Ever had your scope objective dew up? Frustration eh? After it happened to me I went shopping for ready made heaters.
Well… being frugal (i mean really cheap), I just couldn’t bring myself to part with the money they wanted for the heaters. So…. I looked for ways to do it myself. The solution I ended up with: an Electric Blanket!!
Actually, the heating elements from an old electric blanket. I think this stuff is really cool. Within an old electric blanket is something like 70 feet of rubber insulated, finely wound Nickel Chromium resistance wire. I cut up the blanket to get the wire out. The wire has a resistance of about 2 ohms/foot and seems to be about perfect to make low power heaters from.
I decided about 6W of heat would be a good figure to start with, figuring I’d lose 30 to 50% to the outside, so only 3 to 4 watts would get to the scope. So, 6W @ 12v = 0.5 amps, which sounds low enough. Now, since W=(E x E)/R then it would mean I’d need a resistance of R = (12 x 12)/6 = 24 ohms which meant 12 ft of wire would do it.
I also planned to try them first with two heaters connected in series so I’d use half the power from the battery, and put just 1/4 the power to the scopes. It turns out that usually the “in series” configuration is enough to keep the objectives clear. Only on the wettest nights do I have to hook them up as single heaters.
The way I made the heaters:
- Cut two strips of duct tape long enough to wrap around the scope near the objective.
- Lay the first strip on a flat surface with the sticky side up, then lay the wire back and forth lengthwise until I’d used up my 12 ft.
- Put the second strip of duct tape over the wire to trap it between the two strips of tape.
- Hook up some wire leads to the heating elements with crimps (NiCr wire doesn’t solder well) and put on RCA connector on the wire for hooking up to a power supply.
- Wrap the duct tape heater with some scrap strips of the blanket material and it’s done.
This is what it looks like in process of making one these “el cheapo” heaters:
I power with either my usual 12V battery or AD/DC adaptor. I’ve built a little connector box so I can easily use either supply and also hook my heaters in serial or parallel, or just a single unit.
I hold the heaters to the scope(s) with elastic bands and velcro. Here’s a picture of them in use on the “equipment” gallery on my main website.
See the heater bands in use here:
There are heaters on both scopes. Black elastic holding the heater on the SCT, and some white elastic holding it to the refractor.
This project was simple to do and works fine.