The light is on with cooling

iceman (Mike) · #1 07-08-2006, 07:23 AM

It's common knowledge that for the best views/images, the mirror has to be at the same temperature as ambient.

People like Bird, and later David Pretorius, have gone to great lengths to actively cool their mirror using peltier coolers and fans to blow cold air onto the mirror to help the temperature come down quicker than just blowing ambient air.

I've had the cooling fan on my scope for about 2 months now, just blowing ambient. I've also got the temperature sensor to measure ambient vs mirror temperature, and even just a normal fan blowing air onto the mirror does make a difference and helps the mirror cool quicker than just leaving it out on its own.

But if the ambient temperature keeps dropping, the mirror will never catch up. I've only ever seen the mirror get to within 0.8° of ambient once ambient has stabilised, but it usually sits 1-1.5° above.

Last night, the ambient didn't drop for quite a while - it was sitting on around 12°. I started imaging at around 6:30pm and as usual, the seeing was ordinary with large "waves" going across the preview screen. Over the next half hour, the mirror temperature actually came down to within 0.3° of ambient - the first time it's been that close.

And then it happened - the image stabilised incredibly. The seeing still caused the image to vibrate and distort a bit, but the large waves had completely disappeared. The image was much steadier and there was a much larger percentage of crisp frames.

It showed me for the first time, without a doubt, how important it is for the mirror to be at (or as close to) ambient temperature. What had been blamed on bad seeing was in part due to the boundary layer causing distortions in the image.

I had thought that a difference of around 1° was close enough, but now realise that it has to be even closer. Last night the change happened at around 0.3-0.4° difference, when the air in the tube stabilised and the image became incredibly still.

This experience has now fast-tracked my plans to actively cool my mirror, as Anthony and DP have done.

RB (Andrew) · #2 07-08-2006, 08:15 AM

Very interesting indeed Mike.
It'll be great to see the difference it'll make.
How long before you do the mod?

iceman (Mike) · #3 07-08-2006, 08:18 AM

Don't know Andrew, I've gotta figure out exactly what I need, how to assemble it, how to power it, etc.

Expect a thread in ATM soon asking all the questions

Lee · #4 07-08-2006, 09:48 AM

Always a problem with cooling things isn't it..... heating - simple, easy, cheap.... cooling a way different proposition!
Same with tropical fishtanks - heater $50, cooler $hundreds+

Good luck Mike - I can see you hurrying along so as not to miss Jupiter as it disappears!

IanW · #5 07-08-2006, 01:11 PM

Quote:

Originally Posted by plasmodium

Always a problem with cooling things isn't it..... heating - simple, easy, cheap.... cooling a way different proposition!
Same with tropical fishtanks - heater $50, cooler $hundreds+

Good luck Mike - I can see you hurrying along so as not to miss Jupiter as it disappears!

Active cooling isn't expensive to implement, a peltier device and control electronics can be had for around 50 bucks if you're handy with a soldering iron. The hard part is determining the method of mounting and how exactly to spread the cooling to maximise it's effect.

netwolf · #6 07-08-2006, 05:32 PM

Something on my todo list that keeps geting put off. Is there any good sites with information on the ins and outs of cooling. I know a while back Bird had a link. I would like to learn more about what is happening and understand it better before working on a solution. What are the facts and are they measurable and repeatble or random?

IanW · #7 08-08-2006, 01:23 AM

Quote:

Originally Posted by netwolf

Something on my todo list that keeps geting put off. Is there any good sites with information on the ins and outs of cooling. I know a while back Bird had a link. I would like to learn more about what is happening and understand it better before working on a solution. What are the facts and are they measurable and repeatble or random?

Here's a great starting point:

http://www.acquerra.com.au/personal/...onomy/cooling/

Cheers
Ian

iceman (Mike) · #8 08-08-2006, 05:31 AM

Yep, that's Bird's site. A wealth of information.

asimov (John) · #9 08-08-2006, 01:14 PM

Dave P has some very interesting posts in this forum on his experiments with a peltier cooler on his 10" scope too Wolfie.

netwolf · #10 08-08-2006, 11:13 PM

Ok so i just spent the last hour+ reading through that link and the rest of Bird's site. I understand know the importance of equilibrium in such systems.

Question1: What causes the mirror temp to rise when cooling is turned off? Whats heating it?

Question2: Without cooling, If the mirror is hot why does the tube temp drop bellow ambient would not the heat from the mirror rise up the tube like a chimney? Hence should not the tube temp follow in sync with mirror temp?

I still am missing an aspect to this, i think i need more information about the flow of energy in this system.

Dujon · #11 09-08-2006, 10:52 AM

I wondered the same, netwolf. Thinking about it I wondered if the mirror's "core" is still well above ambient while the "surface" has cooled to almost that. The inference here is that the cooling system is sufficient to keep the surface at a reasonable temperature while the "core" energy is being released but as soon as the cooling effect is removed the surface once again begins to heat up relative to the ambient air temperature.

I'm probably wrong - it wouldn't be the first time.

Photon · #12 09-08-2006, 12:30 PM

Interesting questions! I'd imagine if the core of the mirror was still warm (a good suggestion BTW) optimum viewing would not be acheived as there would still be some surface distortion due to differences in expansion/contraction rates coupled with the temperature differentials(??)

IanW · #13 09-08-2006, 12:58 PM

Quote:

Originally Posted by netwolf

Question1: What causes the mirror temp to rise when cooling is turned off? Whats heating it?

Dujon's answer is right, it's heat energy stored in the core of the mirror. Pyrex and most glasses used in mirrors act a bit like a bucket of hot water, that is they loose the heat energy from the periphery first and then the energy gradually flows out from the central core region as the periphery cools.

Quote:

Originally Posted by netwolf

Question2: Without cooling, If the mirror is hot why does the tube temp drop bellow ambient would not the heat from the mirror rise up the tube like a chimney? Hence should not the tube temp follow in sync with mirror temp?

It's a very complex issue involving the concept of black body radiation and the fact that a telescope radiates heat to space (ie: to 0 Kelvin). Also you have to factor in the specific thermal radiation properties of both tube and the material the mirror is made of (glass or ceramic).

It's also an area that gets extremely complex to model mathematically, so rather than bogging this discussion down in formulae and miles of terminology here's the lay version

Each component (mirror, tube, mirror cell) absorbs and radiates heat at different levels. In general a tube will absorb heat and cool faster than a mirror because it's got a greater surface area. If the mirror was in direct contact with the tube there would be heat transferrence, however there's usually about 20-30mm air gap between the mirror and tube and this air isn't sufficiently warmed by the mirror to transfer much heat to the tube. It is enough to produce eddy currents in the air column in the tube though (tube currents).

One thing Bird didn't mention in his article was the accuracy of his measuring gear.

bird (Anthony Wesley) · #14 09-08-2006, 01:50 PM

Hi guys,

regarding the electronics and stuff that I use for measuring temperatures, it's explained in a seperate article on my site, here:

http://www.acquerra.com.au/astro/equipment/temp-logger/

The temp sensors are quite accurate, reading accuracy of 0.16C, digital output.

As for the reason that the temp rises when the cooling is turned off, there are really two heat sources to blame - one is the core of the mirror as has already been pointed out, and the other one is the hot side of the peltiers (the heatsinks). As soon as you turn off the power the "heat pump" effect stops, and heat can flow back into the system from the hot side.

The fans that are normally used to blow the cold air off the coldplate and onto the mirror then turn into fans that are blowing warm air as the heat from the hot side gets back into the system via the peltiers.

In more recent experiments I've tried turning off the cooling in 2 stages, first turn it down to about 50% for a few minutes and then turn it off. This gives the hot side time to cool down a bit before the heat pump is turned off completely.

Something to remember is that these mirrors are made in a temperature controlled environment, where the mirror as a whole is always at a constant temperature, so the mirror will only be at it's most accurate when it is at the same level of thermal stability.

If you're using (or thinking of using) peltiers then you have to remember that they will introduce a temperature gradient through the glass, from cold on the outside to warm on the inside. You have to turn them off and let the mirror equalise for about 20 mins to 30 mins before it can equalise temperature throughout the glass, and then you'll get the sort of spectacular views that Iceman has found :-)

cheers, Bird

bird (Anthony Wesley) · #15 09-08-2006, 01:55 PM

Also, the aim of the game here is to end up with the mirror as a whole at exactly the ambient temperature (ie the temperature of the air next to the mirror). To achieve this you have to remember that the temp sensors on the mirror are only reading the surface temperature, and you can be sure that the core of the mirror will be warmer, so it's normal practice to cool the mirror until the temp sensors on the glass are reading a few degrees *below* your expected air temperature.

That way, when you turn off the cooling and let the mirror equalise, you end up at the right temperature overall.

In particular (in Canberra) the air temp can keep falling almost all the way through the night, so I have to take that into account as well. I try and overcool the mirror by a couple of degrees so that it will be at the ambient temperature in about 30 minutes time, after it's had time to equalise :-)

cheers, Bird

bird (Anthony Wesley) · #16 09-08-2006, 02:10 PM

Following up on Mikes original post, it seems from my experimental data that you have to get the mirror to within 0.5C of ambient for the boundary layer currents to stop. Interestingly Damian Peach has also posted the same observations a few months ago, also claiming that the mirror had to be within about 0.5C of ambient.

The difference that it makes is very dramatic - details that were completely invisible suddenly pop out and the image as a whole becomes very stable.

cheers, Bird

Photon · #17 09-08-2006, 02:13 PM

Hi Bird,
Do you need to fine tune the collimation when this equilibrium point is established?
Regards Ian Gillespie

bird (Anthony Wesley) · #18 09-08-2006, 02:35 PM

Hi Ian,

Normally I'm waiting impatiently for the equilibrium point so I can start imaging - this means I have camera gear + filter wheel + barlow all fitted into the focusser and ready to go, so I don't want to pull it all out and check the collimation :-)

I've implemented a "collimation scope" which is an old 6x30 finderscope (eyepiece removed) + my firewire camera from last year, this is such a small and stable scope that it never shifts out of collimation, so I can compare the views through it to the view through the main scope, and adjust the collimation screws on the primary to keep them in agreement. In practice this works quite well, and means I can keep the camera gear in the scope all through the session.

I can have both video windows open at the same time on my laptop, and it's easy to compare the images coming from both scopes. The collimation scope fits onto the main scope just like an extra finderscope.

An extra benefit, the larger field of view in the collimation scope makes it a breeze to find things that are close-to but not-quite-in the field of the main camera.

cheers, Bird

Dujon · #19 09-08-2006, 02:43 PM

Thanks, Ian W and bird for your answers cum explanations.

I must admit that it was only yesterday that I realised that the fan on the base of my Newtonian was moving air away from the mirror. To me that meant that most of the air passing through the fan would be from 'outside' the telescope and very little would pass over the base of the mirror. Perhaps I could fabricate a shroud to ensure that the air being moved came from within the tube, over the mirror and out the back but I'm somewhat dubious about doing that.

Two other options would to be reverse the polarity of the electrical connections or to physically reverse the fan. The latter is a problem; the screws securing the fan (which are as loose as a coward in a bun fight) are on the mirror side of the mount. Sheesh!

A question for all:

Some years ago I did a couple of repairs on what are termed 'car fridges'. These, as best I can gather, use something like the 'peltier' to which some of you have referred to produce their cooling effect. Are we talking about the same thing?

Also from memory, the replacement cost of these cooling/heating units was quite reasonable - well, the people for whom I was doing the repairs were quite happy to pay for my labour plus the replacements.

bird (Anthony Wesley) · #20 09-08-2006, 02:45 PM

John, peltiers have become very low cost in the last 5 years or so, now you can buy the peltier chips from places like jaycar etc for about $10, but my first peltier (bought back in about 1996) cost me well over $100...

So peltiers now show up in all manner of "portable fridge" devices, exactly like you might find running off a cigarette lighter in a car.

In fact I think that's where DaveP got his peltiers from?

cheers, Bird