Created September 2010




Telrad Dew Heater

This is a very simple modification of the classic zero-magnification Telrad finder, the Telrad is a superb finderscope and is an essential part of my deep sky observing with my dobsonian, but the only problem is that it is very exposed sat on top of the telescope and is prone to dew problems. To solve this you can buy a hood and even heaters but all that is needed is a strategically placed resistor and a 12v DC power supply which will gently heat the underside of the telrad viewing glass and keep the optical window free from dew all night long.

The Modification

Firstly remove the top battery cover and the batteries, and remove the 4 screws from the backplate of the telrad, this will allow you to access the inside of the housing, take care not to touch the tiny reticle plate inside

Then remove the glass optical window, I found that mine was stuck quite well into the telrad and a bit of persuaion was required to shift it, just be careful not to break the glass while removing it

Drill 2 small holes approximately 10mm apart from each other and just big enough to be able to poke the wires through, again take care not to damage any parts inside the telrad when the drill goes through

Next step is to solder a resistor onto two lengths of wire, I cant remember what size resistor this is, and I can never can work out the coded bands, so I just tried out some different resistors I had until I got one which got quite warm, but without getting burning hot, with a 12v DC supply

This is then inserted into the two holes and the resistor wire bent to suit the holes, make sure that the resistor wont actually touch the glass, but instead alow the heat to rise up underneath

For the electrical connection, another hole was drilled in the side of the telrad to suit a 12v connector, which again was soldered together to the other end of the resistor wires and secured in place with the locking screws

And Voila!

the unit was reassembled, and a lead made up to connect to my power supply

September 2010