Homemade Anti-dew Heater

by Richard Snow

Schmidt-Cassegrain telescope with homemade dewheaters installed

My solution to my scope's dew problem (and my lack of cash problem) was to make my own anti-dew heaters (all except for the correcter plate which would require more time and effort than I was prepared to invest). The construction is simple and consists of watt resisters connected in parallel to provide even heating. The cost is about $4.00 Can. for each heater if you go to an electronics supplier for your parts. If you must get your supplies from Radio Shack, the price will skyrocket and your selection of parts diminished. All you need is a soldering iron, solder, speaker wire, heat shrink tubing, RCA plug, electrical tape, some proper resisters(see link), a little patience, and knowhow(again, see link). I opted to buy the Kendrick controller (and 10" correcter plate heater) and use a portable 12v power supply(ES2500 booster pac sold at Canadian Tire), but you can use a variable voltage battery pack(sold by Canadian Tire) and follow the design found here: Dew Heater Design

I Followed the instructions at this page and made a few modifications to suit the eyepieces and accessories that I own, but the formulas and basic calculations remain the same.

A basic schematic

Diagram of parallel and series parallel resistor circuits

One of 4 of the parallel circuits to be soldered in series

One of four parallel circuits to be soldered in series

The circuits soldered in series, ready for finishing(always test the circuits for proper resistence and for shorts with an ohm meter before assembly).

Soldered circuit ready for finishing

Place the 3/8 camping mat foam to one side of the circuit and wrap the entire assembly with electrical tape (just enough to insulate the wires and hold it all together and mold the circuit to the diameter of the eyepiece as you wrap or it will never bend to shape when you are finished). Cut the elastic banding to length and attach the hook side of the velcro to one end and include it to the last few inches of the foam so that it is held in place by the tape and sticks out 3-4 inches after you have finished the wrapping. Place the blue heat shrink tubing (optional) over the electrical lead wires ( to the battery or controller), then solder the RCA plug in place. Attach(contact cement) the loop side of the velcro to the outside body of the wrapped foam(insulation out, resisters facing in).

The finished product

Finished dew heater

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