Newtonian Cool-Down Times?

[DeejayP999]

I'm considering buying a Skywatcher 150PL, having previously mainly owned refractors, and I was wondering what the cool down times are like?

Obviously, I know it won't be as fast as a refractor but that's OK as long as we're not talking hours and hours!

Any comments appreciated. Many thanks.

[Nexus 6]

Hi DJ.

I think you may need to leave for around half an hour at least, I usually leave my 925 cooling down for around an hour, I just open the conservatory doors and so don't have to worry about a passing shower.

Alan

[NGC 1502]

The cooldown time will depend on the temp at which the scope is stored.

If you can store it other than in a heated room, that will help.

If you put the scope out to cool, then by the time you have gone back in for

your extra layers and get your eyepiece box etc, then you can use it at low

powers straight away, while waiting for full cooldown, max 45 mins at a guess.

Best regards, Ed.

[reddoss]

The rule of thumb for reflectors is 5 mins per inch of aperture to cool down. So 30 mins for the EXPLORER-150PL.

[Mikea]

I didn't know that. That's the first thing I've learned today - thanks reddoss!

[Priastro]

5 mins per inch. If there is a fan behind the primary mirror it should be a shorter cool down time or is the recommendation taken with the respect to the fan on? I have read that it is negative to have the fan on when the mirror has reach the ambient temperature.

Martin

[Patbloke]

I didn't know that. That's the first thing I've learned today - thanks reddoss!

Oops me also... Great site this! I shall remember that next time which will be as soon as these skies clear... However, it's not looking good for tonight me thinks

[Moonshane]

I'd agree with the approx 30 mins on average for a 6" scope but I have found that with my f11 6" dob (which has a 1600mm tube) the cooldown seems a little longer than that. Cooling starts as soon as you take the mirror outside so by the time you have set up, collimated, found your eyepieces, got dressed for outside, decided what your targets are and done a cup of tea, you won't be far off. I often use my bins to have a scan about while the scope is cooling - helps with getting yours eyes adapted to the dark too.

As has been said though you can use low powers quickly.

I tend to use a fan and then turn off when I get to ambient. I cannot really see a big difference in the image with a rear fan only. I feel the side fan on my bigger dob helps more with this but remember that if you run a fan too long, the mirror can (potentially at least) become cooler than ambient which may lead to fogging - unlikely generally but when it's humid, certainly possible.

[DeejayP999]

Many thanks everyone. 30 odd minutes doesn't sound too bad at all.

[Rob_Jn]

Its a bit more complicated than a set time. It depends on the rate the ambient temperature is falling and whether or not you have fans fitted. This is especially true for larger mirrors. There is an excellent calculator here Telescope Mirror Cooling Calculator Check out the links at the bottom of the page also.

Rob.

[LukeSkywatcher]

I let my 130P cool down for the same amount of time as it takes my eyes to become fully dark adapted. This is usually 30-40 mins.

I spend this time scanning the skies with my bins.

[Parus major]

My 150PL took about thirty-five minutes to cool from about twenty-two to about minus ten. I used to take the mount out and set up which can take the same time and then put the tube on to cool but I've found taking the tube ount and laying it on a three folded picnic rug to keep it from damage but in the cold. Having it cool while you fiddle with the leg length, levelling and polar aligning means you can get to business right away.