rod

[Logieberra (Logan)]

Oops, missed the 12v Laurie. I'm just a bit paranoid about refractor glass

[Larryp (Laurie)]

Originally Posted by Logieberra

Oops, missed the 12v Laurie. I'm just a bit paranoid about refractor glass

As I said before, Logan, I would be dubious about a 240volt dryer, but many people use 12 volt ones in the field to control dew, and I find its marvellous for getting rid of that condensation when you bring the scope indoors.

[casstony]

I keep optics capped when bringing them into a warm room then remove the caps after the scope has reached ambient temperature - otherwise a dry, cold lens gets covered in condensation within minutes of being inside.

[hotspur (Chris)]

I just have two 4 inch dew zappers at objective end of Vicky,at the end of the night,I just roll the roof back on the observatory and point the objective towards the carpeted floor and throw a sheet over her.I put dessicant bags on the inside of the cap,also take out the flip mirror a shove a bag up in tube.

Interesting point re hair driers and very cold scopes,I am unsure if anyone has broken a objective on this forum by using a hair drier-but it could be possible-I know the word in the bush re cast iron camp oven is -never put cold water in a hot oven,as it could crack-someone we know put a cast iron pot that had been in oven, on a cold stainless steel bench and it broke (guess they had no tea),but it might be the same for the above cold objective and hot drier.I know astronomers talk about it.

[OzEclipse (Joe Cali)]

Originally Posted by Logieberra

I worry about 'shocking' the glass after a sub-zero night of imaging. The telescope tube is basically frozen, and the glass isn't far behind. Get a hair dryer too close to cold glass and I think a crack is possible...

Sorry, I have to disagree there. Good quality optical glass is carefully annealed to avoid stresses. In my lab, I've put annealed pyrex in liquid nitrogen and then put a hairdrier or heater on it raising it to 100-150C thousands of times without a crack. Cracks only occur when there is a fault in the glass or it's badly or not annealed.

By comparison with the thermal stress of going from -196C to + 100, going from sub-zero night time temperatures to hairdrier temps is quite negligible. Use a hair drier without hesitation.



Joe Cali

[Profiler (Profiler)]

Apologies for what might seem like a fairly straightforward question but can anyone provide a few suggestions of where (in terms of regular stores that can typically be found in most Oz cities) satchels of silica can be purchased.

[Larryp (Laurie)]

Originally Posted by Profiler

Apologies for what might seem like a fairly straightforward question but can anyone provide a few suggestions of where (in terms of regular stores that can typically be found in most Oz cities) satchels of silica can be purchased.

They are available on Ebay in many different sizes

[icytailmark (Mark)]

have a fan over the top of your telescope it helps to blow away the dew.