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 a DewBuster™ Controller is
being used a dew shield, the reduction in heat loss to the night sky will allow
a lower setting of the controller 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.
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 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, then a 2" heater will fit either. I like to put the
heater strip on my star diagonal rather than the eyepiece because it warms the
diagonal which then heats the eyepiece by conduction and makes changing
eyepieces much easier (for a Dob you can put the heater on your Paracorr).
How do I heat a Telrad since it has a plastic housing?
Heaters which fit the outside of the Telrad are not efficient or effective
because the plastic sides of the Telrad do not conduct heat well. The best
Telrad heaters apply heat directly to the glass.
Dew-Not and Telescope Solutions both sell
heaters of this type. You may also wish to consider
building your own Telrad
heater.
Where is the best location for the heater strip?
Around the aluminum tube just behind the corrector plate casting as shown in
the photo below. This warms the tube keeping the air within from getting cold
and robbing heat from the corrector plate. The corrector plate will then receive
warmth from the air inside the telescope tube. See
Telescope
Cooldown for more information.
If my scope has rails or bars that interfere with
placing the heater strip just behind the Corrector Plate Casting then where
should I place the heater strip?
In many cases the Corrector Plate Casting has a larger outside diameter than
the telescope tube so there is about a 1/16" gap between the tube and the rail
or bar as shown in the upper portion of the diagram below. Try sliding the
thinner Velcro end through the gap first and then work the heater strip through
by sliding back and forth along the rail. To avoid wear on the fabric, just
leave the heater in place rather than removing it after each observing session.
If however there is no gap or it is too small, then you may put washers or
spacers between the rail and the casting as shown in the lower portion of the
diagram below to create a gap. Adding spacers or washers will most likely
require longer screws than those supplied with the rail (short screws will not
fully engage the threaded holes and will strip out). In general the screws must
be longer by the same amount as the spacer or washer thickness you added, but to
be sure you should check the length by looking through the corrector plate while
turning the focus knob and make sure the screws protrude into the scope a little
but not so far that they hit the primary mirror as it moves back and forth. If
the screws are too long you can either use a thicker spacer under the rail or
cut the screw shorter (run a nut onto the screw before cutting and filing the
burrs, then check the threads by unscrewing the nut).
Why not place the heater around the corrector plate
casting or inside the dew shield to warm the air where the dew forms?
The corrector plate does not make very good thermal contact with the casting
because there are spacers between the metal and glass. Heat applied to the
corrector plate casting usually warms the air inside the dew shield which wastes
energy. The air is not captive so it becomes less dense when heated and rises up
escaping into the atmosphere. It is much more effective to warm the telescope
tube because the heat energy travels into the telescope warming the air within
and putting a blanket of slightly warmer air beneath the corrector plate. The
heat then transfers to the corrector plate offsetting radiation cooling by the
night sky. See
Telescope Cooldown for more information.
What is the Temperature Sensor Cable for?
The Temperature Sensor Cable (see photo above) measures both the ambient air
temperature and the telescope temperature so that the DewBuster™ Controller can
adjust the power output of the Temperature Controlled Heater in order to
maintain the telescope slightly above the air temperature as set by the control
knob. For easy attachment, the Scope Sensor clips to the heater strip and relies
upon the heater to keep it in contact with the telescope. The Air Sensor is
located on the end of a brown extension wire which suspends it for an accurate
temperature reading. Since the two sensors are combined into one assembly with a
single 6-foot cable, wiring clutter can be minimized by routing the sensor cable
alongside the heater strip cable and you may tape them together if you wish.
What is the hole in the white plastic clip
for?
If you do not need to remove the Temperature Sensor Cable from the heater
strip then you may wish to lock it in place on the heater strip. This is done by
passing the brown extension wire through the hole in the white plastic clip. The
brown wire is stiff enough that it can be bent tightly to hold the sensor firmly
in place.
How do I attach the temperature sensor to
a heated dew shield?
As shown above, 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. Insure that the Scope
Sensor (two little bumps under the black heat shrink band on the clip) are able
to make contact with the telescope tube.
Why are there two Temperature Controlled heater outputs
for each Sensor Cable?
The extra Temperature Controlled 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 Temperature Controlled output
usually runs at lower power than the Medium Power, so it can be used if you want
a heater to run cooler. Most users will only need one Temperature Controlled
Outputs. You may request that your DewBuster™ Controller be configured with
fewer Temperature Controlled outputs if you would like (or more).
Should I buy or build my heaters?
If you enjoy building projects then making your own heaters can be fun and if
you don't like the results all you've wasted is your time. 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 you would not have bought a
heater for. One particular advantage of
heaters
built with the instruction on this web site is that they produce the
same amount of heat for a given diameter as do commercial heaters. That means
you can mix commercial and home made heaters on your telescope without worrying
about one creating too much heat and the next being underpowered. It also means
that you can build heaters for everything and if you like the commercial heaters
better then your can replace them one or two at a time as your budget allows.
One exception is the corrector plate heater,
commercial heaters are usually thinner than
home made heaters and thus easier to slip between the rail and telescope tube so
many Schmidt-Cassegrain owners opt to buy the corrector plate heater and build
the smaller heaters themselves.
Can I use an RCA splitter to plug 2 heaters into one RCA jack?
Almost all commercial splitters are designed for use with stereo/audio
components and ARE NOT DESIGNED TO CARRY MUCH CURRENT. These are typically
referred to as "shielded" because the signal wire is surrounded by many tiny
wires to block electrical noise. If shielded cables or splitters are used with
dew heaters they may get hot, melt, and short out. If you need a splitter, it is
best to make your own. For small heaters use at least #24AWG wire. For larger
heaters use at least #18AWG wire.
