Why cool your mirrors?

[Bmanners]

Greetings All,

I have been reading these forums with great interest, alot of good info in here. I'm curious however about the cooled mirrors I have read about. How does the mirror temperature affect the image of the telescope? Does an uneven temperature distribution cause the shape of the mirror to change? or is there some other reason. Is it a large affect or only a real issue with the photography (which I have to say is way cool!). Is colder better or close to ambient better?

I work with Peliters and heat flows so I find this stuff interesting.

Cheers
Brett

[RAJAH235]

Hi Brett. :hi: & :welcome: to IIS.
The temp of the mirror is a major factor in either visual or photographic mode. A 'hot' mirror will not come to it's correct shape for it's best performance.
There are others who will give a more detailed explanation, i'm sure. Just hang on to your hat... :windy: Hope you have fun here... :2thumbs: :D L.

[circumpolar]

G'day Bmanners.:)
And welcome to the clan.

[acropolite]

I think there are also air movements close to the mirror which are at minimum when the mirror is at ambient temperature,

[bird]

Hi Brett, there are two compelling reasons to try and get the mirror as a whole to the same temperature as the ambient air:

1. If the mirror is not at the ambient temp then there will be a thermal gradient through the glass that distorts the shape of the mirror away from a perfect parabola, giving the mirror some amount of spherical aberration.

2. If the mirror is warmer than the ambient air then a layer of unstable air forms over the mirror (called a boundary layer) which distorts the image very badly, as light refracts differently through air at different temperatures.

Either one of these effects will destroy the ability of your scope to give high power views of the planets, and both effects together (the usual case) contributes to the poor reputation that newts have for planetary views.

With some effort it's possible to correct both these problems, and then you can get really spectacular views and/or images of the planets.

regards, Bird

[davidpretorius]

welcome brett, yup what bird said!!

[iceman]

Hi Brett.

Welcome to the forum! How did you hear about us?

I was gonna say what Bird said, but he beat me too it and his answer is much more accurate than mine would've been :)

What type of telescope do you have?

From Anthony's experiments, it would appear that at ambient temperature is better, but he usually cools to 0.5deg below ambient, so that by the time he's ready to image it's settled down.

The boundary layer and distortion of the mirror will affect the views both visually and when doing imaging. Obviously it's much more noticable when doing imaging, because you're working at much longer focal lengths (higher magnification).

Non DIY newt imaging people like myself are going to start by just working out what temperature the mirror is compared to ambient, so wombat_in_space is making me a unit that measures the temperatures and sends the data to my laptop. To start with, i'll know when the best time to image is.
Following that, i'll make an active cooler like Birds, to cool the mirror faster so that I can start imaging sooner.

[Starkler]

Think of the heat haze effect coming off a hot road in summer. The principle is the same with a mirror that is not at the same temperature as ambient. To get maximum image sharpness and stability, we must get rid of this boundary layer.

[Bmanners]

ahh the old varying index of refraction with temperature trick. I would have never thought such things could have a big affect on the image quality. Those mirrors must be extremely well shaped.

Bird: Your telescope looks brillant, I'm very impressed! How long did it take to build? Have you measured the hotside of the Peltiers? It looks like getting the heat out of the Al plate on the hotside will be a restriction in the heat flows. Peltiers can be great coolers but you HAVE to get the heat away from them VERY efficiently. This is where all those cheap powered eskies fall down. In a good system the hotside should be 5 to 10 C above ambient to get the most out of the peltiers.

Iceman:
I found this forum from a magazine, Sky & Telescope I think it was called. I tried to google for an Aussie astromony forum but never found The forum.

Now I'll tell you want sort of telescope I have but you have to promise not to laugh. My telescope, purchased at great expense, cost me a whopping $76 for a 76mm reflector from **** Smiths. (you promised!) Hey for $76 you cant go too far wrong and when I was able to see Saturns rings it blew me away, by far exceeding my expectations of the telescope. I have however seen speed boats that are more stable. Once I have read these forums a little more and I have a better idea of what I want I will be asking some questions about what to get as my first real telescope.

Cheers
Brett

[bird]

I've just finished a new 13.1" scope, probably took about the equivalent of a couple of weeks work spread out over the last 6 months.

For the hot plate I was happy to use my hand to check that it wasn't getting too hot - I am running the peltiers at a duty cycle of around 75% so that they are on for 45 seconds and off for 15 seconds. I had made an error when I first wired it up, and the exhaust fans on the hot side were running off the peltier power, so they would switch off when the peltiers switched off. That wasn't too good as the residual heat would leak back through to the cold side, so I've changed it around and now the exhaust fans are running on an independent supply. The cooling curve is much better and the hot plate runs about 15 or 20 degrees above ambient, close enough for me.

The 3 peltiers together draw about 10 amps @ 12v, so there's an extra 120W of heat that I have to get rid of :-)

All in all I'm very happy with the combined mirror cell + cooler, it seems to be the best part of the new scope, the collimation adjusters are smooth and very precise as well.

regards, Bird

[jjjnettie]

Thats one tiny reflector Brett. What ep's came with it?
I'm not laughing cause I started off, like a few others here, with a 60mm refractor.
Jeanette

[Bmanners]

It came with 25mm, 15mm and 10mm EP's and a 3x and 1.5x Barlow lenses. I find the 15mm with the 3x Barlow is the best for clarity/magnification. I cant seem to focus the 10mm with the 3x Barlow (on Saturn anyway).

For those of you using Peltiers to cooler your mirrors using computer CPU coolers as hotside heatsinks works well. They are about the right size and do quite a good job. They also often come with 12V fans however some of these can be rather noisy.

Bird: You might find those Peltiers work better if you drop the voltage a little. This sometimes helps as it reduces the load on the hotside heatsink. Maybe run them at 7V (between 5V and 12V rails on a PC power supply) and at a 100% duty cycle?

Cheers
Brett

PS Saw Jupiter for the 1st time last night. Its a big one!

[bird]

Yep, a couple of old slot-A style heatsinks stolen from Athlon-450's made up my previous model, seen here: http://www.acquerra.com.au/personal/bird/astronomy/cooling/2/



Thanks, but I think they're working ok, I've been experimenting with different duty cycles, I didn't have this option on the previous model so it's something new for me to explore. With the relatively warm nights during summer I don't want to overheat the hot side so I run a dutycycle around 75%, but in winter with colder nights I'll be back at 100%.

This new model is more slimline than the old one, since it's an integral part of the scope, and one of the things I sacrificed was large heatsinks on the hot side, so it's definitely something I have to keep an eye on.

The cooling graphs I'm getting so far are quite satisfactory.

regards, Bird

[mickoking]

G'day Brett and welcome.

My first 'scope was a crapmatic 60mm refractor :lol: It had no finderscope and a zoom eyepiece x15-x60 but I did see (and share with others) Saturns rings, Jupiters moons and the orion nebula. I also saw Halley's comet with it. My next scope was also a 60mm refractor but a bit better with naaaasty interchagable 25mm eyepieces and useless finder in 1985 it cost $220.

BTW brett where in Perth do you live?

[ving]

welcome aboard brett, i started with a 60mm ebay bought refractor :) wobbled like a bowl of jelly but it was good enough to get me into this hobby :)

[Bmanners]

Mickoking, I live in Claremont - Cottosloe end. You have put up a challenge. I have to see if I can see Jupiters moons and the Orion Nebula. Finding Jupiter wont be too hard, staying up late and focusing the telescope will be the hard part. Find the Orion Nebula.....I have some research ahead of me....



Cheers
Brett

[rmcpb]

Orion nebula is the middle star in the handle of the saucepan.

BTW welcome to the site :)