Re: [ATM] fans

[Mark Holm <mdholm@telerama.com>]

Jerry Reddell wrote:

>One thought is that it probably does significantly alter the radiation
>characteristics (emissivity ?) of the front of the mirror, making it more
>reflective (less receptive) to radiation energy arriving while reducing the
>ability to radiate the heat from the mirror to deep space. This reduction of
>radiation to space could possibly explain the difference in measured
>temperatures of the front and back.
>
>  
>
It may well play a part.  It is well known that polished metal surfaces 
have low emissivity.  Since most of the sky is significantly colder than 
most telescope mirrors, a mirror's emissivity will dominate the exchange 
of energy in that direction.  How much it absorbs is of relatively 
little importance since the sky emits so little.  All objects at normal 
Earth surface temperatures, open to the night sky, will lose energy to 
the sky.  A shiny object loses less.

The back side of a mirror typically has both higher emissivity and 
higher absorptivity than the front.  This makes it likely to be in 
better  thermal equilibrium with the objects it faces.  Since the back 
of a mirror usually faces the ground, the mirror back side will tend 
toward ground temperature.  This tendency will be stronger than the 
tendency of the front of the mirror to cool to sky temperature because 
of the emissivity and absorptivity differences.

In many situations, we don't think much about radiative heat transfer, 
largely because convective and conductive play a larger role in many 
situations.  Under the open sky, on a still clear night, radiative 
effects are more noticable.  It is possible to largely interrupt 
radiative heat transfer with a simple device: the parasol.  A thin, 
radiation barrier with lots of surface area, allowing it to readily 
exchange heat with the air by convection and conduction, greatly reduces 
radiative heat transfer.  This is the whole working mechanism of an 
unheated dew cap.  One part of a telescope that can benefit from this 
approach is the reflex finder.  A Telrad, or similar finder can be kept 
free of dew for longer, if it is shrouded, and the sky opening of the 
shroud closed when the finder is not in use.  The best finish for the 
shroud, as well as all other sky facing radiation interrupters for night 
use, is shiny metal.

I have written before, that one should have a small parasol, with a 
shiny metal sky facing surface and a black scope facing surface, on the 
sky end of the diagonal mount.  It will help keep the diagonal from 
cooling below air temp and delay its dewing.  The parasol needn't be 
curved, a flat one would work well in this application.  Curvature in 
parasols is largely a structural thing.

Spider vanes, especially ones in scopes with little tube extension, 
might profitably be made shiny.  Most black finishes are pretty 
reflective anyway at the low angles that matter for this application.  
Why not go all the way and make them shiny?  Of course, the wire spider 
is probably a more effective cure for the temperature problems of spider 
vanes.

-- 
Mark Holm
mdholm@telerama.com


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