61 replies to this topic

[photonovore]

Obviously the local climate has much to do with the length of cooldown necessary..and if the OTA is exposed to daytime heat or not--food for thought for those who store their ota's in outdoor observatories year round i imagine...

Looking at the temp historical graphs (available on wunderground.com for most locations) i found that at the observing location i have most used reflecting telescopes in a serious manner, the temperature, after sunset, rarely varies more than about 4degrees C all night and during the hours before midnight is usually stable to within a degree beginning shortly after sunset. Where i am at now the temp stabilizes after midnight to within a degree and before that falls about 3 degrees C over the space of several hours. Without controlled data to the contrary i have a hard time believing that a mirror cannot adequitely dissipate heat natively in that sort of timeframe.

If one stores their telescope indoors in an air-conditioned area, the optics will be stabilized at that temperature. If say, that is about 72degreesF, taking them out into the rather typical summertime after sunset temperature of about 70degrees would make the two hours of cooldown time commonly heard about right. This probably explains why i have less reason to consider this a serious issue than others in different locales might...

Some useful information would be to obtain the native heat dissipation *rates* of various mirrors made of various materials,sizes and dimentions and then testing the theoretical heat dissipation models of those masses with some actual telescopes containing them and equipped with temperature monitoring equipment. This information would allow one to make a more informed estimation as to whether there is actually an issue/and or how much of an issue really exists with the mirror mass tracking the typical nightime temp drops in one's particular observing location.

Anecdotal evidence is never sufficient for anything more than the most broad conclusions--there are far too many relevant vaiables between individual mirrors to make an effective generalization about how long it takes a mirror to dissipate x calories of heat.

The external (peltier) cooling idea is sure a neat one for areas with extreme drops in nightime temperature or to hasten cooldown generally--or for scopes stored outdoors which absorb the heat of the day. Tight thermostatic coupling and multiple chips would reduce the possibiltiy of thermo-optical distortion resultant from uneven cooling.


What's ironic is that telescope mirrors are made so that the very surface that is most sensitive to the effects of heat dispersion is also the surface optimized (with a bonded aluminum coating) to do so! This creates the situation that when a mirror cools it releases the majority of it's heat through it's aluminized surface--exactly where it is most noticable! What an utterly counter-productive situation! This effect could be mitigated to a great degree simply by affixing aluminum heat sinks to the *back* of the mirror, with CPU paste and epoxy around the edges to hold them in place & contain the paste, thus optimizing as much as possible (through passive means) a non-critical surface to share heat dissipation with the front of the mirror. I wouldn't be suprised if someone hasn't done this before as well. I also would not be suprised if this is enough to eliminate, in most cases, a lengthy cooldown as a practical consideration.

Telescope tube color-- is a factor only as concerns heat absorbtion from a radiant heat source. At night, in the absence of radiant heat (sun) color is irrelevant. What *is* a factor is that common paint, of any color, is an insulator. That's why the poster that mentioned a bare aluminum tube as capable of achieving thermal equalibrium sooner than one painted was correct. Same goes for tube material- between sonotube and fiberglass i don't know which is worse but both are far inferior to aluminum in this regard. The best tube is no tube at all.

Bare aluminum has a problem though and that is oxidation...and alum oxide happens to perform as a really good insulator. Once bare alum attains an oxide coating the heat it contains is then convected through an alum oxide/air interface and that isn't very efficient at all...better than paint, perhaps, but you've lost the advantages of an aluminum/air convective interface. If one really wanted to go hightech in tube performance, you can get an aluminum tube coated with a thermal dispersant coating. LOL! This is commonly used to enhance heat disipation in high perfomance motorcycle aluminum engines (another hobby of mine) by replacing the inevitable alum oxide layer, which almost immediately forms on bare aluminum, with a protective coating, electrostatically applied, which is composed of material less insulative than paint or aluminum oxide and prevents the latter from forming of course.

Then there is the thermal energy which the mounting itself accumulates, especially if left outdoors throughout the hot day. That has to go somewhere too...and since the mount is attached to the telescope OTA itself, that will be the preferred route of conduction except--- mitigating that may be another (unintentional?) minor function for those felt pads lining the rings than simply to prevent scratches to shiny tube paint after all.

No one has mentioned (i think) cool-UP time..going from a cooler to a warmer atmosphere--like from an airconditioned environment to a hot summer evening. Now you have a mirror mass absorbing heat to reach equalibrium instead of discharging it. What are the effects then? probably a lot less annoying. LOL! What a merry-go-round this cooldown issue is... maybe a solution would be just to get an old refridgerator set at about 60degrees to store your OTA's in? The resultant surface condensation would disperse in a short time (in a relatively dry climate anyway) and you'd have a nice cool mirror substrate--giving you a miniature cold front in front of your mirror instead of that nasty little warm front to look through! ah, just kidding...sort of.

[Bird]

Telescope tube color-- is a factor only as concerns heat absorbtion from a radiant heat source. At night, in the absence of radiant heat (sun) color is irrelevant. What *is* a factor is that common paint, of any color, is an insulator. That's why the poster that mentioned a bare aluminum tube as capable of achieving thermal equalibrium sooner than one painted was correct. Same goes for tube material- between sonotube and fiberglass i don't know which is worse but both are far inferior to aluminum in this regard. The best tube is no tube at all.

The tube material is also important as a radiator of heat during the evening when no heat sources are around. Before saying that it's not relevant at night, have a dig around on the net for the results of studies done into paints used on various installations.

Here's one link that I found: http://edhiker.home....cope12inch.html

That page contains a link to here: http://www.ctio.noao...pteng/dome.html

but I remember (when I did this a few months ago) that there were several more. I found that reading these studies was very informative.

The fundamental result of these studies is that tubes painted with white paint that uses TiO2 will cool up to 5 degrees C *below* the ambient air temp, adding to seeing problems.

regards, Bird

[photonovore]

The tube material is also important as a radiator of heat during the evening when no heat sources are around. Before saying that it's not relevant at night, have a dig around on the net for the results of studies done into paints used on various installations.


The fundamental result of these studies is that tubes painted with white paint that uses TiO2 will cool up to 5 degrees C *below* the ambient air temp, adding to seeing problems.

regards, Bird

Ok, first the structural thermal load of an observatory dome housing a 4 meter telescope, and the net effects of it's various construction material interactions, is not comparable, transferrable in concept, or least of all in direct 1 for 1 relation, like you're trying to do, to a small telescope tube. I mean come on, where on earth would 5C worth of extra calories needed for such below ambient cooling even come from in a structure of so little mass as a telescope tube? Regardless of what it's made of, let alone aluminum? You're literally claiming that paint has refrigerative capability! I don't mean to get on your case (the peltier cooling scheme was really innovative) but how on earth can you consider a massive dome composed of a myriad of materials, all differing in thermal character, acting in unknown (to us) relation to the effects of a climate controlled atmosphere within the structure, and the whole works all sitting out baking in the sun all day long---with a telescope tube??? Think about it. The information just is not transferrable--at all.

If there are "studies" that suggest that tube paint can cool the tube it's painted on to below ambient temperature without any other source of *active* energy, they are examples of net pseudoscience-- as the whole idea contradicts *very* elementary physics--every law of thermodynamics that there is in fact. It's specious.

As far as tubes go, they are of so little mass and typically made with a material which has such supurb heat dissipation characteristics (aluminum) compared to-- a pyrex mirror-- that their contribiution, either caloricly or in dissipation time to the performance of the system as a whole while in the process of cooling down, becomes so negligible in comparison as to be irrelevant. If we had a material with the structural characteristics of glass but with the thermal dissipation and conductivity characteristics of aluminum, no amateur astronomer would even have heard of "cooldown". It's only because glass is the best substance for holding an optical figure and that it also unfortunately happens to hold onto it's calories like a miser holds onto his pennies that cooldown even exists as an issue. Obviously cooldown of an aluminum tube simply cannot be a significant system issue--there aren't enough calories present and the dissipation rate of what there are is too great by magnitudes (133 times the rate of pyrex as a material) for it to factor enough energy at the levels necessary to have a discernable effect-- and neither is what it's painted with a relevant issue (solar scopes excluded) no matter how much web pseudoscience is drug up--or actual science missapplied--to the contrary.

Even sonotube (wood fiber and resin glue), probably the worst tube material, in this respect, there is in common use, still has several times the thermal disperision capabilities of pyrex and ten times the surface area available to radiate whatever heat load it possesses away into the atmosphere. Painting a cardboard tube silver, grey, black, white whatever just isn't going to make a measureable difference in it's heat dissipation rate. Do the math? The mirror may have enough excess calories and holds onto them long enough to become an issue, but nothing else a telescope is made of--except wood-- can even come close.

[jandrew]

Just thought I'd toss in a touch of empirical mirror data to add to
Bird's data -- I just picked up a simple indoor/outdoor thermometer and
affixed the probe to the back of my 4.5" mirror. Then I shoved the mirror
in the fridge and recorded the temperature changes over time. (actually
repeated the experiment with a different model thermometer as well). The
attached gif plots temperature (degrees C) over time (minutes) for both
trials (with slightly different starting temps).

The plots here go down to 4.8 degrees (to within half a degree of ambient
which was 4.3C). Two full hours for a little 4.5" x 1.5cm mirror -- fully
exposed (no mirror cell or tube walls) except for the insulating patch
covering the probe on the back -- to passively fall through most of a
17-18C temperature differential. At 60-minutes we were still 3.5C above
ambient and don't cross the 1-dgree theshhold until 105 minutes. A little
longer than some (myself included) might have expected.

I'll repeat with my 8" using the roomier chest freezer (which will also
better simulate current outside temperatures) sometime soon, and then
test with a fan.

andrew

Attached Thumbnails

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[sixela]

I mean come on, where on earth would 5C worth of extra calories needed for such below ambient cooling even come from in a structure of so little mass as a telescope tube?

5° C is not a lot -- just express that temperature in Kelvin and you'll see that relative temperature drop isn't that large.

The heat is lost by radiating into the night sky - which, I can assure you, is much colder than the immediate surroundings of the scope minus 5° C (at least I hope it for you).

Also, temperature is not expressed in calories - you're comparing apples and apple trees. Where do you get the idea that something that can't store much heat is difficult to cool? Quite the opposite, actually...

[Bird]

Well, I'm not going to try and change anyones mind or start a flame war... this site is well known for its hospitable people and polite company...

If you want to learn something, anyone can get hold of accurate thermometers and do the experiment themselves :-)

btw the articles I referred to were talking about the properties of the materials, and the paint. I doubt that it matters what the structure is that they are used on, their properties are unchanged.

Don't be confused into thinking that a 5C difference in temperature equates to much usable energy - as soon as you try and extract that energy the temp difference will disappear, just as sixela said. But it's true that there is a little energy there (not enough to be useful for anything). It's not free, of course, you're just finding a way to extract a bit of thermal energy from the surrounding air that was imparted during daylight.

regards, Bird

[Tonk]

If there are "studies" that suggest that tube paint can cool the tube it's painted on to below ambient temperature without any other source of *active* energy, they are examples of net pseudoscience-- as the whole idea contradicts *very* elementary physics--every law of thermodynamics that there is in fact. It's specious.

If the paint is a very effecient IR radiator then it will bring the temperature down to some equilibrium point involving "ambient" IR input vs output as well as smaller convection and conduction contributions. This could well be below the temperature of the surroundings if the surroundings are poor at loosing heat via radiation and the conduction between the surroundings and the paint material is also poor. All this is saying is that the "system" is not at equilibrium yet - give it long enough and you will find the air temp eventually reach that of the paint! The paint just got near to potential equilibrium temp first.

The assumption here was that "ambient temp" meant equilibrium temp. I think it should be read as "current surrounding temp".

[photonovore]

sixela, simply put, transferrance of heat energy is what we are talking about and heat energy is measured in calories, storage, gain or loss thereof; temperature is a measurement of the effect thereof. It's not apples and oranges, it's apples and the apple tree. Both terms are factors of the same basic equation. The "immediate surroundings of the scope" is the ONLY conductive mass relevant to the telescope's ambient temperature and internal air stability--certainly not the air masses anywhere else.

5C *below abmbient* is very significant when discussing passive amounts of heat transfer in mass the size of a telescope tube--or an observatory dome massing thousands of pounds for that matter, even more so when below ambient cooling is concerned. 5C is also 5K (20/25C = 293/298K)---and 9F. Using a larger scale measurement standard does not make the effect quantatatively less significant.

Tonk, your post is correct and quite relavant applied to the observatory dome situation, for example, where there are significant passive masses contributing to the localized equation (as well as some active sources such as HVAC effects). But the amounts of stored heat energy in such a small heat reservoir as a telescope tube are incapable of reifying the degree of sub-ambient temperature differentiation claimed for the passive effects of a coating in this case. The claims of "5C" and secondly "below ambient" are what i have issues with...not with the existance of discreet differences between heat dissipation rates of materials and coatings. Like i said earlier, coatings can have a dramatic effect upon the conductive rate of heat dissipation (and concordantly surface temperature) re; active heat sources like an internal combustion engine. Upon small passive sources, of high conductivity/diffusivity (like aluminum), taking place at environmental scale temperature differentials, however, the difference a coating could create would be infantesimal and of extremely short duration and, lacking contributory masses below ambient acting as a sort of heat sink, could never extend below ambient. As i said, the caloric storage capacity of the masses involved at the temperatures involved is far too small to create any such effect of the character or degree claimed.

Bird, you might want to take a closer look at the relationship between caloric energy, materials, temperature and time and review the first and second laws of thermodynamics. As far as useful energy is concerned, reducing a temperature 5C--from 80F to 70F- has a name: it's called air conditioning and requires either an active external energy source to achieve or, as in the case of a nice cool basement for example, a cooler significant mass, provided by the earth surrounding it. A simple telescope tube is subject to neither of these effects-- in any relative way-- and that is why a 5C temp drop below ambient due to a simple coating of any kind concerning such a physically simple and insignificant mass is just physically impossible.

BTW, I complimented you on your ingenuity and attempted, at least, to correct your misapplication of data. I can't see how simply correcting a factual error while attempting to explain the science behind it qualifies as anything personally derogatory in any reasonable way(flaming).

As for the effects of a 17C temp differential concerning a mirror, such a massive temp differential is unnecessary to endure and easily avoided. All one has to do to vastly shorten the time required for cooldown to within a much more reasonable timeframe is to store the OTA in an environment closer to the current nightime ambient temp. This is easily accomplished...in winter simply store it in an unheated structure instead of inside the home; like a garage or garden shed. In the summer store it in a cooled structure, the basement of an airconditioned house for example. Take it outside after sunset. A couple of heatsinks properly attached to the rear of the mirror will substantially mitigate the proportion and degree of front surface convection and the cooling rate of the mirror generally--which is the issue at hand is it not?

Just a little forethought as to storage location will eliminate the lion's share of temperature differential that must be overcome and thus the time until the optics are within the desired 1C of ambient temperature of the air which surrounds them. It really needn't be either a substantial nor particularly complex issue to solve in most cases.

[sixela]

It's not apples and oranges, it's apples and the apple tree.

I wonder where you got that figure of speech -- read my post again.

And it's all fine and well to lecture us about the laws of thermodynamics, but you're still simply ignoring our point that there is such a thing as *radiation* that affects the thermodynamical system.

The system defined as the air surrounding the scope and the scope is not a thermodynamically closed system. The night sky, or if your prefer, the universe, is still there.

That doesn't only apply to scopes, but also to the ground, by the way. I'm sure you are well aware that ground frost does occur, and does occur at air temperatures of up to 4°C.

If radiation into the night sky did not exist, but only convection and transmission did, that phenomenon would not occur.

And, by the way, cooling just about anything 5° is not called air conditioning. Cooling *air* is, and air does not radiate warmth efficiently.

[Derek W]

Bird probably said "flame war" because you come across with a relatively confronational tone. (at least for Cloudy Nights)

This would be eliminated by just stating your claims and not telling someone how wrong they might be.

It's been my experience in life that good science, data, information - whatever - will speak for itself.

The messenger only detracts from the effectiveness of that information when they interject commentary of a personal nature. When this happens we might start seeing an emotional response, and this is when logic takes a hit and the argument heads in a direction nobody wants.

Certainly, I've never been convinced I'm wrong because someone simply tells me I am.

[sixela]

5C is also 5K (20/25C = 293/298K)---and 9F. Using a larger scale measurement standard does not make the effect quantatatively less significant.

K is the only real unit here, at least if you want to plug anything in elegant formulas.

And expressed in those units, let's just agree that 278K and 273K are a lot closer than 273K and 2.75K: as far as the universe is concerned, winter and summer and night and day on Earth are all really hot.

Mind you, I'm not saying that OTA tubes are cooled to a temperature down to 5K colder than ambient air because they're very effective heat radiators -- just that it's not a ridiculous proposition.

There is evidence for some cooling below ambient temperatures, though: dew. I think you'll (sadly) have a hard time convincing most people on this forum that dew never occurs before ambient air temperatures reach the dew point.

[David Knisely]

Bird probably said "flame war" because you come across with a relatively confronational tone. (at least for Cloudy Nights)

This would be eliminated by just stating your claims and not telling someone how wrong they might be.

It's been my experience in life that good science, data, information - whatever - will speak for itself.

The messenger only detracts from the effectiveness of that information when they interject commentary of a personal nature. When this happens we might start seeing an emotional response, and this is when logic takes a hit and the argument heads in a direction nobody wants.

Certainly, I've never been convinced I'm wrong because someone simply tells me I am.

Uh, I hope you aren't referring to me (the post referred to at the top of your reply names one of my postings). As for cooldown, the "rules of thumb" quoted here are, I think, enough to convince people that one needs to allow the scope to cool down to at least something close to ambient air temperature. I consider my SCT to be pretty much "ready to use" anywhere from 30 minutes to 1 hour after it is brought outside and the Cat Cooler started (depending, of course, on the temperture difference between where I stored the scope and what the outside temperature is). All optical surfaces may or may not be at ambient at that time, but the scope will perform at least in a fairly satisfactory manner (significantly better after this cooldown time than it would have if I had brought it outside and tried to use it immediately). My Newtonian cools somewhat faster (*if* I use the fan), and does not appeared bothered by its tube composition very much. Some of my best seeing has occurred just after sunset or up to late evening twilight. Clear skies to you.

[Bird]

David, I think he was referring to photonvore, who seems to be a bit confused.

If I read photonvores argument correctly, he is saying that nothing in the backyard can ever be cooler than the surrounding air, at any time. I'm sure that's what he is saying, cause when I stated that I had *measured* my tube as a couple of degrees C below that of the surrounding air then he jumped in to tell me that this was not possible.

Clearly he must be right, and my digital thermometer is wrong :-)

photonvore, you can see a typical temperature graph collected by me on my website in the article where I talk about the peltier cooler:

http://www.acquerra....ronomy/cooling/

...and I think your reference to "pseudoscience" and "specious" might have been the bait to start a personal attack. Since I am a scientist by trade, I found that rather offensive.

Just for fun, here's some more links for you:

http://www.weatherqu...auses_frost.htm
http://amsglossary.a...browse?s=d&p=21
http://amsglossary.a...tional-cooling1
http://www.islandnet.../whys/frost.htm

etc. I think there is plenty of evidence that surfaces exposed to the open night sky can and do fall significantly below the temperature of the surrounding air.

regards, Bird

[bierbelly]

would anyone care to explain Peltier coolers to me? Maybe a link?

[markf]

I believe it's a kind of metal that, when current passes through, one side gets really hot and the other side really cool.

Kinda like those coolers that you can plug into your car that can both heat and cool, depending on which way you flip the swich...

Just a guess though

Mark

[Derek W]

Sorry, wasn't speaking of you David.

Just about our newer member.

[markf]

My question:

I have a 6" mirror, and after reading this thread, I was thinking of using and old PC case fan to aid in cooling (as well as take the cover plate off the back that Celestron puts in place)

So, if I don't want to design a fully enclosed baffle system, which way should I direct the airflow? Blowing onto the back of the mirror?

Mark

[David Knisely]

would anyone care to explain Peltier coolers to me? Maybe a link?

A Peltier cooler is a device which cools without having to be a mechanical refrigerator. The Peltier Effect was first noted in 1834 by French watchmaker and physicist Jean Charles Athanase Peltier, and takes place when an electrical current is sent through two dissimilar materials that have been connected to one another at two junctions. One junction between the two materials becomes warmer while the other becomes cooler, thus transering heat from one side of the device to the other. The cooler usually uses semiconductors in an array, with current flowing through the junction of the device which gets warmer. With the proper choice of materials, you have, in effect, a solid-state cooler which requires no moving parts or refrigerants. It is commonly used in Astronomy for cooling things like CCD arrays (or soft drinks on hot days when you are observing the sun :-)). For more information about how such a cooler works, try the following URL:

http://arstechnica.c.../peltier-1.html

Clear skies to you.

[Bird]

bierbelly - Peltier coolers are interesting semiconductors which transport heat from one of their junctions to another when you pass current through them.

The usual design process is to create a sandwich of many junctions in parallel.

So, one surface gets cold, and the other surface gets hot. The hot surface gets *very* hot cause it's receiving not only the heat from the other surface but also the heat created by the peltier device itself.

Google should be able to find lots of links for you - they're properly called "thermoelectric devices" or "thermoelectic coolers".

In Australia we find them used in all sorts of places - like the "cooler bag" that you take in the car to keep things cold.

Links:

http://www.electroni...96/sep96_04.htm
http://oemagazine.co...1/tutorial.html

cheers, Bird

[sixela]

I think there is plenty of evidence that surfaces exposed to the open night sky can and do fall significantly below the temperature of the surrounding air.

Aha -- a line of thought: photonvore is from Venus (where they've never heard of open night skies), so she is a woman!

[sixela]

So, if I don't want to design a fully enclosed baffle system, which way should I direct the airflow? Blowing onto the back of the mirror?

Mark

If you had only one fan, my guess would be *pulling* air *across* the *face* of the mirror.

That should allow you to get rid of the boundary layer faster than at thermal equilibrium - if you're prepared to mount the fans in such a way that you can leave them running without causing vibrations.

Ideally, you'd also want to destroy the boundary layer at the other side of the mirror. That boundary layer will slow down mirror cooldown times, but will not directly influence viewing. For that boundary layer, you don't really care whether airflow is laminar (in fact, if my memory serves me right but that's been too long, you'd even prefer airflow to be turbulent), so just blowing air at the back of the mirror should be OK.

[Bird]

My question:

So, if I don't want to design a fully enclosed baffle system, which way should I direct the airflow? Blowing onto the back of the mirror?

Mark

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.

fwiw my opinion is that you're better off trying to bring the mirror itself down to the correct temperature, and this is probably done by blowing air onto the centre of the back of the mirror. The closer the mirror is to the surrounding air the less boundary layer effect there is to worry about. Also reduced the chance of damaging the mirror surface by blowing some grit across it, and keeps it a bit cleaner.

That's how my current iteration of the peltier cooler works, mostly by accident cause at the rear was the most convenient place to put a fan.

When in doubt, try all of them and see what works :-)

regards, Bird

[gazerjim]

as the whole idea contradicts *very* elementary physics--every law of thermodynamics that there is in fact. It's specious.

Now I know why Bird's images are so crummy

I'm picturing Bird's imaging environment as filled with IR radiation from energy collected during hot, sunny days. It's therefore not difficult to imagine that a white tube might remain cooler than adjacent heat "absorbers."

Jim

[Bird]

Thought you guys might find this interesting... here's the temperature graph from my session a few nights back:

http://www.acquerra....050303-temp.png

The colours are:

* RED and YELLOW mark the mirror temp. The RED sensor is on the back of the mirror and the YELLOW sensor is on the side up near the face.
* GREEN is the temp of the aluminium tube
* PURPLE is the air temperature.

Watching the RED and YELLOW lines shows the peltier cooler in action, and you can see when it switches on and off, controlled by a small computer, to try and keep the mirror close to ambient temperature.

Things to note:

* The rapid temp drop at sunset when the graph was started

* How the peltier + fans was able to cool the mirror at a rate that was fast
enough to keep up with the dropping air temp. You can see the mirror temp level off from time to time when the peltier is switched off.

* At 4am I started imaging, and had to switch off the peltier cause it creates too much vibration. You can see the immediate result is that the mirror is no longer able to cool fast enough. Remember - this is at *4am* when , by all normal accounts, everything ought to be in equilibrium.

Unless you live in Canberra

At 5.30 the thermal shimmer was so bad that I was forced to take a break and turn the cooler on again for a while.

I have Plans for a modified peltier than can be switched down to a lower voltage so I can leave it running while I image. The fans would be turning much slower, so hopefully no vibration, and yet it can keep cooling.

regards, Bird