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Why buy a DewBuster™ Controller when cheaper ones will
keep dew off?
There's more to it than just preventing dew. As long as the lens is
slightly warmer than the air dew can not form on it, but if it is any warmer
then the excess heat will warm the air near the lens and cause destructive air
currents in the optical path. You bought excellent optics to see sharp highly
magnified images of objects in the night sky, yet blindly applying heat to your
lens will blur those images and your telescope will never perform at its best.
As humans we have difficulty sensing temperature and we can not rely on "feel"
to adjust dew heaters, if it feels warm then it is definitely too hot. The only
way to be sure we are not heating the lens any more than absolutely necessary is
to accurately measure both the telescope temperature and the air temperature and
then constantly fine tune the power going to the heater strip so that the
telescope is kept just a little warmer than the air temperature. The DewBuster™
Controller does exactly that and it does it automatically leaving you free to
enjoy your telescope
Why does dew form at night?
During the day, when warmed by the sun, the air is like a sponge absorbing
any moisture it comes into contact with. After sunset the dropping air
temperature is like a hand squeezing that sponge and most of the moisture
absorbed during the day is deposited as dew. The air must rid itself of this
moisture so dew will form, it is just a matter of what surfaces it will form on.
Objects which radiate heat will cool faster than the air and dew up first, while
objects which are warmer than the air will not dew up.
Why does dew form on my lens or corrector plate?
As described above, dew can form on any object which gets colder than the
air temperature. Since the corrector plate is thin, it does not have much mass
to store heat and since it is exposed to the night sky it quickly radiates that
heat and cools below the air temperature. A dew shield will reduce the area of
sky that is robbing heat from the corrector plate, but it will eventually cool
below the air temperature and if the air is near the dew point temperature then
dew will quickly form.
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).
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.
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?
Every situation is different so I suggest starting at 1 degree per inch of
aperture and try a lower the setting each night until you find the lowest
temperature that never allows dew to form. So if you have an 8" telescope set
the DewBuster™ Controller to 8 degrees. If you do not experience dew then
try 7 degrees the next night, and 6 the next. The ideal setting is the one that
works every time, so if 5 works all the time but 4 works most of the time then
just use 5 so you never have to worry.
How can the DewBuster™ Controller apply heat without
causing tube currents?
See
Telescope Cooldown for a full discussion.
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.
