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What ambient cooling rates can a mirror handle?

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#1 sprinter

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Posted 21 August 2014 - 12:54 AM

I know that mirror and tube cool down is important.  It seems like most of the questions revolve around bringing a scope from a warm house into the cool outdoors requiring a certain cool down period.  But what if the scope is kept in an unheated environment like a garage?  Around here in the summer time, the air temp can cool from, say 85 F down to 55 by viewing time, maybe a 5 or 6 hour period.  That is something like a 6 degree per hour rate.  Winter evening cooling can be even steeper when the sky is clear.   I'm especially interested in 8 or 10" scopes,  but would welcome a general discussion of ambient cooling rates because I intend to keep my scope in an unheated environment year-round.   

 

I guess my question is, is there a certain cooling rate in terms of degrees/hr for a given mirror size or telescope type that is acceptable to avoid problems?



#2 Asbytec

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Posted 21 August 2014 - 05:27 AM

I guess my question is, is there a certain cooling rate in terms of degrees/hr for a given mirror size or telescope type that is acceptable to avoid problems?

 

 

I would think a good cooling rate to avoid problems would be beginning at the same temp and maintaining the same ambient cooling rate.

 

If you asking what the cooling rates are for 8" to 10" mirrors, gosh, dunno. Have to measure it, maybe search the internet for data. They retain some heat because they are somewhat thermally stable to maintain their figure as I understand it. But, that creates boundary layers and thermal effects.

 

So you will likely need some active method of increasing the rate the mirror cools so it can at least reach ambient, initially, and cool as ambient temps fall. Under clear nights, even summer nights, quicker ambient temperature drops might pose a problem. And will pose a problem if the mirror is warm to begin with.

 

It would be nice to have a glass that cools to ambient instantly, yea? Maybe?

 

I suppose the can "handle" pretty rapid cooling without much fuss, cooling a little faster than ambient. I "ice" mine down prior to observing with a reusable medical ice pack. It kills off residual heat fairly rapidly (say over half an hour) and down to more moderate ambient temperatures. I doubt you can get it to freezing easily, but cool summer nights prove to be no problem.


Edited by Asbytec, 21 August 2014 - 05:35 AM.


#3 dotnet

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Posted 21 August 2014 - 08:01 AM

I think the cool down rate depends on several factors, like the scope construction (ventilated?), the environment (windy?), whether the scope is exposed to dark sky or covered, where it's pointing, etc. I don't think anyone could give a useful degrees per minute per inch of aperture figure.

 

However, it is easy to spot when the time of thermal equilibrium has come, especially if you know that your scope is collimated. Observe a defocused bright star image at 150x or higher magnification and you can tell whether the mirror is still cooling. Extra-focally you'll see shimmering diffusion on the face of the diffraction figure, and it will be out of shape (looking like out of collimation). Intra-focally you'll see a tornado-like apparition from the centre to the edge that slowly swings about on one side of the diffraction pattern. (EDIT It's actually the other way around: extra-focal tornado, intra-focal shimmer.)

 

Cheers

Steffen.


Edited by dotnet, 22 August 2014 - 06:24 AM.


#4 Eddgie

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Posted 21 August 2014 - 01:16 PM

Google "Thermal Management in Newtonian Telescopes". There was an article on this in Sky and Telescope a long time ago. Bottom line is that if temps are falling faster than about 2.5 degrees o'er hour, a bigger mirror my never reach ambient. Article shows how boundry layer fans make it unnecessary for the mirror to reach ambient. Image is stable in about 30 seconds. Don't know if the title above is accurate, but I think close enough.
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#5 sprinter

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Posted 21 August 2014 - 08:23 PM

Thanks guys.  Yes, I was mainly interested in how a scope might behave in terms of thermal issues when left in a naturally cooling ambient environment.  I read the S&T article which was very informative.  I'm not sure I'm quite up to cutting holes in a new scope, but knowing the issues helps.

 

Asbytec:  How or where do you apply the icepack on the mirror?  Does in cause any water problems due to condensation?  This is the first time I've heard of this approach.  Sounds like it could work if it didn't cause any other problems.



#6 Asbytec

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Posted 22 August 2014 - 05:47 AM

Asbytec:  How or where do you apply the icepack on the mirror?  Does in cause any water problems due to condensation?  This is the first time I've heard of this approach.  Sounds like it could work if it didn't cause any other problems.

 

 

I simply lay it along the top of the OTA with a hand towel underneath for any condensation. I make sure part of the ice pack lays over the primary mirror end (if your talking about a CAT) where the majority of the heat retaining mass is.

 

You can use one or two depending on your needs. It cools nicely down to modest temps and so far condensation has not been a problem. 

 

If you're talking about a newt primary, I dunno how to best place it without looking at the primary mirror and cell construction. But surely we can be creative.


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