Beginner Question - Why let a telescope cool to ambient temperature?

[AstroAccountant]

I've noticed from several posts and websites, that it is advisable to let a telescope cool to ambient temperature before observing.  I am new to using telescopes and was just wondering why this is done?  I have a couple theories:

a) In Maksutov Cassegrain telescopes I've read that it is due to the corrector plate needing to cool, I'm guessing so that it is not slightly bent or pressed against the metal of the optical tube.

How does this help in a Newtonian?  Is it for the same reason?  Does allowing cooling of the primary mirror allow for better collimation?  Is this only necessary with larger parabolic primary mirrors?  Would this type of cooling be advantageous to a smaller (say 6" or less) or a spherical mirror?

Thanks for any advice!

[Schorhr]

Hi,

a mirror is a big junk of glass, and like every material, it will expand / contract a little depending on the temperature.

This will cause problems at high magnification. Plus tube seeing, when warm air rises and causes yet more problem.

SOmething like the open Heritage 130p is cooled down in, say, half an hour (depending on temperature, shroud...), a larger, thick mirror will need more time.

A Mak in a closed tube is better isolated, it will need even longer. Even my small mak needs a long time.. It's best to get it cooled out asap, for example on the balcony, basement...

[CodnorPaul]

In Newtonian it reduces tube currents which form as the mirrors and tube cool down

[John]

The glass changing it's figure slightly as it cools is one issue but I think that the dissipation of warm air currents trapped in the scope tube is a much more significant issue that cooling time addresses. You can certainly see those currents on a de-focussed star image when the scope has yet too cool !

[Moonshane]

the layer of warm air above the mirror creates a weak lens effect and therefore softens focus and contrast until the layer of air is the same temperature as the mirror.

I tend to work on the rule of thumb of 5 minutes per inch of aperture regardless of type but in reality low power (circa 50x or so) observing is possible even with large scopes almost immediately. the possible magnification increases with more time.

[AstroAccountant]

Thanks for the information! 

[ollypenrice]

the layer of warm air above the mirror creates a weak lens effect and therefore softens focus and contrast until the layer of air is the same temperature as the mirror.

I tend to work on the rule of thumb of 5 minutes per inch of aperture regardless of type but in reality low power (circa 50x or so) observing is possible even with large scopes almost immediately. the possible magnification increases with more time.

The boundary layer on the mirror is a big issue and, oddly, nobody follows the research which says that you should use cooling fans blowing acorss the surface. The all blow up or down, which may not be the best way.

Olly

[Moonshane]

agreed Olly. I suspect the reason is that surgery is required for cross mirror flow but not for end of tube fans.

[AstroAccountant]

I'm guessing this is only a real issue in very long focal length scopes with long optical tubes right?  Mine is about 450mm.  Still I'll give it a try this weekend, I'll let it sit out an hour or so before I view Saturn at 100x.

[Moonshane]

not really, it's the mass of the primary mirror or objective that matters. e.g. a 1" thick 12" mirror may cool quicker than a 2" thick 10" mirror.

[John]

agreed Olly. I suspect the reason is that surgery is required for cross mirror flow but not for end of tube fans.

There is an interesting modification sometimes discussed on forums which involves a fan drawing air in at the back of the mirror and a thin baffle around the inside of the tube just above the top surface of the mirror which directs the air flow across the top surface of the mirror.

In my case my 12" dob has a fan but I don't use it as the scope seems to cool reasonably quickly on it's own.

[jetstream]

The boundary layer on the mirror is a big issue and, oddly, nobody follows the research which says that you should use cooling fans blowing acorss the surface. The all blow up or down, which may not be the best way.

Olly

The boundary layer issue and thermal management are interesting,but complicated subjects-along with the micro blurring rigid mounted fans cause.http://www.fpi-protostar.com/bgreer/sep2000st.htm

[michaeldurban]

I just like the feel of the telescope when it's the same temperature

as outside..

makes me giggle..

[RichM63]

But why do we insist on calling it 'Cooling the Telescope'?

Surely more accurately would be 'Temperature Equalization'.

Right now my kit is far cooler inside the building than it is outside. 16° vs 21°

So if I take it outside it needs to Warm Up.

This is even worse if I set up earlier in the day.

Just being pedantic, I understand all the reasons behind, trying to avoid internal turbulence.

[michaeldurban]

normally...(when one lives in the European district), a scope is stored inside..

When weather permits...the scope is carried outside, and therefore the

environmental temperature as opposed to the inside ambient temperature

creates a rather large delta T.

Hence, the scope needs to cool down.

[jetstream]

Aluminum tube dobs can create their own problems with small delta T's,part of the tube can end up cooler than ambient.Truss dobs may be easier to control,but not many use 8"-10" planetary truss's.Once equalized there is no boundary layer,but how often is a mirror truely equalized...or not affected by ground source thermals.A fan is a big asset to decrease cooling times for large temp differences I find.

If you look closely when you think your scope is cooled,defocus and see if any air "crawling" around.A light breeze blowing down the tube can eliminate it briefly,for fantastic views.

[choochoo_baloo]

Most helpful advice; answered some of my queries.

[LukeSkywatcher]

You are all wrong.

The reason we allow our scopes to cool down to the outside ambient temp is to allow enough time for our eyes to become fully dark adapted. The bigger the scope the more time needed.

   

[m37]

I thought it was so we had an excuse to mod our Dobs! 

Is it the same as the wobbly road heat haze mirage thing? Warm currents of wobbly air etc.

[Charic]

Most things already covered here,  but to put in another way if it helps anyone, warm air will interfere and mess up your image, especially at higher magnifications. We allow the telescope to cool in order to reduce/remove this warm air. The warm air you find in the Newtonian telescope is often referred too as the "tube current". This  warm air  effects the light entering the telescope, despite what your looking at has just completed its 3 million light Year journey? and  so effects your final image, as these "tube currents" rise up the inside of the tube assembly, but why and from where does this warm air come from or stored? Its from the heat stored in/on the primary mirror and no-doubt associated fittings, but primarily the main mirror.

Just as our atmospheric seeing conditions are affected by particulates of dust, water, sand and hot and cold air currents and other disturbances, the same is so for the inside of the telescopes optical tube assembly ( OTA ). The amount that light is disturbed is dependent on the airs refractive index, and that index is based on the airs density. which in-turn, depends on the airs temperature. (With me still?)  When you take the telescope outside, there will be a difference in temperatures between the OTA and the outside ambient temperature ( as the warm air is removed, the cooler air is replaced ) therefore what's happening inside the OTA is not stable. As the light passes through these unstable air currents, the images become unstable to almost poor quality as if you had installed a 'Cling-film' lens.  Defocus a high magnification image on a cooling scope, and it looks like a grey kaleidoscope of dancing "wobble"? This seriously degrades high power images. Its fine if I start using my scope straight away with my 32mm EP, but gets worse as I increase the magnification. But to be honest, I've never checked after a session. I'm not certain the "wobble" stops altogether. I doubt it does. But less is far best. 

The area most affected in the OTA is across the face of the Mirror, known as the 'boundary layer' This area can be improved by cross-flow fanning, having both  input and output  porting, to allowing dispersion of the air currents. Again, more modifications that can be achieved by some precautionary drilling and fitting two fans, but fans can cause vibrations if left in use. better to cool then switch of when you think conditions are best.