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Re: [OM] Freezer Film!

Subject: Re: [OM] Freezer Film!
From: Kennedy <rkm@xxxxxxxxxxxx>
Date: Fri, 10 Jul 1998 01:48:22 +0100
In article , Pauls0627@xxxxxxx wrote
>In a message dated 7/9/98 4:27:39 PM Eastern Daylight Time, image66@xxxxxxx
>writes:
>
><< BTW, anybody got any solutions to fogged lenses when shooting sunrises?  If
> your equipment is cold (as it would be if you started shooting while still
> dark) it will fog up and moisture will condense as the morning air warms. >>
>
>Actually, the problem of fogging (dew) occurs because the temperature dips
>BELOW the dew point, and since it's not only darkest, but also coldest, right
>before the dawn, condensation is most likely to occur at that time. This is a
>common problem encountered by amateur astronomers, who use things called dew
>caps over the front elements of their telescopes to prevent it. Essentially
>they are just long lens shades. I'm not sure I understand the physics, but I
>think the long cap prevents air circulation across the front elements. 

Partially the reason, but the main effect is that the mirror or lens
surface is radiating in the infra-red region according to Planks law
into 2Pi steradians, and receiving black body IR radiation from the same
angle but at a much colder background temperature - hence it is
radiatively cooled reaching an equilibrium between the emitted and
absorbed black body radiation temperatures - forming condensation
surface if its temperature falls below the dew point.  The use of the
lens hood limits the solid angle which the lens is absorbing IR from the
cold sky temperature and replaces it with black body IR from a surface
closer to ambient - consequently the radiative cooling is much reduced
and the probability of reaching the dew temperature reduced.  Instead of
the lens or mirror forming the condensing surface, the hood does
instead.  

It's the same effect as parking a car next to a wall on a moderately
cold night - condensation forms on the side away from the wall but not
on the side next to it - the car is radiatively cooled on the open side,
but not nearly as much on the side next to the wall.

Incidentally, lens hoods should be light coloured on the outside and
black on the inside to reduce their radiative cooling - but I don't know
of many that are, although professional telescope housings usually are.
-- 
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers         (replace 'nospam' with 'kennedym' when replying)

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