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Hinged Shelter for Dobsonian Telescopes

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#51 aeajr

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Posted 14 May 2022 - 10:56 AM

I don't know about humidity I your area but my garage is unheated and get below zero often. Never had a problem. My club has an SCT in an unheated dome and there have been no problems.

I don't think you need to heat it in the winter.

#52 Stevegeo

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Posted 14 May 2022 - 12:17 PM

I dont heat mine either , but then my scopes mirror is 4 ft off the floor and floor another 3 from ground . 

Humidity has never been an issue . 

 

I learned about borax from an old timer and found it keeps ants and most ground bugs away . Its cheap, in most laundry soaps ( powdered) . 



#53 Pierre Lemay

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Posted 14 May 2022 - 01:40 PM

I don't know about humidity I your area but my garage is unheated and get below zero often. Never had a problem. My club has an SCT in an unheated dome and there have been no problems.

I don't think you need to heat it in the winter.

First a caveat: When having this sort of discussion on dewing and cooling of a telescope, and so forth, it's important to take into account one's geographical location. What works for someone in Montreal may not at all be relevant for someone else living in Arizona, the Sahara desert or humid locations like Malaysia or Singapore. It's also important to consider the size of a telescope and the mass of the glass involved: a thick 20 inch mirror in an opened mirror cell will not react the same way as a 12 inch closed tube Schmidt Cassegrain.

 

For me, here in Montreal, humidity and outside temperatures vary greatly throughout the year: -35 C to +35 C, and 50% to 100% RH variations are common over the year. 

 

I've rarely seen a primary mirror dew up during a night of observing because the mirror is:

  • at the bottom of the tube
  • mirror is normally chasing the falling air temperature (primary stays warm enough to keep dew away)
  • the reflective mirror surface is facing the sky, reflecting the cold firmament temperature back into space.

The diagonal is another story, of course. It must often be heated to prevent fogging.

 

On the other hand the problem of condensation on things like the primary mirror (and other large masses in the telescope) are likely in my part of the world when, at the end of the night, air temperatures begin to rise as a new day begins and the sun is about to or begins to rise. For a few hours, in the early hours of the morning, the air temperature is rising faster than the glass temperature is (the reverse of what happens when the evening progresses).

 

Depending on ambiant RH, the mirror may fog up for a few hours until the sun has warmed the morning air enough to change the local air conditions around the telescope and move the dew point inside the shelter, where the telescope is stored, to "safer" dew territory.

 

After storing his telescope inside its storage location, an observer would not likely notice this morning dew because, after a long night of observing, he is probably sleeping. This morning dew problem, of course, does not affect observing since the night is over when it happens but, it will have an effect on the life of the coating if it is allowed to happen several dozen nights per year when the atmospheric conditions are susceptible to generate the event. Again: depending on where you live: "your mileage will vary".

 

By the way I've opened my observatory a few times, after a few days of heavy rain, to find many parts of the telescope dripping wet inside (truss tubes, mirror shield, mirror). It wasn't wet because the shelter was leaking (it doesn't leak: the floor was perfectly dry). Parts of the telescope were wet because the high RH water vapor in the ambiant air was condensing on colder parts of the telescope. 

 

So I know my primary mirror can fog up in storage. I've seen it happen. I'm trying to prevent that from happening as much as possible to keep the aluminum coating in good shape (recoating a 20 inch is expensive). Keeping the mirror exposed to a little bit of heat during storage sounds like a low cost insurance policy.


Edited by Pierre Lemay, 14 May 2022 - 03:56 PM.

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#54 archer1960

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Posted 14 May 2022 - 06:39 PM

A little heat to keep it a few degrees above outside ambient makes sense to avoid condensation. Trying to keep it above freezing in the depths of winter does not.




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