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  • Lubos Kuzma
    What is the sign of the mirror not having the same temperature as the outside? (I mean before the temperature stabilizes) What difference do you see? And by
    Message 1 of 2 , Apr 2, 2007
      What is the sign of the mirror not having the same temperature as the
      outside? (I mean before the temperature stabilizes) What difference do
      you see? And by how much does the fan cut the time for mirror to
      stabilize?
    • jeff_swick
      from Rick who is still behind a corporate firewall: Remove the eyepice from the holder and look down through the holder at the mirro at a decently bright
      Message 2 of 2 , Apr 2, 2007
        from Rick who is still behind a corporate firewall:

        Remove the eyepice from the holder and look down through the holder at
        the mirro at a decently bright star.. If you see swirling turbulance,
        the mirror has not cooled down enough. (Distinguish this from lots of
        scintillation, which could be a warm mirror or a bad sky - shadowed
        swirling turbulance all across the mirror is alway 'tube currents' due
        to a warm scope).

        What difference do
        you see? And by how much does the fan cut the time for mirror to
        stabilize?

        Glass cools at a rate of 1" thickness per hour to reach ambient (a 1"
        thick mirror takes 1 hour, 2" thick - 2 hrs, etc). This is a
        guideline and it depends from how warm to how cold. It is best to
        store your scope in a shed so the temp is always near ambient. Taking
        scopes in and out of the house assures a long cool-down time, and also
        assures condensing moisture on the optics at some time,which helps to
        degrade mirror and lens coatings. Quick temperture changes during
        observing periods also creates turbulance; settling to temp can be
        done quicker with a fan if you see seeing change abruptly and stay
        bad. Scopes should also be designed to have air flow along the tube
        wall and past the mirror - ie - not a closed tube for Newtonians, some
        venting for Cassegrains, but this does not happen in Schmidt Casses,
        and some close the backs of their Newts (which they shouldn't do), but
        if having air-flow past the mirror, you have to baffle the gap between
        the mirror and wall somehow, so your eyepice doesn't see the snow on
        the ground past the mirror edge. :-)

        rick



        In rascstoon@yahoogroups.com, "Lubos Kuzma" <lubos_kuzma@...> wrote:
        >
        > What is the sign of the mirror not having the same temperature as the
        > outside? (I mean before the temperature stabilizes) What difference do
        > you see? And by how much does the fan cut the time for mirror to
        > stabilize?
        >
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