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Cool down time, the longer, the better

Started by Derek W , Feb 26 2005 09:54 PM

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61 replies to this topic

#51 sixela

James Webb Space Telescope

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Joined: 23 Dec 2004

Posted 07 March 2005 - 06:00 AM

I think its still an open question... Some people will say to blow air across the face of the mirror, and others will say to blow onto the back of the mirror. Honestly I have no idea which is the best.

These two things are trying to achieve different things - they're not mutually exclusive.

Blowing air to/from the back serves to destroy the boundary layer there. Doing so aids in achieving thermal equilibrium between the mirror and the ambient air faster, but otherwise does not influence the view - so you don't care that much if the flow around that back of the mirror is turbulent. In fact, the best way to cool the back of a mirror, in my opinion (given in by the way people design things to get 130W off a small chip) would be with a heat sink covering that back - the mirror would transmit heat into the sink, and the sink would have a much larger surface and be less prone to develop a boundary layer.

It doesn't have to be a heat sink like the ones on a chip. Anything that sticks to glass, transmits heat efficiently, and has a very, very rough and spiky surface would be better than the back of that mirror...any ideas?

The same would hold for any cooling device you'd use on the front of the mirror that you would only use when you're *not* viewing. That will also help the mirror to cool down, given you want to use both faces of the mirror to transmit heat. And anything, including blowing air straight at the mirror, would be acceptable as well (though heat sinks and Peltier elements on the front of the mirror would be slightly troublesome

).

If you blow air across the face of the mirror, chances are that at the other side you're going to blow air onto a wall, which may give you a very turbulent airflow in front of the mirror. Not a problem if you're just using that fan as above, but you won't be able to view well with the fans on either. There's little point in replacing a small boundary layer by a huge layer of turbulent air.

If you try to make the airflow across the face of the mirror laminar, then you can actually start to observe *before* the mirror has reached the temperature of the ambient air if you keep fans running - you're constantly destroying the boundary layer.

I do agree with others that it's easier to keep airflow *in front* of a fan laminar than to keep airflow behind a fan laminar, though, which makes it preferable to *pull* air from the mirror and let the scope figure out where to get that air from (it won't pull it out of a wall of the scope structure

), than to *push* air across the mirror and hope for it not to bounce back and cause eddies.

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#52 gazerjim

Cosmos

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Joined: 12 Feb 2004

Posted 07 March 2005 - 05:41 PM

As a layman thinking intuitively, I would question whether true laminar flow--virtually at the molecular level--can be achieved across the mirror face; especially given much difference in temperatures. Perhaps laminar is a relative term. However, based on empirical evidence with my 8" Newtonian, image quality is usally improved somewhat by blowing air through the tube from the back, especially when the mirror is slightly warm. This makes me think that some "large scale" turbulence on and ahead of a warm mirror might be preferable to a layer of warm, stagnant air. I think this is especially true near the Zenith when the tube is vertical. I have not tried "transverse cooling".
This is "for what it's worth". I haven't taken measurements, so these observations are stictly anecdotal.

Jim

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#53 markf

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Posted 07 March 2005 - 06:05 PM

Probably ideally, the truss guys (and for that matter professional observatories) have it right: open air design rules!

As I only have a six inch, I'm guessing it should be simple to get my mirror down to ambient (or in the summer, up to ambient). But after seeing Bird's data (and the pictures he's consistently putting out), even my mirror may take some time to cool.

So I'm thinking a simple PC case fan, wired with a car lighter plug, blowing onto the back of my mirror should help speed things along. Or, build a fully enclosed baffling system with air outlets at various points to fully bathe the mirror with fresh air :grin:

Or a petlier cooler....

But for visual all this might be over kill....

In any case, unless I'm working in the daylight, that cover on the back of the tube is coming off!

Mark

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#54 gazerjim

Cosmos

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Posted 07 March 2005 - 06:18 PM

Sounds like a bad cartoon, but I wonder what might be done by ducting air through an ice chest stacked with "freezer" modules, and then onto the mirror. Hopefully, the air would become de-humidified in the process. Such a setup would be cheap, but certainly not pretty.
Or I wonder if Peltier cooled air could be used the same way?

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#55 wes

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Posted 07 March 2005 - 06:57 PM

Hi Jim,

A crude way that I cool my primary in the Spring and Summer is with a small room air conditioner ($100.00 from Lowes) blowing to the back of the primary, it works very well and will cool my 18' down very quickly :grin:

it does not work during the Winter though because it will only blow out air at about 60 degrees F.

Thanks,

Wes

Sounds like a bad cartoon, but I wonder what might be done by ducting air through an ice chest stacked with "freezer" modules, and then onto the mirror. Hopefully, the air would become de-humidified in the process. Such a setup would be cheap, but certainly not pretty.
Or I wonder if Peltier cooled air could be used the same way?

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#56 sixela

James Webb Space Telescope

Posts: 17,794
Joined: 23 Dec 2004

Posted 08 March 2005 - 03:51 PM

As a layman thinking intuitively, I would question whether true laminar flow--virtually at the molecular level--

That's not really necessary - all of this is quite macroscopic. Someone posted a link to movies showing the effects of the boundary layer on a grossly defocused star - very educational. Can't remember where it was, though.

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#57 gazerjim

Cosmos

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Joined: 12 Feb 2004

Posted 08 March 2005 - 10:07 PM

Was it maybe Bryan Greer? His work on the subject has been extensive.

Jim

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#58 sixela

James Webb Space Telescope

Posts: 17,794
Joined: 23 Dec 2004

Posted 09 March 2005 - 04:27 AM

Yup.

http://www.fpi-protostar.com/bgreer/

They were the videos from his 2000 and 2004 (part 1) articles.

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#59 geoff

Mariner 2

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Joined: 08 Nov 2004

Posted 23 March 2005 - 07:32 AM

I think it is kind of neat that the mathematics which best describes the cooling curve for a Newtonian telescope is Newton's law of cooling. Two of the many ideas from this great man.

That is to say, the rate of cooling is proportional to the temperature difference between the telescope and its surroundings. This is because the heat flow is approximately proportional to the temperature difference driving it. This is exactly true for thermal conduction and a good approximation for convection and radiation at the small temperature differences involved here.

So what does Newton's law of cooling predict? Well, if the temperature difference between telescope and ambient halves in time t then it will fall to 1/4 in time 2t and to 1/8 in time 3t and so on. However, as Bird points out above, the situation is more complex when you have a changing ambient temperature. In this case the telescope may never achieve thermal equilibrium.

To some extent, full thermal equilibrium may be unnecessary. Temperature differences cause 2 main problems, air currents within the tube which cause unwanted refraction and temperature differences within the optics which cause mechanical distortion of lenses or mirrors. These effects are quite separate. A mirror which has a constant temperature profile may perform well even if this temperature is a little different from the ambient temperature.

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#60 Guest_DONOTDELETE_*

Posted 23 March 2005 - 08:58 AM

After a couple of years struggling to fix the tube currents in my 12.5” f/30 Cass I happened to look into the focuser without an eyepiece once to see the secondary shadow with a plume -- of when could only be heated air -- coming off the secondary. It looked like a negative image of smoke from an old fashion chimney on a cool but calm morning in the hills of Tennessee. :cool:

This plume was mostly from the secondary and holder, so I attached some steel wires with alligator clips to the tube ends of the spider and dangled them off the side. This reduced the plume from the secondary, so the first consideration was to find a way to eliminate the heat reaching the secondary mirror. Easier said than done. What fixed the problem was replacing the heavy fiberglass tube, with a thick coating of white paint on the outside and black inside, with an rolled and welded aluminum tube – blackened inside and outside. Then the spider attachments screws were replaced this heavy-duty Nylon screws.

Now, a typical Cassegrain has a closed primary end and a lot of metals indirectly contacts the primary mirror. Since most of the points toughing the mirror were Nylon anyway, I went ahead and replaced the other steel screws with Nylon screws and installed a small DC fan from a hole in the rear of the tube that was attached by an old bicycle inner tube. It was stretched and fitted and screwed to the fan then a similar arranged at the tube to reduce vibrations. The fan pulled air out for the tube. After all that I could then peer in the focuser to see the shadows and reflections of the optics and components without plumes of warm air. The images were stable and the telescope would cool down in a mater of a few minutes as opposed to having to open up two hours in advance of using it.

While finding a good place for the fan in the old fiberglass tube I cut some holes at various positions and watched the tube currents from the focuser with each experiment. When the fan blew air across the primary the turbulence and plumes was horrific. Then reversing the fan it appeared more stable. The fan was moved to the front, middle and end of the tube and reversed directions with each move. In the end, literally, the fan that exhausted warm air out the back of the tube worked best.

A friend and I ran experiments on our long Newtonians using an IR sensor poked into the focuser. As the Sun set we measured the temperature signature of the optical path as the telescope began to cool. Measurements were made on different days with the fan placed at the rear end and blowing up the tube and exhausting the air. We found that the telescope would cool off faster it the fan sucked the cooler air down the tube and out the back, carrying with it the warm air it snatched up on its way down. No turbulence was seen an the temperatures came down faster. Also, my bare aluminum tube at first took much longer to cool than it did after I pained it black. Like three or four time faster as a black telescope.

I wrote up several lectures and articles on this and while at the 1994 RTMC a guy showed me the ultimate telescope tube he made for his 8-inch f/8. It was a 0.1-inch thick aluminum tube that was anodized flat black! All Nylon mounting screws where the optics may be indirectly connected and a small fan built in the primary cell, exhausting air, and this telescope had absolutely no signs of tube currents – even after he had moved it from his hot van out into the cool air.

So, that’s my two cents worth of hot air :jump:

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#61 RobSter

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Posted 23 March 2005 - 09:56 AM

I keep my telescope in the garage, and most of the time i take it out and observe straight away, and i've had really good views of the planets. Leaving it in the garage seems to get it cool enough. And the weather's always too unpredictable to be able to plan my observing sessions anyway.

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#62 Guest_DONOTDELETE_*

Posted 23 March 2005 - 11:05 AM

I keep my telescope in the garage, and most of the time i take it out and observe straight away, and i've had really good views of the planets. Leaving it in the garage seems to get it cool enough. And the weather's always too unpredictable to be able to plan my observing sessions anyway.

Probably the best way is to leave a scope in a large area in free air so it only has to adjust to outside air when taken out. That too involves some engineering :grin:

The problem with expounding on such engineering problems as this, and that is what it is, too many variables, i.e., size, type, etc. I live in central Florida and store my tall 16-inch f/6.9 Newtonian out under silver tarps. While they are not as good as in the past they do block IR or Sunlight heat very well and need no cooling off time to use. I can go out and work on the scope under the tarp and not keel over from the 90-degree heat in summer. Hum, it gets even hotter at times. :jump:

For the brief few years living in northern Virginia my engineering skills had to be honed back to Earth and revive the learning once again. The conditions here are all too easy to get used to and much different than up north. Temperature swings are high up there and it seems to be the same, day or night, here. A never ending process of learning.