Homemade Telrad and Eyepiece Dew Heater


Electrical modifications and construction of electrical accessories can be dangerous. Do not attempt such activities unless you are qualified to do them safely, without injury to yourself or your property.  Anyone not qualified for these activities should secure the help of a licensed electrician. It is always recommended to consult with the appropriate manufacturer prior to modification of any equipment, as this may void warranties and/or alter the equipments performance.

Telrad and Eyepiece Dew Heaters

Purchasing Telrad and eyepiece dew heaters will cost about $50-$70, depending on the model and electronics options. I made mine from a pair of battery operated hunting socks that I purchased from Cabela's for about $25 (below left photo). I used components from this sock because it uses a low amperage to eliminate the possibility of electrical shock.

Each sock contains a 10 cm strip of coiled, high resistance wire (most likely nichrome wire, but I'm unsure about this) in each toe.  The wires were easily removed without damaging the socks, and the socks were retained for cold starry nights. The high resistance wire is packed in cloth tape and insulated on one side (below right photo).

I set one of the two heating wires aside and did not remove it from the cloth tape (this wire was used for the eyepiece dew heater).  I carefully removed the other resistance wire from its cloth tape packing (this wire was used for the Telrad dew heater). The below left photo shows the resistance wire after removal of the cloth tape. The below right photo shows the same wire after straightening, re-soldering to the copper wire, and insulation of the soldered connections with heat shrink tubing. 

I cut a small piece of 3mm polycarbonate to the same width as the Telrad window. I filed two grooves into the polycarbonate and glued the wires into place (below left photo).  The below right photo shows how the heating element installs into the Telrad. The heating wires are bent upward, so they give direct contact with the glass Telrad window. I will probably purchase a small clamp to clamp the polycarbonate strip against the top of the Telrad window. Total fabrication time was only 1-2 hours!

Following are a few additional photos that show the heating element installed in the Telrad.

The eyepiece heater is much simpler. The resistance wire that was not removed from the cloth tape is just wrapped around an eyepiece. The below left photo shows a first test. I will insulate the side facing away from the eyepiece, pack the resistance wire in a waterproof-electrically insulated outer shell, and add some Velcro to make attachment to the eyepiece easier. Alternatively, this resistance wire could possibly be used to make a secondary mirror heater. The below right photo shows the complete Telrad and eyepiece dew heater system. Only about 1-2 hours fabrication time and approximate cost $25.