Cooling down

[Citizen]

When I recently purchased my new telescope, and asked the bloke in the shop about cooling down the scope, he said it was not really necessary and to ignore a lot of what I'd already read elsewhere about the need to do this.

Do all scopes need cooling down or just certain types? The lad was very helpful and even though he was obviously trying to sell me a telescope and avoid putting me off buying it, was he right? Already read on here tonight about people getting their equipment cooled down (ooo-er missus).

I did put the scope for 20 minutes or so before using it tonight, as it was freezing, and it had been in a fairly warm living room for several days. How long is a cooling down period and do Celestron Nexstars (SCT) even need to go through the process?

Thanks in advance

[LeeB]

Hi all scopes need cooling a sct with a closed tube will take a while depending on temp change an hour should do it. The larger the scope and the bigger temp differnece the more cooling time. My smaller newt takes a hour but the 10" can be 90minutes or so and believe me a scope thats not cooled is almost impossible to use the image bubbles and shimmers. Its best if you can store the scope cool (not damp).

[John]

It depends on the temperature difference between where the scope is stored and the outside temp. The greater the difference, the more cool down time is needed.

Assuming moving from a house to a normal winters night, SCT's and Maksutov-Cassegrains usually do need at least 30-60 minutes to cool. The larger ones can take longer than that.

Newtonains need a bit less - perhaps 20-40 minutes and refractors less again.

You can use scopes while they are colling for low power views but to use higher powers you need a cooled scope with no warm air currents in the tube is possible.

I think the bloke in the shop was being an optimist !

[Rob_UK_SE]

Totally agree with LeeB.

I have a Celestron 9.25" SCT and it needs a least an hour to cool down and get half decent views. After 2 hours things really start to get exciting. Because of this I have a refractor for those 'grab and go' moments.

SCTs take a longer time to cool as the corrector lens effectively seals off the tube. They do suffer badly from thermal currents if they are not properly cooled.

I find starting with wide field / low mag observations is best and then gradually pushing the magnification when boiling is no longer a problem.

Previously, I have owned an Nexstar 6SE and found that reasonable results can be achieved after about 40 minutes.

A cold place is the best storage option, however, you should bring it in occasionally to ensure it completely dries out and no moisture is settling inside the scope... A dew shield is a must and ideally, a heated strip too.

Rob

[Citizen]

Cheers guys. Rob, I am using the 6SE, so that's useful to know. I don't have anywhere other than the warm house to keep the scope when not being used.

Will get it out for an hour before serious useage in future then. A dew shield is in the post from FLO as we speak

Got some fairly decent views tonight considering the temperature drop and very little cooling down time

Thanks again. This site is going to be invaluable to me!

[Tim]

I store my telescopes in an unheated observatory. The temperature inside is typically only a couple of degrees warmer than outside while the roof is closed, but even then the scopes take some time to cool down to get the best out of them.

Also, if the temperature suddenly drops quickly through the session, you might find the view deteriorates again until the glass/mirror cools at the same rate or reaches thermal equilibrium.

The effects of an uncooled scope on the image can be quite dramatic, I have the images to prove it

Glad you find the site useful

Tim

[todd8137]

at least 10 mins per inch of mirror,i usually leave mine about two hours to be honest or when its still light i cook tea square that away, and then go out

[sanmatt]

"Square that way"?

Sounds like a Military man to me!

[hobbesy]

"Because of this I have a refractor for those 'grab and go' moments."

Do refractors not need to cool down then?

[umadog]

Refractors need a lot less of a cool-down. The air behaves better in their tubes for some reason (probably due to internal baffles) and only passes through the tube once. So it only has one chance to get screwed up by thermals. The boundary layer above a Newtonian primary causes most of the trouble and here the light passes through it twice, so things are worse. In an SCT it's even worse because you have two curved mirrors, the primary is very fast, and a closed tube. The closed tube means they take longer to cool, but also the squat bucket shape they have may promote tube-wide convection currents.

Cooling down definitely makes a difference! How much you notice it depends on the telescope design you have, the size of the mirror (the thickness being particularly relevant, but also the diameter matters), the temperature difference, and what you're observing. You can happily set up the scope and do low power observing with a warm mirror. However, the higher power planetary images can be horrible with a warm mirror.

[Matt 82]

The first night I used my 8" dob it wasn't cool at all. I was in a rush to catch Jupiter before it disappeared behind buildings. I could still see it and 2 bands but it was a good bit blurrier than when the scope is cooled.

[Citizen]

Thanks for the input everyone. I will be cooling down the scope from now on and my dew shield arrived from FLO today.

Clouds are back obviously though

1 half clear night out of 5 so far. I apologise to everyone

[kirkster501]

I take my 11" SCT out of the garage three hours before if its clear and I am working from home.

I live in a reasonable area but need to insure my scope as a named item.

Steve