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Using a chest freezer/refrigerator for cooling telescopes?

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22 replies to this topic

#1 chadrian84

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Posted 14 July 2019 - 06:07 AM

I'm thinking about buying a chest freezer/s for my SCTs.  I'd use a thermostat like this to set the freezers to whatever the outside temp will be after sunset.  Has anyone tried this or have thoughts on it?  One consideration is making sure the air in the freezer is dry - I'm not sure if that will be difficult or not.  Perhaps having a dehumidifier in the room with the freezer/s would help.

 

OTA fans are not a good option where I live due to frequent high humidity.  They would pump moist air in and cause internal dewing.



#2 John O'Hara

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Posted 14 July 2019 - 06:33 AM

One drawback might be that your corrector plate would dew up instantly when you took the scope outdoors, unless you have your dew suppression strap in place and working for some period of time prior to removing the dust cap.  



#3 Jon Isaacs

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Posted 14 July 2019 - 06:35 AM

Have you been following the insulation threads in the CAT-CASSES forums?  The idea there is that by insulating the scope, it cools much more slowly and thus the tube currents that damage the image are suppressed.  It's an idea from the Netherlands and seems to be effective.

 

Jon


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#4 TOMDEY

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Posted 14 July 2019 - 07:20 AM

Yeah, it's not just temperature, but also humidity one needs to contend with. A scope can wind up either too hot or too cold. Too cold nearly guarantees dew... too hot guarantees no dew... just waiting for the scope to come to ambient. Ideal target is scope at ambient and (hopefully) low humidity.    Tom


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#5 Dynan

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Posted 14 July 2019 - 08:07 AM

Equilibration (temperature equalization) is the goal here. The only time refrigerating a scope would be a good idea is if you were about to take it out into cold (refrigerator) or freezing (freezer) temperatures. Chilling the scope is, as mentioned, only inviting condensation in all parts of the scope.



#6 HarryRik9

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Posted 14 July 2019 - 08:40 AM

I put my C8 telescope OTA in the trunk of my car about 30 minutes before nightfall.

#7 MikeTahtib

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Posted 14 July 2019 - 08:46 AM

I was thinking of doing the same thing for my dob.  Regarding being too cold, the key is, as you said, to match the temperature of the scope to the expected air temperature at dusk.  I was going to make a new cover for the top and bottom of the mirror box, and run something like dryer duct.  Input would be bottom, so the cold air would hit the back fo the mirror first, in case of the cold air being too cold and causing dew.  

However, I dont' think condensation will be a problem during the cooling cycle.  Any condensation will occur on the iceblock or refrigerator coils in the cooler, pumping saturated air, at worst, into the dryer duct.  But the air would pick up heat along the way, drying it out.  By the time it hit the hot mirror, it would not be able to deposit condensate on it.  Condensation occurs when dap air hits something colder than the air.  The coldest thing in the system will be the block of ice in the cooler, so no problem.  I was planning on drilling a couple small drain holes in the cooler to let the water out, and hopefully prevent the air from becoming even more saturated.  

The risk for condensation happens when you disconnect the cooler and expose the cooled parts to ambient conditions.  But if your thermostat has worked properly, your mirror wil be at or slightly above ambient, which is the goal of fans to begin with.

It seems to me the biggest thermal problem with mirrors is the big chunk of glass in the middle.  Fans can only cool the surface to ambient.  If the ambient keeps falling all night, you have a hard time catching up, and there will always be a big gradient between the mirror core and surface.  If you can start with a mirror uniformly cooled throughout to the temperature you will start observing at, it should make thermal equilibrium easier throughout the night as temps fall.  

I did have a backup plan, though, incase the primary does dew over.  I always bring some chemical hand warmer packs with me, because ti gets pretty darn cold here on a clear night, even in August.  I could put these in the cooler in place fo the ice to warm up the mirror a bit to get the dew off.  I am also working on boundary layer fans, which will include a few watts of resistors for the same situation.  From what I've been reading, I shouldn't need them, but resistors are cheap, and trips to dark sky sites are rare and valuable to me.



#8 CharlesW

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Posted 14 July 2019 - 10:34 AM

I think putting your scope in a freezer is a solution looking for a problem. Just get the tube into the backyard an hour before you are going to use it. 


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#9 chadrian84

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Posted 14 July 2019 - 10:49 AM

The problem is my outdoor temps are much lower than indoor for much of the year. I’ve tried reflectix without success. The status quo has been to leave scopes out for 2+ hours before sunset but I still get tube currents.

I’m surprised no one else here has tried what I’m proposing. I’ll give it a shot this fall. I think I can make it work.
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#10 Usquebae

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Posted 14 July 2019 - 11:13 AM

I’m surprised no one else here has tried what I’m proposing.

Me too, and the skeptical / negative replies in this thread are also a bit puzzling.  My winter practice is to put my scope in the garage when I leave for work.  There is still sometimes a 20-30 degree difference from garage to outside, but that's better than 50-70.

 

I've got three wraps of reflectix on my C9.25, with a very thin layer of packing sheet foam between the OTA body and the reflectix.  My seeing is typically so atrocious I can't guess how well this is working.  The lack of consensus among knowledgeable CNers on how to use insulation, and whether it works at all, doesn't help.  I've also got an unwrapped C6.  I should test them against each other and see if there's a discernible benefit.

 

In other news, I just learned how to spell discernible.  Thought there'd be an 'a' in that word.


Edited by Usquebae, 14 July 2019 - 11:14 AM.

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#11 chadrian84

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Posted 14 July 2019 - 11:30 AM

I have two identical edge 11 rigs. I’ll put one in a deep freezer and directly compare the result of freezer vs leaving outside. If it works I’ll buy another freezer for the 2nd OTA. If it doesn’t work I’ll keep the deep freezer for food :).

#12 csrlice12

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Posted 14 July 2019 - 11:39 AM

Get a big enough freezer, convert the lid to pop/roll off.....instant observatory!


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#13 Jeff Morgan

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Posted 14 July 2019 - 03:17 PM

Yeah, it's not just temperature, but also humidity one needs to contend with. A scope can wind up either too hot or too cold. Too cold nearly guarantees dew... too hot guarantees no dew... just waiting for the scope to come to ambient. Ideal target is scope at ambient and (hopefully) low humidity.    Tom

 

I have put small freezer packs in the sealed Dob mirror box in the past and had some success. The idea was not to freeze the mirror, but rather give it a head-start on the night. We're talking 55 degrees, not 35 degrees!

 

The packs were attached to the top cover above the mirror with about 8" airspace, and a folded cotton towel placed over the mirror in case of condensation on the packs dripping onto the mirror.

 

As Tom says, one has to be watching the local dew point. Cool the glass below your dew point, and it will dew. Living in a dry climate, I was absolutely comfortable with it.

 

Your local Goodwill store probably has old refrigerators sufficient to handle an entire SCT, could be a good project modifying the thermostat.


Edited by Jeff Morgan, 14 July 2019 - 03:18 PM.

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#14 starman876

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Posted 14 July 2019 - 03:41 PM

You will need sensors to know when the OTA is the same temp as the outside.  However, this time of year is not the time to be cooling things before you bring them outside unless you live at the north pole.



#15 John Fitzgerald

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Posted 14 July 2019 - 06:14 PM

All around bad idea, IMO.  Freezers and refrigerators tend to be moist inside, if they are ever opened.  That's why the ice buildup.



#16 chadrian84

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Posted 14 July 2019 - 08:25 PM

I appreciate the skepticism but if humidity inside the freezer can be controlled I'm not sure it's a bad idea.  It would become a choice of whether it's worth the cost and effort.  Some initial thoughts on controlling humidity:

 

- I would place the freezer/s in an unheated four season porch and run a desiccant dehumidifier in the room.

 

- Place something like Eva-dry inside the freezer chest - probably multiple units

 

- Make sure there's a good seal on the freezer

 

- Open the freezer lid only when needed

 

 Even if I'm able to control humidity down to ~30 degrees Fahrenheit it would be helpful.  It will become progressively more difficult for lower temperatures.



#17 MikeTahtib

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Posted 15 July 2019 - 06:37 AM

Humididty may  not require such heroic efforts.  How does your food look in your regular kitchen refrigerator?  Is it all covered in dew when you take a bottle or jar out?  If not, then you should be fine.  If it is (and it may be, depending on your local humidity & dew point), remember that you won't be cooling as much in your telescope fridge.  As long as you're not cooling below the dew point, you should be fine.  If you are cooling to predictd ambient at darkness, and that's below the dewpoint, it probably won't be a good night for observing anyhow. 

Really, it won't be much different than leaving it in an unheated shed or observatory.  The difference is that you will be at thermal equilibrium right away, whereas someone leaving a scope outside won't each that until 3:00 in the morning.  Of course, you will need dew control if the ambient approaches the dew point temperature, just as anyone else would.  I would say if the investment in refrigerator and thermostat isn't too painful if it ends up being a total flop, give it a shot, and report back with your experience.  Either way, you'll be providing the rest of us with important information.  If it works, we'll call it a Chadrian cooler!!



#18 Sketcher

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Posted 15 July 2019 - 11:38 AM

I shared the topic with my wife.  Her take on it:

 

"A freezer doesn't get cold enough." (She knows what kind of temperatures I've observed at!).  "It'll waste energy."  "Why not just find a secure place to keep the telescope outside?"  Plus (She was making a joke!) "Every time you open the freezer the light will come on." lol.gif

 

My thoughts:

 

Well, I've been observing in the cold outdoors for a very long time -- often at temperatures well below 0-degrees F.  My record observation (with a sketch!) was made when the air temperature was minus 50 degrees F.  I've never considered using a telescope freezer.  After learning a few lessons early on, telescope cool-down ceased being a problem for me.  Better solutions are available.



#19 John Fitzgerald

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Posted 15 July 2019 - 01:56 PM

I'm fairly certain that my recent experience is typical of the SE U.S.   The dew point has recently been reached at sunset, or even slightly before, for objects near the ground.  This happened on at least two clear evenings last week, and made observing nearly impossible.  Might as well have been in a fog.


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#20 Jeff Morgan

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Posted 15 July 2019 - 03:24 PM

I'm fairly certain that my recent experience is typical of the SE U.S.   The dew point has recently been reached at sunset, or even slightly before, for objects near the ground.  This happened on at least two clear evenings last week, and made observing nearly impossible.  Might as well have been in a fog.

 

Well, it all depends upon where you live.

 

Checking NWS observations (dew point) at 21:08 Z on July 15th:

 

Fayetsville, AR: 70 degrees

Prescott, AZ: 45 degrees (it's the start of our monsoon season)

 

Nothing John in NW Arkansas can really do here with pre-cooling. The good news for John is that with his humidity the diurnal temperature change will be relatively small. Cooling fan(s) should keep the mirror within a couple of degrees of ambient given a reasonable starting point.

 

For me in northern Arizona, pre-cooling will be effective provided I do not go below the current dew point. And I need all the help I can get since my diurnal temperature range is 30-35 degrees every clear day of the year. Without water vapor to hold the heat in, at sunset the temperature here drops like the ice planet Hoth.

 

Since I am only looking to give my mirror a 10-15 degree head-start (I am not putting it the freezer!), I am golden.

 

Now, if I chill my mirror to 55 degrees and take it out into 75 degree ambient, the mirror will still have have a thermal problem, at least temporarily - but it will not dew because it is warmer than the dew point.

 

Understand that simple principle and the observing environment and you will be ok.


Edited by Jeff Morgan, 15 July 2019 - 03:25 PM.

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#21 John Fitzgerald

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Posted 15 July 2019 - 03:33 PM

Jeff, the worst evening last week, the dew point was 78 degrees at 9 PM, given by the weather service.  In our little valley, the dew point was 80 degrees.  The sky was clear, but observing was impossible with any kind of glass, unless one wanted to keep constant heat on it with a hair dryer, so I gave up.  We are southeast of Fayetteville.

 

Couple nights ago, I went out with the grab and go to look at a few things under a clear, but humid sky.   Clouds started moving in about 10 PM.  I moved the scope inside.  By 10:20 or so, it was raining.


Edited by John Fitzgerald, 15 July 2019 - 03:37 PM.


#22 Jeff Morgan

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Posted 16 July 2019 - 01:37 AM

Jeff, the worst evening last week, the dew point was 78 degrees at 9 PM, given by the weather service.  In our little valley, the dew point was 80 degrees.  The sky was clear, but observing was impossible with any kind of glass, unless one wanted to keep constant heat on it with a hair dryer, so I gave up.  We are southeast of Fayetteville.

 

Couple nights ago, I went out with the grab and go to look at a few things under a clear, but humid sky.   Clouds started moving in about 10 PM.  I moved the scope inside.  By 10:20 or so, it was raining.

 

Yikes! With that kind of dew point, even keeping the mirror in an air conditioned house could be too much!



#23 MikeTahtib

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Posted 16 July 2019 - 07:21 AM

I think the real enemy with this approach will be radiational cooling.  When exposed to the clear sky, things cool below ambient and dew up. This will happen anyway, but since you are starting colder, it will happen sooner.  Fans might help this.  I'm a dob person, not an SCT person.  I've read about dob owners using fans to blow air across the primary to keep it from dewing (I'm working on this for my telescope).  Would the same kind of arrangement would work for an SCT, where you would blow air across the corrector plate to keep it from dewing?  Is this something that SCT owners do?  Just an idea I'm tossing out there...




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