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Heat Plumes

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#1 speedster

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Posted 23 February 2019 - 01:51 AM

I need my obs floor about 8' off the ground to clear obstructions and have a really good view of the sky.  The best place, in terms of geography, is in my attic with the dome sticking up above the roof.  Attic has 20" of blown fiberglass but there is still heat loss in the winter, even though much less than typical.

 

The only potential issue is heat plume coming off the roof of the house in winter.  Does anyone have a dome on their roof and have you had heat plume issues?

 

Thanks!



#2 CharlesW

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Posted 23 February 2019 - 11:04 AM

Frankly, it doesn’t matter if other folks are seeing heat plumes, or not. You should assume that you will, and is that a deal killer? Can you put your obs on the ground on another part of your property? If you are automated, does it matter that you are imaging three objects a night from 60 degrees to 60, or just one from 30 to 30?



#3 StarWolf57

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Posted 23 February 2019 - 12:07 PM

I think you would have to expect heat plumes to one degree or another. If you plan using anything but a small refractor for visual use, you should probably really try to find another location.



#4 MCinAZ

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Posted 23 February 2019 - 01:03 PM

  Perhaps you could set a telescope on the ground in a few locations around the house and make some observations over the roof to determine how the escaping heat will affect your view. Also, if your dome is located to the south of the roof peak, you may not be as badly affected since most of the objects you are likely to observe will not be over the largest area where plumes might be a problem.

 

  Be sure to consider the consequences of this type of modification with respect to home insurance and resale value. In my experience, it's not a matter of whether a dome will leak or not, just a question of where and how much. Any potential buyers would almost certainly stipulate that the dome be removed and the roof restored to it's original state as part of a sales contract.



#5 speedster

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Posted 24 February 2019 - 12:47 AM

Thanks for the input.  I'm blessed with fairly clear west Texas skies and light pollution that could be much worse.  Also blessed with a wooded back yard.  The 8' height gets me over trees, wires, and neighboring lights.  All the north sky is over the house regardless of where I put the obs.  City light dome to the west.  Good idea about setting up and trying over he house to see what the heat is doing.   I've shot over the house but never compared it to a south shot on the same night so I've never known what might be plume and what might just be general seeing.  I'll shoot both north and south right after each other and see if I can see a difference.

 

The other thing to balance is, if it's too cold or windy to set up, I might use the obs instead of doing nothing, and suffering the a plume might be better than not doing anything.



#6 KLWalsh

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Posted 24 February 2019 - 02:14 PM

Consider the prevailing winds, too. If the wind blows from the West (likely in TX I’d think), build the obs at the western end, to let the prevailing winds blow the heat plume away from you instead of toward you.

#7 Elektronkind

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Posted 25 February 2019 - 01:03 AM

Replace the blown fiberglass crap with 3-4 inches of closed-cell expanding foam (such as Icynene) applied to directly to the underside of the roof decking and trusswork. Say goodbye to excessive heat loss and make the attic an insulated space to boot. It also makes the house much quieter and extends the life of asphalt shingles. If you're worried about heat loss at 3am after the sun's thermal energy has long-been sloughed off, this is what you do.

 

I did this to my 50yo house which had seen its original blown fiberglass become a settled and compacted, knotted mess over those years. I had it vacuumed out and replaced with foam after getting a 30 year roof put on. It's probably the best thing I've ever done for energy efficiency and comfort. It might be that your state has programs for home efficiency improvements that can help defray the costs. But would I do this just for a roof-top observatory? Probably not.




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