RE: ATM helium filled OTAs

["Antonio de Ugarte Postigo" <ugarte@mad.servicom.es>]

Hi Peter,
    I think that the idea of making vacuum makes much more sense for a small
scope than filling it with helium. For solar telescopes making vacuum in
them is a very common practice, and it is usually cheaper than introducing
helium. I've read about a proyect of a solar telescope that was going to be
filled with helium, but this was becouse it was going to have a very big
mirror (~2.50m) and it would need a very thick window so it didn't bend with
the vacuum.
    The vacuum solar telescopes usually have a pressure of a few mbars in
them. I don't know how would this pressure afect the shape of a typical
corrector lens.
    Anyway, this can be a useful technic for a solar telescope which has to
stand abrupt temperature gradients in its interior and also has a long focal
lenght. For night scopes I wold try to find easier solutions.
    Hope this helps,

Antonio de Ugarte Postigo
-----Mensaje original-----
De: Peter Hirtle <notpete@scn.org>
Para: atm@shore.net <atm@shore.net>
Fecha: lunes 9 de noviembre de 1998 23:08
Asunto: Re: ATM helium filled OTAs


>
>In a book on high vacuum I noticed that helium has about 10 times the
>thermal conductivity of air.  That led me to infer that a vacuum of 1/10
>atmospheric or better should be comparable to helium filling.  It would
>also be easier to maintain.  The logic here was probably all goofy but
>maybe it is worth more thought.
>
>Peter
>
>
>--
> Peter Hirtle   notpete@scn.org   Seattle, WA.
>
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