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What does "cooldown" mean for your telescope?

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#1 Goldwing

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Posted 14 February 2015 - 06:03 PM

I see the word "cooldown" but not completely sure of it's true definition

 

I remember going to Haleakala on Maui with a 11" GPS Celestron, after going back down to sea level the scope was sweating and blamed it on the humidity?

Temperature change from 37 degrees to 75?

 

Or is it leaving your scope out before dusk to get the scope accumulated to the cooldown in temperatures as the night fell?

 

OK, stupid question :wacko:

Thanks

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#2 nwcs

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Posted 14 February 2015 - 06:19 PM

The basic idea is the same for both photography and telescopes. It's allowing your lens/telescope to reach the same temperature as the ambient temperature. Having a difference in temperatures can cause different issues with getting a good view. Most of the time it's cooldown because you're going from a warmer day into a cooler night but certainly that's not always the case.



#3 Goldwing

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Posted 14 February 2015 - 06:44 PM

Can I assume a lager mass telescope takes longer to acclimate to temperatures then a small scope as the night approaches?

Thanks



#4 russell23

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Posted 14 February 2015 - 06:45 PM

That depends on a lot of factors.  Indoor vs. outdoor temperature difference, scope type, and scope aperture are several of the big ones.

 

One of the reason I like refractors is there are many nights with small windows of clear skies in my area.  I'm better able to take advantage of those windows with a refractor because of the fast cool down times.

 

But there is a large variations in the temperature differential at my location. In the summer there is maybe a 10 minute cool down time.  Last night I put the scope out into -1 deg C temperatures and after an hour the scope could only support 60x magnification.  Seeing might have been that bad too. 

 

I usually try to get the scope out just before dark.  The thing I have to balance in the spring, summer, and fall is putting the scope out with enough time to cool down when it gets dark but not too long before so that I don't lose observing time to the objective dewing up.  On really nice nights when I am not too tired I will bring my scope back into the house and blow it off on low temp with a hair dryer if it has dewed up.

 

Dave



#5 Goldwing

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Posted 14 February 2015 - 06:50 PM

Thanks!



#6 Achernar

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Posted 15 February 2015 - 12:27 PM

Cooldown refers to the telescope cooling down to the ambient temperature, and thus no longer heating the air around it. Until that happens, rising currents of warm air off the optics and other components will degrade the view.

 

Taras



#7 paulymo

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Posted 15 February 2015 - 01:25 PM

Can I assume a lager mass telescope takes longer to acclimate to temperatures then a small scope as the night approaches?

Thanks

 

As Dave implied, it isn't the mass so much as the type of scope.  SCTs like yours and MAKs typically take longer to cool down as a rule than reflectors and refractors.



#8 Goldwing

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Posted 15 February 2015 - 03:23 PM

Thanks all for the clarity of the meaning :waytogo:



#9 sg6

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Posted 15 February 2015 - 05:36 PM

Getting the scope to match the ambient temperature.

But for mine it means it gets put out first, I go get some other bits, leveling the mount, putting it in the start position, going through alignment, remembering to other bits I have forgotten, getting them, deciding what to look at, going to that and by my definition it has, or had better have, cooled down. I do not rush through it all so the time it is cooling is a bit more extended.



#10 Tony Flanders

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Posted 15 February 2015 - 05:45 PM

Can I assume a lager mass telescope takes longer to acclimate to temperatures then a small scope as the night approaches?
Thanks

 
As Dave implied, it isn't the mass so much as the type of scope.  SCTs like yours and MAKs typically take longer to cool down as a rule than reflectors and refractors.


Size and telescope type both make a big difference. If you scale every dimension of a scope up 50%, it will take a lot longer to cool down.

Even within a given telescope type, some are designed to cool faster than others. For instance, reflectors that are open at both ends cool much faster than ones with a solid back end. And some scopes have built-in fans to aid cooling.

Edited by Tony Flanders, 15 February 2015 - 05:47 PM.

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#11 jgroub

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Posted 18 February 2015 - 10:08 PM

I remember going to Haleakala on Maui with a 11" GPS Celestron, after going back down to sea level the scope was sweating and blamed it on the humidity?

No, the scope was just relieved to be down off of the top of a volcano, as any of us would be.  Scopes have feelings, too, ya know!  

 

Okay, kidding, of course.  Everyone else has already chimed in with the correct answer.  



#12 Feidb

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Posted 19 February 2015 - 10:05 AM

Most of the time, I don't really care. Since planets and double stars aren't my thing, I just slap it together, and wait until dark. I have a cooling fan but rarely if ever use it.

 

I usually get there before dark to set up so I can see my mirror spot for alignment. When I do that, I can sit back and watch the sky get dark. By then, the mirror has stabilized anyway and for deep sky, I'm all set. If there are any planets out, I've already checked them out just to gauge sky conditions and even then, if my mirror is swimming a bit, I don't bother with the fan.

 

By the time it's dark enough for deep sky, I can usually go to 390X without any problem and I don't like going above that, even on planetaries because I have to nudge too much, so any magnification below that isn't much of an issue with cool down on the stuff I look at.



#13 russell23

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Posted 19 February 2015 - 10:17 AM

Most of the time, I don't really care. Since planets and double stars aren't my thing, I just slap it together, and wait until dark. I have a cooling fan but rarely if ever use it.

 

I usually get there before dark to set up so I can see my mirror spot for alignment. When I do that, I can sit back and watch the sky get dark. By then, the mirror has stabilized anyway and for deep sky, I'm all set. If there are any planets out, I've already checked them out just to gauge sky conditions and even then, if my mirror is swimming a bit, I don't bother with the fan.

 

By the time it's dark enough for deep sky, I can usually go to 390X without any problem and I don't like going above that, even on planetaries because I have to nudge too much, so any magnification below that isn't much of an issue with cool down on the stuff I look at.

Wow - wish we had seeing that good around here.  Last time out I could not push the scope above 60x - not per inch - just 60x.

 

Dave


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#14 gene 4181

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Posted 20 February 2015 - 01:17 AM

 UPSTATE NEW YORK, Jetstream city, same here.



#15 russell23

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Posted 20 February 2015 - 07:07 AM

 UPSTATE NEW YORK, Jetstream city, same here.

Exactly.  Cloud city too.



#16 johnwarm

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Posted 06 December 2016 - 04:21 PM

I know this thread was last active in 2015 but does this cool down pertain to a telescope in an unheated roll off roof observatory i.e. should I open the roof an hour before I want to start observing?

 

I finished the observatory in October 2016 and just bought and setup on a pier a CGEM DX 1400.



#17 jgroub

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Posted 06 December 2016 - 04:27 PM

It does, but not as much.  There is always going to be a temperature difference, even in an unheated roll off observatory that has received the rays of the sun all day.  However, the temperature difference is going to be much smaller than otherwise, so no, I don't think you'll need an hour before starting to observe.  20-30 minutes should be plenty.  



#18 Redbetter

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Posted 07 December 2016 - 03:23 AM

johnwarm,

 

It depends on how well ventilated your observatory is during the day.  If it is getting quite toasty inside compared to ambient then you will have a hot mirror/etc. that will take longer to cool.  This will likely matter more in the summer than winter.  You probably want to let the thermal mass of the observatory itself cool some before use as well.

 

I've got some cheap battery powered electric temp/relative humidity meters that I used while balancing the HVAC on a multi-level home that had distribution/stratification issues...and some drafts...and missing insulation.  They show the highs and lows for a little over 24 hours.  You could find something like this in the store and put it in the observatory to get an idea for peak temps compared to ambient.   

 

With my big scope in summer it can get quite hot in the garage (and I often keep a meter in there), so I will roll the mirror box into the house to better match night time temps when I plan to take it somewhere that night...particularly when that somewhere is up a mountain where night time temps will be at least another 20-30+ F below the temps in the house even in mid-summer.  It takes a 2" thick mirror some time to cool even with a fan on. 


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#19 sg6

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Posted 07 December 2016 - 04:14 AM

Seems to just mean geting the scope to match the ambient temperature, at least as best or close as you can. Will say that if it takes an hour to "cool down" then the scope does not get that before I start observing.

 

Reason is simply that the choice seems no observing to observing but not at "perfection". Since with my eyes perfection is not going to happen I observe whenever I can. Having slight astigmatism means I can happily think "So what?"

 

Usually the scope goes out, bits gets found and taken out, scope gets set up and aligned and checked then I select a target and get observing. probably the scope gets 20 minutes cool down maybe 30. But I am not going o wait another 30-40 minutes.

 

Having a goto however means that during the late afternoon (still light) I can take scope out, attach power, level it, point North and then cover and leave it until dark to align. Saves trying to do it all in the dark. In this case the scope gets one or two hours cool down.



#20 Jon Isaacs

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Posted 07 December 2016 - 05:55 AM

Seems to just mean geting the scope to match the ambient temperature, at least as best or close as you can. Will say that if it takes an hour to "cool down" then the scope does not get that before I start observing.

 

Reason is simply that the choice seems no observing to observing but not at "perfection". Since with my eyes perfection is not going to happen I observe whenever I can. Having slight astigmatism means I can happily think "So what?"

 

Usually the scope goes out, bits gets found and taken out, scope gets set up and aligned and checked then I select a target and get observing. probably the scope gets 20 minutes cool down maybe 30. But I am not going o wait another 30-40 minutes.

 

Having a goto however means that during the late afternoon (still light) I can take scope out, attach power, level it, point North and then cover and leave it until dark to align. Saves trying to do it all in the dark. In this case the scope gets one or two hours cool down.

 

It does mean that the scope needs to match the ambient temperature to provide the good views.  There are two issues, as the optics are cooling, they can change shape, not a good thing.  Secondly, when the optics are warmer than ambient, there are convection currents in the tube, these are like mini-mirages or bad seeing so looking through these "tube currents" blurs the view at the eyepiece. 

 

How important thermal equilibrium depends on the size and type of telescope.  Small telescopes generally cool faster than large telescopes and refractors are much less sensitive to thermal issues.  Based on my experiences here in the mild climate of San Diego, I think it's very unlikely that a 10 inch Telescope without active cooling (a fan cooling the mirror) will be close to ambient in an hour.  Even with effective cooling, it will take an hour..

 

Certainly one can observe with a telescope that has not cooled down to ambient, with larger scopes it's probably rare that they are fully cooled.  At low powers, the effects are minimized and may not be visible, where it is important is at higher magnifications when viewing the planets and splitting close double stars. 

 

Realistically, your eyes would have to be very poor for the views of the planets and doubles to not be affected by a cooling scope.  Slight astigmatism generally disappears as the magnification is increased because the small exit pupils only uses a small portion of the eye lens..

 

To get the best our of a larger scope, attention must be paid to getting it cooled down.  At the 200 inch at Palomar, they keep the observatory cooled to what they believe will be the outside temperature during most of the night..

 

And ambient, it's a moving target, on clear nights, the temperature drops over the course of the evening so the ability of the telescope to track the air temperature is important.

 

Jon


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#21 dboeren

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Posted 07 December 2016 - 10:51 AM

So you know how if you look at a blacktop road in the hot summer, you see the air above it is sort of wavy?

 

That's what's happening at a smaller scale in your telescope when it is a significantly different temperature than the surrounding air, you get "tube currents".

 

Bigger scopes take longer to equalize than small ones.  Closed scopes (SCTs for instance) take longer than open ones like a reflector where the air can vent out, but the mirror still has to cool too.  Many larger dobs have fans that can blow on the mirror to help cool it faster.

 

I have a 12" truss dob.  It cools fairly fast for its size because the truss makes it more open and I take the tube cover off too so it's super-easy for air to escape.  Therefore, the mirror is the main limiting factor.  If I'm in a hurry I use the fan, but most of the time I just set the tube out on the porch an hour ahead of time.



#22 astrosky123

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Posted 07 December 2016 - 11:24 AM

Can I assume a lager mass telescope takes longer to acclimate to temperatures then a small scope as the night approaches?

Thanks

yes



#23 astrosky123

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Posted 07 December 2016 - 11:27 AM

also 2nd part to that is a triplet will also add alot more time for cooldown or maks since they have a thick front corrector. i had a 7" mak and even with vents and powers airflow its took 2 hrs to good views but 3hrs is where it was amazing views for me. so its doesnt have to be huge scopes even in medium scope cooldown can take awhile.



#24 aeajr

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Posted 08 December 2016 - 11:14 AM

Others have defined the term and time for them. No need for me to rehash.

I have no cooldown time issues as my equipment reside in an unheated garage so it is always near ambient temperature and ready to go.

#25 mike89t

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Posted 08 December 2016 - 04:52 PM

So you know how if you look at a blacktop road in the hot summer, you see the air above it is sort of wavy?

 

That's what's happening at a smaller scale in your telescope when it is a significantly different temperature than the surrounding air, you get "tube currents".

Yeah I read about this when I did my research for a scope.  However I didn't think it would be that noticeable. 

 

Then I took my new scope out for a quick view of the moon on one of the first nights that I used it and clearly could see the "heat waves" or "tube currents" while looking through the eyepiece.




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