Do I still need a dew shield with electric heaters?
A dew heater is a supplement to a dew shield, not a replacement. The dew
shield reduces heat loss to the night sky and also blocks stray
light improving image contrast. To be effective, the dew shield should extend at least one
lens diameter in front of the lens. Most
refractors have a built-in dew shield, but if it does not extend at least one
lens diameter past the objective then an extension is recommended. A dew shield does
not have to be expensive either, some of the best are
home made.
When the heaters are controlled with a DewBuster™ controller, a dew shield reduces the loss of heat energy from the corrector plate to the night sky. This allows setting your DewBuster™ controller to a lower temperature without dew forming. This not only saves precious battery power, but also improves optical performance. Many dew shields also act as insulators and when fitted over the dew heater can improve heat transfer into the telescope and reduce power consumption.
Where is the best location for the heater strip?
Around the aluminum
tube just behind the corrector plate casting as shown in the picture below. This
warms the tube keeping the air within from getting cold and robbing heat from the
corrector plate. The corrector plate then stay warm with the heat it receives
through the tube via the corrector plate casting. See
Telescope
Cooldown for more information.
Where is the best location for the DewBuster™ controller's
temperature sensor?
The sensor should be clipped onto the main heater strip (see photo above)
which is plugged into the AUTO heater jack. The Telescope Sensor must rest
against the telescope
tube (see photo above) in order to measure its temperature. The Air Sensor
should be at least a 1/8" away from any object so that it can accurately measure
the air temperature (the Air Sensor is located under the black band, see photo
above). The brown extension wire between the two sensors does not measure
temperature and thus can touch or be fastened to the telescope as desired.
How do I attach the temperature sensor to a heated dew shield?
The brown wire of the sensor should be bent into a U shape so that it will
protrude out the back of the dew shield when the sensor is clipped onto the back
edge of the heated dew shield as shown in the diagram above.
Why not place the heater around the corrector plate casting or inside the dew
shield to warm the air where the dew forms?
Warming the air within the dew shield makes it less dense so it rises being replaced by more cool air. Applying heat energy to the
telescope tube is much more effective because the energy travels into the
telescope to offset radiation cooling by the night sky. See
Telescope
Cooldown for more information.
Should I buy or build my heaters?
If your Schmidt-Cassegrain telescope has mounting rails,
commercial heaters are
usually thinner and can usually slip between the rail and telescope tube. A
popular option is to buy the corrector plate heater and build the smaller
heaters yourself. If you are able to solder then you can easily
build your own heaters. A few dollars worth
of resistors will be enough to build heaters for every accessory you own. This not only saves you money, but
once you see how easy it is to build heater strips, you will probably build heaters for many things that you would not have otherwise put a heater on.
What heaters do I need?
For computerized telescopes, if you don't use your finder scope much then you
may want to just cap it and not use a heater. Otherwise you'll want a heater on
the finder scope objective and possibly another on the finder scope eyepiece
(usually 0.965"). For
the main telescope eyepieces, if you keep them warm in a coat pocket then
you may not need a heater strip, otherwise use an eyepiece heater. If you
have both 1.25" and 2" eyepieces, a 2" heater will fit both. I like to put a heater strip on the star
diagonal too, which helps warm up the eyepieces faster.
How do I heat a Telrad since it has a plastic housing?
The plastic sides of the Telrad do not conduct heat so heaters that fit on
the side waste quite a bit of heat. The best Telrad heater is the
Telescope Solutions Telrad Heater
because it fits under the glass putting the heat where it is needed.
Why three AUTO heater output jacks if there is only one input for
a Sensor Cable?
The AUTO jacks are wired together in parallel. Whichever
heater has the temperature sensor on it must be plugged into one of the AUTO jacks.
The extra AUTO jacks are used in special situations such as when using two
heater strips on the main scope (useful on C-14's) or for a large aperture
guidescope (see next question). The AUTO output usually runs at lower power than
the AUX, so it can be used if you want a heater to run cooler.
Can I plug my guide scope into the second AUTO heater output jack?
Probably, but first find the ideal temperature
for your main scope. After a few observing sessions, try the guide scope on the
extra AUTO jack and if it remains dew free then you are OK.
If it dews up, then you need a little more power so switch the guide scope back
to the AUX jack. If the guide scope has a short dew shield, then fitting a longer dew shield
may allow running it on the second AUTO jack. Do not increase the temperature of
the AUTO just to run the guide scope on the AUTO jack, excess heating will hurt
the main scope's performance much more than it will the guide scope.
Should I use the second AUTO outlet for the finder scope or eyepiece?
No, plug the finderscope and other small heaters into the AUX. The reason is that
the AUX will run at about 40% power even when the AUTO reaches 0% power. This is because these heaters are not thermostatically controlled so
they
need a little extra heat to insure dew does not form.
Will the dovetail bar or side rails of Celestron telescopes interfere with
attaching the dew heater strip?
Many commercial heaters will fit even though the gap may seem tight. Start
by feeding the thin elastic part through the
crack between the telescope tube and the rail.
Once through, slide the heater back and forth along the rail while gently pulling
on the elastic part. Once the thick leading edge gets through, the rest of the
heater will pull through more easily. To reduce wear on the fabric, do not
remove the heater when you store the scope.
If you are building your own heaters you can build it so that the resistors are located in the areas where no side rail is present. Use the same total number of resistors as you normally would but space them closer together so that they will fit in the areas that are not blocked by side rails.
Can I use an RCA splitter to plug 2 heaters into one RCA jack?
Commercial splitters are "shielded" by using many tiny wires to minimize
electrical noise. These tiny wires may heat up and short out if much current is
passed through them. If you use a commercial splitter put it on the your
smallest heaters and make sure the splitter does not get warm after a few
minutes with the DewBuster at full power (unplug temp sensor and turn knob high
enough so DewBuster's red light stays on). Also make sure the heaters get just
as warm as when the splitter is not used. If you decide to make your own
splitter use at least #24AWG wire (#18AWG is even better).
