condensation after bringing my telescope in

[keiwein1]

so i just brought my telescope on from the cold weather outside (was viewing the great orion nebula) adn there is some condensation on my telescope... it reflects from the corrector plate in like a green light and the condensation is pretty thick. do i just cover it up or do i leave the telescope without the cover so it evaporate?

[triton1]

I just bring mine in and let it dry out before packing it away.

[MarsG76]

I third that... Let it dry out.

[seo]

Unless you have a Celestron EdgeHD scope or any other type of scope which has a permanent installed corrector/reducer between the focuser and its main tube thus blocking the air to go immediately inside,  cover the front of your scope immediately then remove anything (eyepieces, filters, etc..) from your focuser and finally leave the warmer air to go in to your scope and dry it.  

Now, IMHO, let's see why this happened...

If you take you scope, lets say for example from a "warm" home which has at least a 6 degrees C higher temperature from the outside air/environment, you'll immediately have cold air touching the warmer external surfaces - optics, tube, etc.. - of your scope. Heat radiated from the warmer surfaces of your scope causes the colder air which comes in touch with them to be more acceptable to water vapor (the dew point of the surrounding air is lowered). From that point and on depending on how much is the total area of your scope which comes in "touch" with the colder air (how big and what type is your scope) one of the three following situations could happen :
1. If the RH (Relative Humidity) of the colder (outside) air is between 96-99 % the amount of water vapor, the air can hold,  is in maximum or critical levels. In this case you may have condensation on your scope, not immediately but very very soon after your scope gets cold enough to start the "opposite" reaction - unless you put your scope on "fire" with dew  heaters. In any case and in my opinion, an RH of 96-99% is a NO go for astronomy - at least for us...

2. If the RH (Relative Humidity) of the colder (outside) air is between 73-95% you have about 5 to 35 minutes to prepare your setup against humidity/condensation. Again, this "positive" delay is because your scope was warmer.

3. Finally if the RH (Relative Humidity) of the colder (outside) air is below 72% usually you won't have any humidity problems.

In the previous possibilities the warmer scope is giving a "delay" in a condensation possibility so getting your scope from warmer to colder environments is not the cause  of any humidity issues. That's the positive part of it... but keeping your scope in a warmer environment will "cost" you the time needed for the scope to come in equilibrium, but that's another issue...

As is understandable, in the opposite case, warmer air will come in contact with cold surfaces causing the completely opposite reaction and if the difference between the outside surface and "inside" air temperature is more than 5 degrees C (depending on how much is the difference) you'll have sure/strong possibility of condensation on/in your scope unless you heat it up very quickly with the proper heat amount - which can be very dangerous due to rapid material expansion.

So, again IMHO, my advice is the following :

In general, don't put your scope from a very cold to a warm (or the opposite) environment immediately - in case of portable equipment. Besides the humidity and condensation problem in which the NO-GO is from cold to warm,  rapid temperature changes put the materials from which each scope and it's equipment is made off, in stress, also you may have possible collimation issues. If you can, make a case/box for your scope with temperature insulating material inside, then have this case/box slightly open (1 cm is enough - not more) for at least 40 minutes near your setup (outside) and under something which will "shadow" it from the direct sky temperature radiation (this way you'll also avoid/reduce condensation on the upper surface of the case/box). Again, "shadow" it - don't cover it. Then, after your session ends and after you shut off the dew heaters (if you have dew heaters) immediately put the scope in the case/box and close it very well. Now you can take the case/box anywhere you like - not beside a fire place...! 

Do the exact opposite (for not stressing the materials..) from warm to cold until your scope is in equilibrium, in this case you don't have to be so "careful"...

If you can't do anything of the previous with a case/box just use a thick towel - but remember, it's got to be in the same gold temperature and dry. In this case, take your time and try to "change" the towel-scope temperature surrounding environment gradually to warmer places at least 1 time before you put it inside your house.            

Hope I helped...

[CodnorPaul]

I put my cover on to the front of the ota whilst the dew band is still on and then it goes straight into a foam filled case. Never had a dew problem that way as it goes away dry

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[Ouroboros]

I put my cover on to the front of the ota whilst the dew band is still on and then it goes straight into a foam filled case. Never had a dew problem that way as it goes away dry

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That sounds a good way to do it. At the end of a session I lock my equipment away temporarily in my warm room (shed) but with the scope and other kit completely open. Dew inevitably forms inside the scope despite the shed being only slightly warmer than the outside. The following morning I 'air' the kit outside in the sun if possible before putting it away in permanent storage. My Newtonian doesn't appear to have suffered at all from this treatment, despite the fact I've been doing it this way for five years or so.

[Stu]

I put my cover on to the front of the ota whilst the dew band is still on and then it goes straight into a foam filled case. Never had a dew problem that way as it goes away dry

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I think that's fine as long as the scope has remained dry during the session. If my scopes are dripping wet then I bring them into the coolest room in the house (normally the hall) and just let them dry off overnight. I usually prop them up in their cases with plenty of room for air to get to them, same with eyepieces. Never put kit away damp.

[CodnorPaul]

I think that's fine as long as the scope has remained dry during the session. If my scopes are dripping wet then I bring them into the coolest room in the house (normally the hall) and just let them dry off overnight. I usually prop them up in their cases with plenty of room for air to get to them, same with eyepieces. Never put kit away damp.

The outside is normally wet but importantly the corrector plate is dry, and the cover that goes onto it is kept dry during my session, so in fact it is dry where it matters.  I then also put some desiccant sachets in a cap in the diagonal just to be sure.  The outside of the OTA being wet is not an issue and that will dry off on its own

[Stu]

The outside is normally wet but importantly the corrector plate is dry, and the cover that goes onto it is kept dry during my session, so in fact it is dry where it matters. I then also put some desiccant sachets in a cap in the diagonal just to be sure. The outside of the OTA being wet is not an issue and that will dry off on its own

I'm sure we all have our own ways of doing things but personally I would not put a damp tube into a sealed case, I let it dry off first.

[CodnorPaul]

I suppose normally I observe away from home so I don't have any choice and by time I get back it is dry anyway

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[souls33k3r]

I had the exact same thing happen to me the other day. When I came inside I left the OTA breath some air to get rid of any condensation. Took about 15 min or so and I then packed everything away