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Cooling of primary mirror

  1. Sep 26, 2012 #1
    As an amateur astronomer I want my primary mirror (diameter 37 cm) to maintain the same temperature as the air. Small fans on the backside do the job. Only, the air cools quicker during the night than the 1.5 cm thick glass.
    Do I obtain a slightly lower temperature on the backside by sticking something wet (cloth?) on the backside, where the fans blow? And thereby cooling the mirror somewhat quicker.
    Wet skin, or wet clothes on my skin feels colder when it blows. But does the same principle apply concerning my solid mirror?
  2. jcsd
  3. Sep 26, 2012 #2
    edit: Read post #3 before #2!

    I can't see water doing anything but insulating it. The glass itself is a much more effective conductor. I'd suggest wrapping the backside with aluminium foil, trying to make as much contact with the glass as possible. I think that would work a lot better than water.
    Last edited: Sep 26, 2012
  4. Sep 26, 2012 #3
    If the humidity is low, then your water idea will promote faster cooling. Google "swamp cooler" to understand why. If humidity is high, then you will have the opposite effect.
    Last edited: Sep 26, 2012
  5. Sep 26, 2012 #4


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    Hi there gloglo

    Welcome to PF

    there are 2 common way to cool the scope and optics before observing

    1) for those fortunate enough --- have the scope set up in its own observatory
    2) for the less fortunate like me, who can build an observatory cuz I rent the property
    I generally take the scope outside several hours before the planned observing time .... ie. before dark... and let the scope and optics cool down naturally along with the surrounding air.
    This has a the advantage that its gradual cooling that doesnt stress the optics and tube assy

    Last edited: Sep 27, 2012
  6. Sep 26, 2012 #5


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    Pointing the scope at a blank section of sky can let the mirror cool by radiation. Just a thought.
  7. Sep 28, 2012 #6
    Thank you for answers.
    I always let the primary mirror cool naturally a couple of hours, before I stow all the parts (Dobsonian) for a short ride to my favourite secret dark observing site, away from street lamps and stray light. A matter of 10 minutes´ride. Then it is assembled, its small fans started, and again it is allowed to cool while I am making time consuming preparations.
    I can not at any time measure the temperature in either the mirror or the thin layer of air just above the mirror surface. But I think I start off with approximately the same temperature those two places.
    However, during the session the air gradually cools a bit. So does the mirror, but slower. An ever so small difference in temperatures mirror / air has a negative impact on the resolution.
    If I could make the backside of the mirror a bit cooler than the surrounding air, I might keep up with the air cooling. I do not want to blow colder air on the backside, since it would slip past the mirror and cool the air in front.
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