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Why not set the DewBuster™ controller based upon the dew point?
Regardless of how low the dew point may get, your telescope performs best
with the optics at the same temperature as the air around it. For this
reason the minimum setting of the DewBuster™ controller is zero degrees
(telescope at same temperature as the outside air) so that the telescope will
not get colder than the air. For humid conditions the
temperature control knob can be set higher to maintain the telescope warmer than the outside
air temperature. This is needed because some heat loss occurs between the
telescope tube where the heater strip is located and the corrector plate. So
unless the telescope tube is kept warmer than the air temperature, the corrector
plate may fall below the air temperature.
Doesn't it waste power warming the corrector when the dew point is well below
the air temperature?
A DewBuster™ controller set a few degrees above 0 only applies a small
amount of energy to keep the telescope tube at the air temperature, much less
than the lowest setting of most controllers. The small amount of energy used is
well worth it to keep the corrector from cooling below the air temperature and
risking dew formation. It takes much less energy to prevent dew than to dry it
up after it has formed.
Why does dew form on my corrector plate?
Air absorbs moisture like a sponge. The amount of moisture in the air
is its humidity which can be specified as dewpoint or Relative Humidity (RH).
When air is warmer than the dewpoint its RH is below 100% and it can absorb more
water. As air cools its RH increases until it reaches 100% and can hold no more
water. If the air temperature drops further it can not hold all of its moisture
and must rid itself of the excess. We call this the dewpoint because the excess
moisture will condense on surfaces (dew) or on particles in the air (fog).
During the night your corrector plate cools to below the air temperature so it
will reach the dewpoint before the air and dew will form on it first.
How can my telescope get colder than the air temperature?
When you first bring a warm telescope outside, the air around it immediately
starts cooling it off because heat travels from warm objects (scope) to colder
objects (air). The greater the temperature difference the faster it cools, so it
may cool 3/4 of the way in the first 5 minutes, but take half an hour to cool
the rest of the way. After that, radiant cooling by the night sky (cold object)
will continue cooling the telescope (warm object) until a balance is reached
where the air (warmer than scope) gives up heat to the telescope to replace what
is being lost through radiant cooling by the night sky. The telescope tube will
also drain heat away from the corrector plate and radiate it to space. A balance
is point is reached with the telescope a few degrees cooler than the air
temperature. The balance point is affected by wind (helps warm scope) and sky
transparency (increases radiant heat loss).
My telescope does not have a metal tube, will the DewBuster™ controller
work with it?
Newer Celestron SCT's use Carbon fiber tubes. While not as thermally
conductive as aluminum, they do conduct enough heat to use a DewBuster™
controller in the normal fashion (heater just behind corrector casting). Plastic
however will not work since it is an insulator. Some economy refractors and finderscopes use plastic lens mounts but the telescope tube is metal. In this
case the heater strip should be place around the metal tube so that it warms the
air within the tube. The lens will be kept warm by the warm air within the tube.
What temperature should the DewBuster™ controller be set at for my telescope?
Start at 10 degrees and lower the setting until you find the lowest
temperature that never allows dew
to form.
How can the DewBuster™ controller apply heat without causing tube currents?
See Telescope
Cooldown for a full discussion.
Can the DewBuster™ controller handle a 14" SCT?
Yes. Click HERE for more
information.
Where would be a good place to mount the DewBuster™
controller?
One popular location on all SCT's is on the optical tube assembly. The
advantage is that the heater wires don't have to flex as the scope
moves, so the only cord you have to worry about is the DC power cord. The power
cord is 6 feet long to allow flexibility in routing. On fork mounted SCT's a
popular choice is to mount the DewBuster™ controller on the fork arm. If you
chose this location, run the heater cables through plastic cable wrap to keep
them organized and out of the way of the moving parts of the telescope.
Radio Shack 278-1654 Split Tubing works well.
