Telrad Dew Heater

Hi All,
After a real scary incident with my dew shield for the telrad, I decided to ditch it and make up a resistor strip for it instead.
The dew shield fell off the telrad and bounced off the cover protecting the mirror, if the cover hadn't been there it would have landed on the mirror. It fell from 8 feet up so it was going some speed as it literally bounced off and landed about 10 feet away.
Anyway, this heater is a simple thing to setup, here is a pic of the resistors in place.


What I have are 5.4 ohm resistors, about 10 of them, 6 are visible and 4 are buried in the white heat sink paste, this past is thermally conductive so it transmits the heat directly to the glass plate. These resistors are 2 watt, so are well able to take the heat.
This gives out about 3 watts of heat, well enough to keep it from dewing over.
I originally had less resistors in there and it was giving out nearly 5 way this is too much as the glass was getting hot, not what you want as its a waste of power.

The thing runs of the scopes power which I run up one of the truss poles.

While at it, I converted the red led in the telrad to also run off the scopes power, so there is now a switch for the heater and one to switch the telrads led over from normal batteries over to the scopes power.


This pick also shows the original resistors which were too big and blocked the fov, they didn't look so big before I put them on.

I also plan to run the laser pen power off this when I get the time to do it.
Comments suggestions and feedback are welcome.

could you post your new wiring diagram for the telrad?Im know its not that complex, but im not the best when it comes to these things. I love the idea of being able to power it off the scope's power.

do you plug the telrad into the dew controller, or how do you do it?

Great post on the dew shield. Im sick of the telrad dewing over. if im ever stuck in the desert, as long as i have a telrad, and a corrector plate from an SCT i will never die of thirst!

I didn't know there was such a thing as a Telrad dew heater. What kind have you got Dave, and do you actually get much problems with dew with it (before the heater, of course).

You had a lucky escape there with the mirror cover being on.

Seanie.

I've a Telrad, and it's the first thing to dew up, and quite badly.
I keep intending to get a heater for it, but the gaps between observing sessions are so long that I always forget!

do you plug the telrad into the dew controller, or how do you do it?

I can have it that it runs off the scopes power directly, but I had a spare type4 kendrick controller so I'm using that now.
Its convenient as it pulses the telrad light and heater since its the same power.

I didn't know there was such a thing as a Telrad dew heater. What kind have you got Dave, and do you actually get much problems with dew with it (before the heater, of course).
Seanie.

Absolutely, my telrad glass plate fogs over to the point where the water is dripping off it, its a disaster really, it doesn't last more then an hour or so.

You had a lucky escape there with the mirror cover being on.
Seanie.

I nearly died of shock before I remembered that the cap was on it!! :shock:

Anyway, here is the circuit diagram, its very easy to make. Just make sure that all the switches and everything else is not in the optical path of the telrads reticle, that's why the switches and connector are so far up near the top of the telrad.

farm3.static.flickr.com/2244/2232448723_3b9b019f7e_o.jpg

I had the diodes handy already so I used them, you could use a small regulator instead, it would take less space also.

Hi Seanie,
Lots of people sell Telerad Dew heaters. Kendricks have a Premium and a Firefly version. Ron Keating of Dew Buster fame endorses an alternative style one from his web site.
Like the previous poster, my Telerad dews up first.
Have a new Premimus one myself and so far so good.

Regards

Donal