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Home made Dew Zapper!


Astroman
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To make a home made dew zapper-you want to generate about 15 to 20 watts of power around the circumference of your telescope corrector plate or mirror. Warm, but not too hot. At 12 volts, this calls for a total of about 10-12 Ohms resistance. You’d like it spread as evenly as possible about the corrector plate, so you’ll fashion the resistors in a ring that matches the circumference of your scope. Buy as many 1W to 5W resistors as will fit in a ring. Divide the number needed into the total resistance. For example, my 8” SCT has a circumference of about 25 inches. I need 12 Ohms to make 10 watts. Each 1 watt resistor is a little under an inch long, so I need 25 0.5 Ohm 1 watt or 12 1 Ohm 2 watt or 6 2 Ohm 5 watt resistors. You want the resistors to meet or exceed your power requirements, so 5 watt resistors work well.

Connect the resistors in a string that will be long enough to wrap around the glass in question and solder them together. Trim off any excess lead. Buy some heat shrink tubing, available at the same place you got the resistors, that’s large enough to enclose the resistor string. Form the whole thing into the ring of correct size, solder some connecting wire to each end of the resistor string to connect to the battery and head the shrink tube with a heat gun, lighter or hair drier. Seal the end with electrical tape, so the resistor ends don’t short out. I added a piece of hanger wire to the end and taped it up, too. This helps keep the ring in the desired shape.

Connect the wire end to a car battery. In a minute, you should feel the heater working.

This arrangement worked well for me. It didn’t drain my battery much, even after an all-nighter. Any questions, just post or email me.

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I think that post will be very useful for people here in the UK where dew is a big problem - especially if you're a SCT owner like we are.

I took the posh route when I first got my scope back in '91 & bought a Tuthill heated dew cap. Still does the job 14 years on......... :)

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I wouldnt mind one for the small refractor the ST102 last night it became heavly dew'd after 2 hours.

Astroman would this system of your be ok for a felt lined dew cap? worried the heat would somehow burn the felt...

Sounds a Great system Astroman!

James

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I made a lens hood for my 300D and was thinking about making a heated ring for that.

So that gives me a sound footing to start, whould resistace wire be any more usefull?

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James: This system will work well with the felt lining. It doesn't really get "hot"-just slightly warm. Enough to prevent condensation, but not really heat things up much.

Ant: Resistance or nichrome wire works too, but it takes a bit more work. You have to somehow measure the resistance in the length of wire to come up with the same total resistance in the individual resistors to get the same power. Insulating each turn of wire saps some of their efficiency, but it does work.

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