What does "Cool down" mean?

[Spacecake2]

 People say that cooling down your telescope is good but what does it actually mean and do. How do I cool down a telescope and why should I do it?

Stupid question 😅

Thanks

Edited October 11, 2020 by Spacecake2

[johninderby]

Basicly just allowing your telescope to reach the ambient outside temperature. The glass or mirrors in the scope can give fuzzy and poor images if they are not the same as the outside temperature. Some telescopes take longer than others to reach the oitside temp and some such as SCTs or Maks are particularily affected. 

There are loads of different ways that can be used to achive this but that is a big subject with different opinions. Try searching for telescope cool down.

The. simplest way is just to leave your scope outside for a while before observing. Allow 1/2 hour to an hour for most scopes and perhaps longer for SCTs and Maks. The bigger the scope the longer it takes to settle to the outside temperature. A 127 mak takes about 1/2 hour minimum to settle down.

Edited October 11, 2020 by johninderby

[ollypenrice]

It's probably more accurate to say that cooling the scope down to ambient temperature is one way of ensuring that it is free from internal 'tube currents' which are what do the damage to the image. You'll be familiar with the way mirages work: light rays passing from a hot road surface, for instance, are 'stirred around' by the heat-disturbed air above the surface and produce the illusion of puddles etc. Turbulent warm air inside a scope tube has a similarly damaging effect.  In an open-tubed scope like a Newtonian the only way to get rid of turbulent air in the tube is to let the whole system reach ambient. The primary mirror is a big source of stored heat.

However, when a scope has a sealed tube there is another school of thought on how to prevent internal currents: you can insulate the tube so that all the components within it stay at the same temperature. I get the feeling that this hasn't gained much of a following in the UK but it's quite widely used here in mainland Europe.

In a nutshell, when all the components in the tube are at the same temperature the light rays will pass without disturbance.

Olly

Edited October 11, 2020 by ollypenrice
Typo

[tomato]

A lot of larger mirror scopes have small fans built into the mirror cell to assist with this cool down process. 

[ollypenrice]

4 minutes ago, tomato said:

A lot of larger mirror scopes have small fans built into the mirror cell to assist with this cool down process. 

They do, but for some reason hardly anyone follows the findings of research published in Sky and Telescope years ago.  It suggested that drawing air in from one side, just above the primary, and blowing across the mirror and  out of the other, was the best way to break the boundary layer over the mirror. This strikes me as far more logical than blowing air up the tube from the bottom, as most do.

The most radical solution I've seen came from the fertile mind of Ralf Ottow who made a watercooled primary mirror for his Newt. The mirror has a second glass blank beneath it, attached by a spiral coil of silicone leading from edge to centre. Cooled water is pumped through this for a few minutes prior to observing. The telescope's images are out of this world. Stunning.

Olly

[Nyctimene]

As a rule of thumb, a reflector telescope (Newtonian, classical Cassegrain) will need about 5 minutes cool down time per inch of aperture; a 41/2" approximately 20 min. , a 10" almost an hour. That doesn't mean, that you cannot observe within this time - use low powers to sweep star fields and open clusters, or start star hopping to your target of the night. With progredient cooling, you can use higher powers. Storing your scope at ambient temperature (garage; garden shed; barn etc.) eliminates any cool down problems.

Stephan

 

[tomato]

2 minutes ago, ollypenrice said:

They do, but for some reason hardly anyone follows the findings of research published in Sky and Telescope years ago.  It suggested that drawing air in from one side, just above the primary, and blowing across the mirror and  out of the other, was the best way to break the boundary layer over the mirror. This strikes me as far more logical than blowing air up the tube from the bottom, as most do.

The most radical solution I've seen came from the fertile mind of Ralf Ottow who made a watercooled primary mirror for his Newt. The mirror has a second glass blank beneath it, attached by a spiral coil of silicone leading from edge to centre. Cooled water is pumped through this for a few minutes prior to observing. The telescope's images are out of this world. Stunning.

Olly

Yes, I notice on my RASA 8 that with the enclosed tube that there are some vents in the cell which allow the air to circulate locally. I’m not sure how well the arrangement works, alas the fan is not powerful enough to blow the clouds away.

[newbie alert]

Acclimatising to the outside temp is more accurate term to use... storing a scope inside then taking outside will cause hot air thermals to distort the image, Olly's description of a mirage is a perfect way of showing air distortions..  so the bigger the tube the longer should be allowed to acclimatise,  sealed tube designs such as sct/ mak are far worst affected.  Fans help but when we are using these instrument the outside temp is  also dropping..

[Spacecake2]

The european extremely large telescope shines lasers into the sky and somehow gets nearly perfect vision.😐

[Spacecake2]

I have never 'cooled down' my telescope before. 

22 minutes ago, newbie alert said:

mak are far worst affected

 

 

[newbie alert]

30 minutes ago, Spacecake2 said:

I have never 'cooled down' my telescope before. 

 

Like I said acclimatising to the outside temp is far more accurate..

Assuming you have a mak? Do yourself a test.. set it up and point at a star, de focus on that star and you will see it distort and dance.. do the same test an hour or more later on the same star

[Alkaid]

I try to plan ahead if I can.  If the forecast is good, I put my scope, diagonal and eyepieces in a plastic storage box in the garden, then cover the box with a waterproof bike cover.   When I get home from work, it's perfectly cooled to ambient and all ready to go.

[Gfamily]

This was brought home to me when visiting Mount Palomar (as a tourist) several years ago. There's a small gallery of exhibits that has a view into the dome - and some small vents, through which you could feel a flow of very cool air seeping out of the dome.  Their weather forecast predicts the air temperature expected for that evening, and the dome and scope is pre-cooled to match so as to reduce the thermals when the dome is opened. 

[RT65CB-SWL]

A few nights ago whilst setting up for viewing Mars, Jupiter & Saturn; I could feel the warm air 'escaping' from my ETX105 and C6/SCT-xlt visual backs whist they were cooling down prior to use.