Re: dew on lenses used for long exposures


"Dave" <Dave@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:2iReg.209810$7a.129440@xxxxxxxxxxx
What's the best way to deal with dew forming on lenses that are left
open at night to capture star trails? Is there such a thing as a 'fan',
like a small battery powered hairdryer, to keep the lens dry? Does it
help to allow the lens to cool down to ambient temp before taking it out
of it's case?

Dave
The problem is that there is a 'balancing act' involved here. Any
lens/scope, that is 'looking' at the cold dark night sky, will actually
tend to radiate it's heat out the opposite way to the incoming light, and
end up cooling fractionally below the ambient temperature. Dew forms on a
surface, when it cools below the 'dew point' temperature, for the air
(which varies with the humidity). A 'dew shield', has two seperate
effects. First, it 'narrows' the window of the sky, the lens is looking
at, and slows the radiation a little. Then seperately, it helps to trap a
fixed volume of air in front of the lens, which slows the rate at which
moisture can migrate to the lens surface. The other solution, is to heat
the lens a little, with a 'dew heater'. The problem is that heating the
lens, introduces thermal currents, so ideally you want as little heat as
possible. Using both the dew shield, and a heater together, allows less
heat to be used.
Cooling the lens first, will help reduce the time needed for the system to
thermally stabilise, but will make dew formation worse.
Hair dryer type heaters, are good where they can be used for a short
'burst' between images, but cause too many thermal currents to be
comfortably used when taking long exposured.
Add a dew shield (preferably at least 1.5* the lens diameter in length),
and a dew heater system, and look at getting one of the latter, that has
good control, so you can set it to give the least possible heat, that
solves the problem.

Best Wishes


.

Relevant Pages

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    ... the scope could act like a dew shield, though the secondary mirror would be ... like a small battery powered hairdryer, to keep the lens dry? ... The other solution, is to heat ... the lens a little, with a 'dew heater'. ...
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