Re: (ATM) ATM Super planetary scope musings.
Chuck Grant (grant@aretha.llnl.gov)
Mon, 21 Aug 1995 16:08:47 -0700
> From: manuell@cadvision.com
>
> I've been shopping by 'net for a maker of fine telescope
> mirrors, and believe I have found one. I am starting a
> long term project -- a 10" f/8 Newtonian, a
> super-planetary instrument, possibly with an optical
> window to support the secondary mirror instead of sheet
> metal vanes. This cuts down on the diffraction 'smearing'
> of the image, yielding beter resolution on low contrast
> objects, ie planets. An optical window also seals the
> scope, keeping out crud, and possibly suppressing the air
> turbulence within the tube as the mirror cools toward
> ambient temp. It also leaves the door open for
> experiments in helium* filling of the tube, again to
> eliminate 'tube currents'.
...
> just those rare nights that have excellent seeing. I know
> the helium will leak out and must be replenished each
> session, unless I'm very lucky in sealing the tube.
I too am planning a Newtonian (145cm f/12) scope with sealed,
helium filled tube. Helium has by far the smallest molecule of
substance you might want to use in a telescope. Only atomic
hydrogen or plasma is made of smaller bits, and these aren't
really molecules. Even tiny hydrogen gas (H2) molecules are
bigger than a helium molecule (He).
Because of the tiny size of the helium molecule, it can leak out
of just about anything. All we can hope to do is slow it down.
I plan on using a steel tube, filling it every session, and hoping
it makes it through the night.
My gut feeling on the properties of 50% air 50% helium is that it
would have good thermal conductivity, but the variation of index
of refraction with temp will probably not be much improved.
Chuck