TELRAD DEW HEATER

A "Telrad" viewfinder is an excellent accessory for a telescope. Basically it's a "heads-up-display" for your telescope. Through it, you can see things in perspective with the whole visible sky, right-side up, and not a mirror image. As such, it can be an invaluable aid when setting up and aligning, when slewing a GOTO scope, or when you're using Telrad Charts to find an object. With a Telrad, you know where the scope is actually pointing with a quick glance, as it projects red LED target rings onto the viewing glass through which you can see the sky.

Unfortunately, the Telrad is also very prone to dew formation, with a large, unprotected glass surface! You can buy or make a little dew shield for a Telrad, but after finding Mark Kaye's pages and seeing what he did for dew control, I decided to follow his lead and make a "dew heater" for it, out of a 100ohm, 2Watt resistor.

Follow these steps to install the heater:

slide the glass pane out with a pair of pliers (using a cloth between the glass and the pliers)
remove the mirror end from the telrad, by taking out the 4 corner screws
bend the resistor's leads on a 90 degree angle, about 1/4" from the resistor
set your resistor in front of the round focusing lens (toward where the bottom of the glass sighting pane was) and adjust so it won't be touching when you reinstall the glass pane
using a pencil,mark where the leads will go down through the plastic when the resistor is mounted vertically
take a small drill bit (snug with the lead size) and drill your marked points
push the resistor leads down through the holes, until the resistor is snug against the plastic
underneath, spread the two wires apart and solder insulated wires to whatever plug or connector that you're using with your heater controller (or just to a fused connector that leads to a 12v battery) - be sure to leave a clear path for the light from the LED reticle projector
reinstall the glass plate
reinstall the mirror

Tada! You're done! back | back to main page