ATM Aluminum Dew

[Larry Manuel <lmanuel@agt.net>]

>From: Jack Schmidling <arf@mc.net>
>Subject: Aluminum Dew
>
[snips]
>Speaking of dew, here is an interesting problem for someone...
>
>When I take my 16" Dob out, I leave it out tiil the moon returns.  I 
>cover it with a Desert Storm cover after use but every morning I must 
>uncover it to get rid of the dew on the primary.
>
>I mounted two 15w light bulbs in two of the holes in the mirror cell and 
>have a muffin fan in the third hole.  If I turn on the lights when I 
>cover it there is no dew the next day and I use the fan to cool it down 
>for use.
>
>Clearly, the mirror cools down to below the daytime dewpoint and when
>I open it to the day time air, it dews up.  No problem.  Lights take 
>care of that now.
>
>However, when not in use, I store it in an unheated building with open
>windows where the temperature and air/humidity is essentially the same
>as outside.  The mirror goes through the same temperature cycle, is
>exposed to the same humidity but never dews up.  Well, almost never.  The
>last time I brought it in (in the afternoon) it dewed up the next day
>but never again.
>
>How come?

[not enough snips?]

Jack, et al,


Your assumption that "The mirror goes through the same temperature cycle."
when in the yard compared to in the shed is incorrect.  Here's why:

The clear night sky has an effective 'black-body radiation temperature' of
about -75F, perhaps cooler in polar and sub-polar regions.  This means that
things can easily drop below ambient air temperature in still, dry air at
night.  If there is no breeze to 'warm' things, no dew condensing to supply
heat by the heat of condensation [or sublimation, if frosty], and little
heat being absorbed by radiation from other earth-bound objects, the object
[telescopes are what we're interested in] cools by radiating its heat to
that cold, 'empty' sky.

I've seen dew form on car roofs even before sunset in humid weather.  Ever
notice how frost on cars forms readily on roofs, windows, etc, but rarely on
doors and sides?  It is because the sides of the car are absorbing radiated
heat from the ground, whereas the roof is absorbing little energy from that
clear sky.

So the scope in a shed, while exposed to the same air temperatures, is able
to retain more heat through the night.   The scope's heat, which is radiated
to the clear sky when stored in the yard, is absorbed and re-radiated back
to the scope by the shed roof.  Air temperatures are only part of the whole
thermodynamic energy balance - the balance of radiant energy given off and
received can be very important in some situations, overwhelming air temp
effects.

Some more examples to illustrate my point:

If you sit in a room with nice warm air temps, but substantially cooler
walls, you will feel much cooler than if the walls are warm.

Modern building heating sometimes concentrates on keeping the structure
warm, allowing lower air temperatures within - great, especially if plenty
of ventilation is required.

Alan MacRobert has written in S&Tel in his article: 'beating the dew' about
how dew-free an observing set-up will be if surrounded by trees, buildings,
or even a simple tarp.  This all has to do with controlling the radiative
contribution to the energy balance.  This balance is the algebraic sum of
all energy given off and all energy received.

I read years ago in an article [Sci American] about Iranian traditional
architecture, how ice was often made in the desert by putting thin skims of
water out in troughs - well exposed to the night sky.

I have often thought about this experiment, but not conducted it yet:

Putting a maximum-minimum recording thermometer out under the clear night
sky, surrounded on five [out of six, for a cube] sides by 2" thick
Styrofoam, and discovering just how much lower the temperature of the sky
side of this box would drop, compared to the over-night air temperature.

Hoping this is some help.


Larry Manuel, lmanuel@agt.net, Calgary, Alberta, Canada.