Frost

[Don't Panic]

Hi, so I am very new at observing and bought my first telescope in the spring, and thus have not had to worry about chilly conditions so far, however last week I thought it would be very nice to take a look at the Orion Nebula so I got the telescope all ready, placed it outside good and early and left it till it got dark enough to go take a look. It was about -12 C at this time (I'm from Canada). Thinking I had covered all my bases, getting it ready and letting it climatize outside I went to take a look only to find everything was covered in frost :doh:..... Well I realized, ok sure that makes sense being inside all day. My question is how does one avoid this? And how much of a problem is it?

Thanx.

[RickJ]

You just discovered radiative cooling. I keep my scopes at outside temperature all the time but enclosed so they don't have a chance for radiative cooling before I use them. That happens when they see the sky which is much colder than the air. This is how a cloud sensor works for instance. Clouds are much warmer than clear sky.

You don't say what frosted over. If it was just the outside of the scope that's normal (I'm in northern Minnesota so frosty here too). Scopes can take this. But you don't want it on the eyepiece, finder optics or objective.

A reflector's mirrors have little problem with frost as the mirrors are well shielded from radiative cooling, they see little sky and usually won't frost or dew over.

An exposed eyepiece will. I keep those in a container in a warm pocket until needed. They are little bothered by the temperature difference this causes though sometimes in extreme cold they will frost over as you bring your eye up to them. I tried a ski mask, but it didn't help. A full beard does. I don't know why but all club members with them have no problem. So a number of years ago I joined their ranks and haven't had a problem since. Wife needed some adjustment time however.

A refractor or SCT/Mak is where the big problem lies. They are helped some with a dew shield. That limits the amount of sky the objective sees and thus slows the cooling but they will frost eventually. I find that one alone isn't sufficient in this climate. Several companies make heaters that go around the objective and provide a very gentle heat, just enough to keep the frost away but not distort the image. Used with a dew shield I have no frost on the optics. Kendrick is the Canadian company making them. Some american companies may be cheaper, I don't know about custom duties and such. Do a web search for "Dew Heater" and you'll turn up a lot of info on the subject.

Mild frost can be kept away with a hair dryer.

Rick

[Don't Panic]

Yeah I noticed a problem when I brought my eye to the eyepiece....however I don't think I can try your beard growing tactic....not very lady like :P I assumed that if the eyepiece and finder scope were frosting then the objective mirror was too. But it is a reflector so maybe it will be ok.

Maybe I will start storing the scope in the garage, that would probly help.

Thanx

[RickJ]

Yeah I noticed a problem when I brought my eye to the eyepiece....however I don't think I can try your beard growing tactic....not very lady like :P I assumed that if the eyepiece and finder scope were frosting then the objective mirror was too. But it is a reflector so maybe it will be ok.

Maybe I will start storing the scope in the garage, that would probly help.

Thanx

The gals in our club keep their eyepieces in a pocket (in a case of course) until used. You have plenty of time to view before they get so cold they frost up. Just keep those not in use in the case in the pocket and it is not a problem.

Only once have I had a reflector mirror frost over -- in over 50 years so it is very rare. It takes very unusual circumstances. Finders don't care if you hit them hard with a hair dryer to keep them frost free. I do it all the time. If your tube is cut too short (most commercial scopes are unfortunately) then on really nasty nights the secondary can frost. If the tube extends at least the tube's diameter beyond the spider then it should remain frost free no matter the conditions, well maybe not that one time in 50 years but close to it. This will increase your contrast for faint fuzzies as well. A trick that seems to have been lost to this generation of star gazers.

Depending on conditions and how sheltered an area you set up in, a scope kept in an unheated garage can be used right off (well maybe not for planetary work but certainly for deep sky). You will have an hour or more in most cases before radiative cooling starts to frost the finder up. Keeping it covered with a loose fuzzy sock when not in use helps too. It reduces the radiative cooling as the finder can't see the sky when its on. A pain but better than viewing through frost when you are far from an outlet for the hair dryer.

And when storing the scope, store it mirror end up if at all possible, even capped dust gets in but it rarely falls up! Secondary mirrors are cheap if damaged in cleaning compared to objectives and sit at 45 degrees limiting buildup compared to the bowl of a mirror. I tried sideways but everything got dustier that way, I don't know why -- static charges?

Rick

[Don't Panic]

Thanx again for the great advice. I will keep my eyepieces warm next time for sure, and I have an unheated garage that will work perfectly for storing the scope. As for storing it upside down....I'm not sure how I would do that, but I can invent some kind of holder for it. Its a dobsonian so it likes to sit right side up. I have a retired hair dryer also which will now be my stargazing dryer :) Now all I need is an old sock....which are always in good supply :)

Thanx again, hopefully we can get some clear skies around here so I can finally get a look at Mars!

[tdvance]

I know a person who made his own posterboard dew shield (essentially extending the length of his cassegrain tube) to prevent dew. That might help with frost too.

Here is a picture of him in Baltimore: http://weblogs.marylandweather.com/Heyn.holmes2.bmp

Todd