Left Field - Ice packs for cooling mirror?

[John K]

Well as I am getting a better understanding of the way my full thickness 12.5" mirror behaves and work out how I can get rid of the boundry layer, use fans, construct peltiers etc etc, just wondering if anyone has used gel ice packs to help cool their mirror down and what their experiences they have had?

The benefit of my set us is that the mirror is in a steel tub and open truss tube, so in theory I can place ice packs around and slightly behind the mirror.

(Image of mirror and tub)
http://www.flickr.com/photos/JohnKazanas/176154967/

My thinking is to then do what I am currently doing which is blast the surface of the mirror with a large household fan until close to ambient and then turn on my webcam. The ice packs would simply make the process faster.

I know that dewing and a reverse boundry layer from a mirror that is too cool or the steel tub being below ambient could be an issue but I will be monitoring the temperatures closely.

Any thoughts?

p.s. also still looking at any radical ideas such as water aided cooling (hoses, tubes etc) or cold rods (have seen these somewhere on the Net used in industrial processing - expensive!) so I dont have to rebuild the entire rear assembly including re-casting counterweights etc.

[davidpretorius]

for the last 2 weeks, i have been thinking that "why not seal the mirror base completely during cooling???"

Ie, instead of trying to fit peltiers and a cold plate to the bottom, simply give it a lid with peltiers and fans recirculating cold air. When it is near ambient / slightly below ambient, then take off the lid and use the scope. Lets make it as much like a fridge as possible. you do not leave the fridge door open and expect it still work as well!!!

You would just need a bigger version of what i am getting made up. ie a perpsex lid (not base) with the aluminium cold plate underneath, blowing air down, (not up like mine)

I have tried gel ice packs held in place by rubber bands, but it was not really satisfactory. Without the moisture drying action of the peltiers and movement action of the fans, the mirror dewed up after a while.

I believe that cold air circulating the mirror is the best / most even method for the mirror.

[bird (Anthony Wesley)]

Dave , a couple of thoughts...

One possible problem with cooling is that it can cause extra dew to form, normally this is on the cold plate since it's the coldest part of the system. If the cold plate is above the mirror then you'll get water dripping onto the mirror.

Ice packs are an interesting idea, but I'm not sure you can control them well enough,. But it might be an idea if you don't have power at your site.

cheers, Bird

[John K]

Just gotta love trying to think outside that square....very interesting discussion.

Dave your idea is so interesting. At one point I was actually thinking of simply taking the whole mirror assembly out of my scope (only 3 large bolts) and just sticking it in the fridge until the mirror was "chilled" to the right temperature. Problem: A bit of mucking about in doing this.

The "sealing" idea could be a great option for my scope as already the steel dust cover does exactly that over the tub, it's basically air tight sealed.

I am going to buy some ice packs in the next few days and try that option first, do some temperature and dew monitoring together with sealing the mirror in it's tub with the ice packs as well to create some uniformity in cooling it like inside a cooler box (I take on board the dewing issue) and I will report back.

Anthony with regard to the potential dripping of dew issue with Peltiers on top, perhaps turning the scope at 90 degrees would overcome this?

Lastly, silly question, but are peltiers a heat pump? and if simply cold air is required does a small heat pump as used in small domestic air conditioners achieve the same thing?

[janoskiss (Steve H)]

I don't see why not. A cage to hold ice packs mounted behind a fan mounted behind the mirror should work well.

[astro_nutt]

Just a thought...how about pumping ice chilled water from a small container via a hose to a cooling coil behind the primary mirror where a fan can blow cooled air..the water temperature can be better monitored via a thermometer and ice can be added or subtracted to suit...this would help reduce the risk of dewing or guesswork...I haven't tried this system..(not yet anyway)..but I know the need for cooling not only to the mirror but the mirror cell so as to minimize contraction rate from thermal diffrences between the mirror and cell..effectivly reducing the temperature so as it would be equal to the coolest part of a particular evening..
Cheers!

[AstroJunk (Jonathan)]

Is this what you're after?

http://tinyurl.com/eqtjv

[davidpretorius]

yup that is what i am thinking but circular to fit on top of the base and seal

[John K]

Originally Posted by davidpretorius

yup that is what i am thinking but circular to fit on top of the base and seal

Now as I am extremely slow when it comes to technical issues being a marketing guy, I dont fully understand this concept very well....

....now the "KoolerAir" unit in the link is sucking warm air in, using ice to cool the air and then blowing it out. ok....I understand this concept.

Your peltier fan idea I assumed did not use ice as your thinking was that the peltiers and cold plate absrob the heat from the sealed mirror area and blow it cooler air back on the mirror surface right? Is the cold plate on top of the perspex and on the outside, or on the inside facing the mirror?

I also thought peltiers worked by taking heat away not blowing cold air back in? is the fact that the ambient is lower on the outisde dealed tub means that the air being blown on the mirror by the peltiers is colder than the mirror it self? therefore the mirror will cool by thermaly equalizing?

[John K]

Originally Posted by AstroJunk

Is this what you're after?

http://tinyurl.com/eqtjv

and one more question..... because I love simple solutions to complex problems.........what about simply using a unit like this to just pump cold air into a sealed mirror tub? In fact this unit is a available with a 1 1/2" tube to direct it's cold exhaust air flow via a tube..... Or is that what you guys are suggesting anyhow? (Because every time I hear "peltier" fans I just get so confused!)

[davidpretorius]

a peltier is a small thin piece of ceramic that has two wires sticking out of it. If you put 12 volt on these wires, one side of the ceramic goes up in temp and one side goes down in temp.

now, the difference in temp between the hot side and cold side stays constant, so... to get the cold side right down in temp, we need to extract the heat quickly from the hot side, ie by a fan and heat sink.

A simple computer fan and heat sink will mean that the cold side can get to -5 degrees in a few minutes, from my tests anyway.

from there, you attach the cold side to a large metal (aluminium) plate and now it will start to cool down as well and being large, it should retain its coldness. You then blow air over this cold plate to cool the air and then this air in turn cools the mirror evenly.

a peltier is simply a very easy way to turn electricity into hot and cold.

[davidpretorius]

so basically, if we can cool the "lid" that will sit over the top, then the internal air will get very cold and cool the mirror.

we need to seal the system so that it is harldy interacting with the outside air too much.

just like a portable 12 volt fridge cooling your beer!

[John K]

Cheers Dave, that's a great explanation!