DIY Dew Heater - 80% complete and working !

[albedo0.39]

Spurred on by a few other folks getting themselves all prepared for winter with the addition of a dew heater, I set about developing a simple version for own scope.

I preferred the concept of heating the mirror direct, so set about the project with some T0220 case 20w power resistors. However, in the current configuration and using the secondary mirror as the heatsink (glass not being a good conductor) I have derated to ~10w each unit.

Right now, the power is limited yet further to ~ 3.5W / side and in this case, it gives me all the heat to drive away some seriously cold nights.

When connected to a 12v 12AH battery, the power drain will give approx 24hrs continuous usage but I'm sure I will not need more than a few minutes because even after ~5mins, the secondary is appreciably warm to the touch.

I tried to keep everything neat and the power devices are bonded to the glass with a special compound / adhesive double sided tape that assures high thermal conductivity but can be removed in the case I need to remove the resistors.

I built in a low power (and quite dim) LED into the circuit so that I don't accidentally leave the thing on without me noticing and for that final touch, have run the power lines along one of the spiders. Postive on the upper edge and -ve on the lower.

The cable is all designed for temperature extremes and is~<1mm diameter - so less than the thickness of the spider.

To route the power to the U OTA, I taped the 2 wires to one of the truss tubes and covered it in heatshrink to match the other tubes - in effect you cannot see the cable.

The final touch of connection will be made by gold flashed phono plugs and a female captive socket (gold being tarnish resistant and very impervious to the atmosphere). In this way, I can run a connection from the battery, via a joint to the truss tube and via another joint at the top of the truss tube to the heater element itself

I'll post the final pictures as the project completes, but for the moment there are some to keep you entertained. Since the pictures were taken, the cabling has been tidied up somewhat but it gives you an idea.

Total cost so far ? £13 - inc everything.

From the front - note the LED - and no obstruction !

One of the two power resistors

From the side

Here you can see the cable routing along the top / bottom of the spider vane

Incomplete but most of the work is done...the cables leaving the U OTA.

[Doc]

Very nice and well thought out Steve.

[George]

That's a well thought out and neat job well done

[kookoo_gr]

i am interested in copying your diy project can you tell me the value of the resistor?

[astroimpulse]

I love it when people come up with solutions that cost a fraction of off-the-shelf products. Love it.

Great photos too

[darkly]

That's a mod that's going to be well copied, it's got everything going for it, cheapness, drilling, soldering and professional looking outcome - exactly what the modders here love.

It's the little details that make the difference such as the grommet in the hole.

One question, I've never done anything with heaters but, does heating the mirror direct affect the optics in any way?

I'm not trying to critiscize, I really don't know the answer but am interested enough to ask as it looks like a really good solution. I think probably not as I seem to remember reading a review somewhere of a >£12000 scope that I'm sure had heating pre- wired in a similar arrangement - but don't quote me on that.

Nice one and thanks for sharing.