Is it smart to insulate a telescope?

[Linda]

After a discussion in another thread, I would like to open a separate thread on the matter of isolating a telescope.

Does it make sense to wrap isolation around a Maksutov (or other telescope that does not cool down quickly)?

I wrote this in the other thread:

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I have been told that this isolation ensures that the scope doesn't cool down so quickly and that you therefore don't get air curling around inside the scope, that can happen during the cooling down. Apparently you don't need cool down time, then. I haven't confirmed this in practice yet. 

And another person wrote this legitimate comment:

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I don't see how this would work. Warm air is less stable than cold air so trying to keep the air inside the telescope warm seems to defeat the object of cooling the telescope down to ambient air temperature to stabilise the air in the tube.

I have tried to find some more information about it on the internet. On a German forum it was recommended to isolate a telescope tube, but only if it was a telescope that could be aired (if I understood the German good enough). Or putting on the isolation after cooling the scope it down. They spoke about tube turbulence. 

Does any of you have any knowledge or experience with the matter?

 

 

[laudropb]

Do you mean insulate the OTA? I have seen this recommended in some sources, but have never found this necessary.

[Linda]

11 minutes ago, laudropb said:

Do you mean insulate the OTA? I have seen this recommended in some sources, but have never found this necessary.

Yes, I suppose I mean that. My English is pretty hopeless lately. Title has been corrected.

[mikeDnight]

I can't see how this would work in practice as the heat within the tube would escape gradually through the lens anyway. I think it would be best to allow the scope to stabilise naturally. Once the thermal balance has been achieved you could then cover the tube to prevent freezing. 

As Maksutov and Schmidt cassegrains are around 25 times  more sensitive to internal heat than Newtonians or refractors, trapping the heat inside the tube doesn't seem a good idea to me. The problem of heat sensitivity in MCs and SCTs is because of their amplifying secondary mirrors. A F2 primary needs a secondary having a amplification factor of 5 to attain a F ratio of 10, and its the square of the amplification factor in such a scope that makes them so heat sensitive. Letting them cool naturally and then lagging them seems more sensible.

 

Mike 

 

[John]

I've always thought that you want to get the warmer air currents out of the tube as quickly as possible, whether through the tube walls or out the ends ?

 

 

[Mr Spock]

If the scope is already at ambient temperature then a layer of insulation on the tube could stop dewing by preventing radiation/heat exchange.

If the scope has to cool down then anything preventing that is a bad idea.

[Northern Soul man]

Never really thought about this too much I just plonk the scope outside - its kept very near to the outside temp anyway - set up takes a while - then probably come inside, load up Stargazers Lounge !! - read all that there is on tube currents !! - by this time the scope has probably cooled enough !! - but, to be honest, I would have thought the best thing to do is go hunting all the low power stuff - have a tour around the sky for a while - pop back in just to check the updated posts on the Lounge !! - ruin your dark adaptation !! - reply to a few posts on the Lounge !! - have a cuppa !! - go out again !! - after just one more check in at the Lounge !! - Re - visit all the low power stuff, wait for your dark adaptation again, increase power - then just thinking, did I reply to all the posts on the Lounge !! - GOTO Jupiter, ramp up the power and ENJOY !!

Naaaah - not really - just plonk the scope outside put in whatever power I want and just OBSERVE !!

Paul.

[ChrisLX200]

You want the scope to reach ambient, the 'damage' to seeing is done at the optical interface where there's a difference in temperature between refracting/reflecting surface and the surrounding air. If the scope is slow to equillabrate then use a fan to circulate air inside to speed things up.

ChrisH

[Linda]

That is what I used to do: set the scope outside, do other things like having dinner, and then observe. I have been told elsewhere that I don't have to do that anymore, now that I have insulation around the tube.

I did make the last version of thecfoam coat removable, so it might be the best solution to set it outside without foam mat and put on the foam later. The scope is stored inside the house in an unheated room, but it might still be 10 degrees Celsius.

[Linda]

On Cloudy Night forum, I found a post by a KerryR, explaining why to insulate:

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Posted 08 October 2010 - 03:00 PM
<...>
Regarding cooling, I've had great success with the 'opposite' approach: I insulated the tube with a foam camp pad. This reduces the rate at which the tube's top radiates heat to the sky, which in turn reduces the speed of the convective currents in the tube. The result is a more stable image, particularly noticeable when the scope is first brought out for quick looks. It's a cheap and easy 'fix', so it's worth experimenting with. Cooling is the Achilles Heal of Cats, particularly for grab and go, for which the scope in question is ideally suited, and I find the insulation method to make the scope more useable, without quite so much of the need to let it cool for an hour or more (or forever if temps drop fast enough that the scope never catches up).

<...>

So the whole idea is to keep it warm... Yes, then you can use it faster. But I must test in practice whether the image is sharper when the scope is properly cooled.

[Stu]

Insist think the idea is necessarily to keep it warm, but just to stop the convection currents which spoil the view.

I do think there might be something to be said for insulating the scope after it has reached ambient temperature in order to prevent radiation effect as from further cooling it below ambient. This will cause dewing, and I assume also keep convection currents circulating in the tube as the warmer outside air affects thing sin the reverse direction?

Never tried it myself though.....

[ollypenrice]

In the UK the thrust is still towards cooling down. On the continent it has gone the other way. Insulation of SCTs is very popular here. Since I don't use SCTs for anything difficult (only casusal planetary visual) I can't add personal experience. I'd be inclined to try a mixture. During twilight I'd have the scope pointing objective down with the OTA back end open to the air. Hot air rises - and so gets out. (Insects get in as well so some tissue to prevent that woud be logical.)

Once up and running I'd insulate to calm tube currents and slow the cooldown.

I heat our Takahashi FSQ106N astrographs with dew heaters even when I don't need to because it slows cooldown and so slows focus drift. This seems to back the continental approach. If you try to let an FSQ cool down it will literally take all night. In other words it never happens. It will drift all night.

I can see the case for insulation and would not jump to any conclusion based on theory over experimant. I think this does need testing.

Olly

[Linda]

2 minutes ago, ollypenrice said:

 I'd be inclined to try a mixture. During twilight I'd have the scope pointing objective down with the OTA back end open to the air. Hot air rises - and so gets out. (Insects get in as well so some tissue to prevent that woud be logical.)

<...>

I can see the case for insulation and would not jump to any conclusion based on theory over experimant. I think this does need testing.

Thanks for explaining this.

An issue for me that might be relevant here, is that we at times have pretty low temperatures outside. They go along with clear skies. I have been observing at -15 or so. Even when I store the scope reasonably cool, the difference with temperature outside is substantial. More then in France for example.

I will try your method of cooling, including the insect protection. ;-) The challenge with the test is finding similar conditions for observing with or without foam. But I'll give it a try, because I really want to find out. The test should involve 1) a not cooled down scope with foam, 2) a cooled down with foam put on after cooling and 3) a cooled down scope without foam. Test 2 and 3 can be tested on the same evening or morning. For test 1 I would need a similar evening or morning.

 

[ollypenrice]

1 minute ago, Linda said:

Thanks for explaining this.

An issue for me that might be relevant here, is that we at times have pretty low temperatures outside. They go along with clear skies. I have been observing at -15 or so. Even when I store the scope reasonably cool, the difference with temperature outside is substantial. More then in France for example.

I will try your method of cooling, including the insect protection. ;-) The challenge with the test is finding similar conditions for observing with or without foam. But I'll give it a try, because I really want to find out. The test should involve 1) a not cooled down scope with foam, 2) a cooled down with foam put on after cooling and 3) a cooled down scope without foam. Test 2 and 3 can be tested on the same evening or morning. For test 1 I would need a similar evening or morning.

 

We see -15/17C on occasion because of our altitude but you're right, your problem is more extreme. Our scopes are all stored in their unheated observatories, though. We never keep them in the house. Humidity is not much of a problem for us so this is OK.

Olly

[Davey-T]

There was a thread on this a while back, you may be able to search it out.

Dave

[laudropb]

Hi Linda. Do not worry, your English is excellent. Thanks for bringing up this excellent topic. It has given us much food for thought.

[badnit12]

Just a thought - aren't most home made OTAs made from cardboard or plywood? At least I've seen pictures of quite a few, and it's what I'm planning to use.

They're both much better insulators than metal tube, does anyone know if people have this cooling problem with these materials?

[Linda]

21 minutes ago, badnit12 said:

Just a thought - aren't most home made OTAs made from cardboard or plywood? At least I've seen pictures of quite a few, and it's what I'm planning to use.

They're both much better insulators than metal tube, does anyone know if people have this cooling problem with these materials?

If you home make an OTA, then maybe you can build in (an opening for a) cooling fan, like the Intes Micro Alter has:

http://www.apm-telescopes.de/en/telescopes/reflecting-telescopes-ota/maksutov-cassegrain/intes-micro-alter-m715-russian-standard-delivery.html.

[ollypenrice]

21 minutes ago, badnit12 said:

Just a thought - aren't most home made OTAs made from cardboard or plywood? At least I've seen pictures of quite a few, and it's what I'm planning to use.

They're both much better insulators than metal tube, does anyone know if people have this cooling problem with these materials?

If you ask the experts how to cool a Newt they'll tell you that you need to break the boundary layer on the primary mirrror and that the best way to do this is to have an entry and exit fan drawing air across it, so the airflow is at right angles to the optical axis. Years ago there was a Sky and Telescope article about this. And yet hardly anybody ever goes for this solution! I'd give it a try! The point about insulating an SCT is that it is a closed tube system. You cannot usefully insulate an open tube one so good cooldown is essential.

Olly

[badnit12]

Linda and Olly, thanks for your input. So far the plans for my build are in my head, and I'm trying to glean as much information as possible before I start (I'm definitely in the right place for that!).

I'm guessing that a couple of fans similar to those found in a pc would be suitable, or would the high speed of these units cause vibration? (or would they not be needed whilst observing?)

Andy

[brantuk]

There's a great page about mirrors, cooling, and fans here. It's mostly related to Newtonians but I'm sure many of the principles apply to closed tubes as well:

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

Hth

[dweller25]

Hello Linda,

This is a really good question and I think Olly gave the right answer - no insulation at first to allow maximum warm air removal and then insulate to stabilize the inside of the OTA.

I have a 7" Maksutov that has vents at the front of the OTA and a fan at the rear - this allows for cool air to be sucked over the primary removing the warm boundary layer and then be sucked straight out - it is very effective at warm air removal.

BUT - if i run the fans too long the inside of the scope dews up, so I am going to try the insulation idea........

[rwg]

22 hours ago, Linda said:

And another person wrote this legitimate comment:

Quote

I don't see how this would work. Warm air is less stable than cold air so trying to keep the air inside the telescope warm seems to defeat the object of cooling the telescope down to ambient air temperature to stabilise the air in the tube.

 

This statement is not correct - warm air is not less stable than cold air. What causes instability is warm air below cooler air - not the warm or cool air on its own.

So, the way to avoid convection is to either cool everything (particularly the lower parts of the scope) down or to keep the upper parts warm (insulation).

Of course, you can't avoid the cold air, so theoretically the best solution is to cool every part of the scope down to ambient air temperature, but this takes a long time and may not even be possible in some cases (consider an SCT pointing roughly horizontally - the underside will be trying to achieve thermal equilibrium with the ground at say zero celcius, while the top will be trying to achieve thermal equilibrium with the sky above which is distinctly colder, which means that the bottom will be warmer than the top no matter how long you leave it).

In practice, insulation seems to work quite well in some cases - I've used it with an SCT and it really cuts down on tube currents within the scope which are a bigger problem than convection around the scope caused by the scope being warmer than the surrounding air.

cheers,

Robin