Jump to content


Photo

Cool down time

  • Please log in to reply
11 replies to this topic

#1 B Brummell

B Brummell

    Lift Off

  • -----
  • Posts: 11
  • Joined: 04 Jun 2012

Posted 08 February 2013 - 05:26 PM

Hi all,

I used to think that my 12" Lightbridge would cool down in about twenty minutes, but now I think it's much longer and more like an hour.
Does this sound about right?

How long does it take before you're satisfied your scope has cooled down and is it less time for a smaller diameter scope?

Thanks!

#2 tomharri

tomharri

    Messenger

  • *****
  • Posts: 467
  • Joined: 19 Sep 2008
  • Loc: USA

Posted 08 February 2013 - 05:37 PM

6" takes 15 minutes
8" 20 minutes
10" 30 minutes
12" 45 min. to 1 hour
14" 2 hours for 2" thick mirror
These are the times for my scope lineup. 6" to 12" are for 1" thick mirrors.

#3 Astrojensen

Astrojensen

    Fly Me to the Moon

  • *****
  • Posts: 5076
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 08 February 2013 - 05:49 PM

My 12" Meade Lightbridge would take at least a couple of hours from a warm house to a cold (+3°C) night. That was with the fan running. I now leave it out in a shed, so it's never more than a few degrees off. Even so, I need to run the fan continuously, if the temp is dropping by more than a degree or so per hour. Turning on the fan makes a very noticeable difference on lunar-planetary details.

Even so, I used it as a sort of grab 'n go scope, as it delivered okay images after about twenty minutes to half an hour of cooldown. Not super crisp, but the seeing is often pretty poor around here anyway and allowing it to cool down for half an hour gave pretty good images at 100x. I usually started out with 75x and it was almost always pretty good, even with zero cooldown, but 100x was pushing it. An hour or so, and I could use 150x and began to hit the seeing ceiling most of the time. 75x on a 12" doesn't sound very impressive, but it's enough to show a bunch of stuff.

Now since I've got it in the shed and cooldown has been minimized, it has really become a grab 'n go scope to be reckoned with. Throw out the rocker box on the lawn, carry out the scope and plop it down in the box, collimate with the laser, collimate the finderscope and we're ready to rock 'n roll, dobson style! The setup process takes less than ten minutes (more like five) from start to first object in the eyepiece. Not bad!

So, if you're willing to observe in a laid-back fashion at low power, no cooldown is needed. If you want all that the scope can deliver, you better start thinking seriously about thermal issues and cooldown. It pays really big dividends.


Clear skies!
Thomas, Denmark

#4 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43410
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 08 February 2013 - 06:33 PM

Hi all,

I used to think that my 12" Lightbridge would cool down in about twenty minutes, but now I think it's much longer and more like an hour.
Does this sound about right?

How long does it take before you're satisfied your scope has cooled down and is it less time for a smaller diameter scope?

Thanks!


I believe cooldown is mostly related to mirror thickness, thinner mirrors cool more quickly.

With my 10 inch GSO Dob, in San Diego's mild climate, with a fan running, it definitely takes over an hour to reach the point where the scope is thermally stable enough achieve maximum resolution, about 1/2 arc-second. Obviously, this also requires excellent seeing.

I think that mirror is about 1.5 inches thick. My 12.5 inch F/6 has 2 inch mirror, "standard thickness." It has a three fan rig and it takes probably somewhere close to 2 hours to be rock solid.

Jon

#5 GeneT

GeneT

    Ely Kid

  • *****
  • Posts: 12629
  • Joined: 07 Nov 2008
  • Loc: South Texas

Posted 08 February 2013 - 06:35 PM

In the summer, I put my 12.5 inch, F5 Dob in the front yard (away from the sun) about two hours before my drive away time. In the Fall/Spring/Winter, about an hour will do. I keep my telescope inside the house.

#6 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4433
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 09 February 2013 - 07:37 AM

If your scope is stored in an unheated garage, then cool-down is easier. Coming from a warm house to rapidly falling winter temperatures can require 1 hour+ cool down times. If the seeing is good you may not reach the scopes full performance for a couple hours.

Using a fan helps.

JimC

#7 HellsKitchen

HellsKitchen

    Apollo

  • -----
  • Posts: 1135
  • Joined: 05 Sep 2008
  • Loc: Melbourne Australia

Posted 09 February 2013 - 09:05 AM

In Melbourne's mild climate, there isn't a big temp differential between the house and outside, so about 90 minutes with the fans on is sufficient to cool my 12" to ambient. Unlike in more inland climates, the temp doesn't drop like a brick after sundown, rather it gradually "trickles" down so the mirror keeps up quite well. In my experience for example, I've observed near Mildura, Victoria in summer a number of times and daytime highs of 115-120F gave way to lows of 65-70F. Don't have that problem here. The skies are beyond epic there, but cooldown is a hassle!



#8 Achernar

Achernar

    Cosmos

  • *****
  • Posts: 9015
  • Joined: 25 Feb 2006
  • Loc: Mobile, Alabama, USA

Posted 09 February 2013 - 09:14 AM

While my 6-inch usally cools off in about half an hour, my larger telescopes take much longer, an hour or even longer. If the temperature is rapidly dropping all night, they are cooling off all night. That is why I have cooling fans on them, which does help a lot but I still have to wait for an hour to get good views at high magnifications assuming the seeing overhead is good. Usually, it's fair to dowrnright bad in my location anyway.

Taras

#9 GaryS

GaryS

    Ranger 4

  • *****
  • Posts: 379
  • Joined: 30 Oct 2006

Posted 09 February 2013 - 12:05 PM

Hi all,

I used to think that my 12" Lightbridge would cool down in about twenty minutes, but now I think it's much longer and more like an hour.
Does this sound about right?

How long does it take before you're satisfied your scope has cooled down and is it less time for a smaller diameter scope?

Thanks!


The situation is a good deal more complex than xx size mirror takes xx minutes, unfortunately. The critical aspects are the mirror's thickness, the temerperature difference between the glass and the air surrounding it, and the rate at which the night time temperature drops.

I think you would benefit from reading the 2-part article "Beat the Heat: Conquering Newtonian Reflector Thermals," in the ATM section of my web site.

Regards,
Gary

#10 John Huntley

John Huntley

    Viking 1

  • *****
  • Posts: 926
  • Joined: 16 Jul 2006
  • Loc: South West England

Posted 09 February 2013 - 07:58 PM

My Orion Optics 12" F/5.3 has quite a thin mirror and, from the house, takes around 45-60 minutes to cool to ambient. I'm not using the fan, as yet.

#11 B Brummell

B Brummell

    Lift Off

  • -----
  • Posts: 11
  • Joined: 04 Jun 2012

Posted 10 February 2013 - 03:37 AM

Hi Gary

Yep, I've read the article now and liked it very much.

Thanks for that. I feel like I've learned something today...

#12 FlorinAndrei

FlorinAndrei

    Viking 1

  • -----
  • Posts: 867
  • Joined: 28 Sep 2010
  • Loc: California

Posted 10 February 2013 - 06:30 PM

Thin mirrors cool down more quickly than thick mirrors.

Fans make a HUGE difference. Running a fan full speed for an hour (or more, depends on the size) before observation, and low speed during the observation (provided the fan suspension absorbs the vibrations), might be close to ideal.

A 6" might do okay without a fan if you take it outside at least an hour prior, and let it breathe out all the heat. A full-thickness 12" without fans will probably never achieve true maximum performance unless it's in orbit above the atmosphere. :)

Gary Seronik's articles are very informative. But also take a look at Brian Greer's work:

http://www.fpi-protostar.com/bgreer/






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics