Re: ATM 3-Vane or 4-Vane Spiders

["Mark D. Holm" <mdholm@telerama.com>]

Arjan te Marvelde wrote:

>As Nils Olof points out, the only (passive) way of obtaining a higher vane temp is keeping the
>radiative flow down, i.e. by using a high reflection surface.

A highly reflective surface helps.  Also, more tube helps.  Dry things cool to below air
temperature at night because they establish an equilibrium between the various sources and
sinks of heat.  The night sky represents a net sink of heat by radiative heat transfer. 
The effective temperature of the night sky when the air is clear and dry is quite low,
much below air temperature, even on the coldest nights.  Anything warmer between the
spider vane (or any other part under consideration) and the sky will radiate to the vane
at it's temperature.  Because the nearer and warmer object radiates  more heat than the
night sky, the vane stays warmer.  The principle is exactly the same as a parasol except
that instead of shielding from an object at high temperature (the sun) you shield from an
object at low temperature (the sky).  Of course the end of a telescope has to be open, so
you are limited in what you can do.  Many scopes are now designed with minimal upper tube
structure.  In addition to letting in stray light, this exposes the spider, secondary,
etc. to more of that cold sky.  Baffling is good for more than just stray light.

(Note: when I say "Anything warmer" above that does not include the air.  Air doesn't
radiate much.)

Mark Holm
mdholm@telerama.com