Do I still need a dew shield with electric heaters?
A dew heater is a supplement to a dew shield, not a replacement. A dew shield helps reduce heat loss to the night sky and can also block stray light to help improve image contrast. To be effective, the dew shield should extend at least one lens diameter in front of the lens and longer is better. Most refractors are already equipped with a dew shield, but if that dew shield 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, some of the best are home made. When a DewBuster™ Controller is being used in combination with a dew shield, the reduced heat loss 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 (see below) 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).
Should I put a heater on my Star Diagonal?
If your telescope is normally used with a Star Diagonal then I recommend putting a heater strip around the diagonal as shown below and then you will not need an eyepiece heater because the diagonal will conduct heat to the eyepiece. Since the diagonal is bigger around than an eyepiece, you will need to use a larger heater. A 1.25" diagonal will probably need a 2" heater and a 2" diagonal will probably need a 3" heater. Another advantage of placing the heater on the diagonal is that the heater does not need to be removed when changing eyepieces. This not only makes it easier, but you won't get dewed up because you forgot to put the heater back on. If you are building your own heaters, take a look at my home-made diagonal heater instructions.
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.
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).
Where is the best location for the heater strip on an SCT?
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.
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 but this air becomes less dense when heated and thus 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 transferring this heat to the corrector plate and offsetting heat loss via 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 (you may tape them together if you wish).
What is the hole for in the Temperature Sensor's white plastic clip?
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 "Y" splitter to plug 2 heaters into one RCA jack?
Most RCA splitters and patch cords are designed for use with stereo/audio components and ARE NOT DESIGNED TO CARRY MUCH CURRENT. These are typically referred to as "shielded" meaning the negative conductor is composed of a braid of tiny wires which shields the center + wire from electrical noise. Unfortunately these tiny wires can't carry much current and if used with dew heaters they will get hot and may even melt and short out. If you need a splitter or extension it is best to make your own using #24AWG (small heaters) to #18AWG (larger heaters) wire.
