Home Made Dew Heater
After putting up with ending several observing sessions early because of the inevitable dew problems, I decided to reduce my frustration in this area. Having researched several different home-brew and commercial solutions, I decided to build my own.
The Kendrick and Orion systems looked attractive, but were expensive and not available locally. Most of the home-brew systems involved soldering a number of resistors together and this was too "fiddly" for my liking. After searching for sources of resistive wire etc I hit upon the idea of using the heating element of an old electric toaster. This would give me the best of both worlds - a neat, effective end-product, yet inexpensive to build and easy to use.
The objective of this project was not to not to "heat up" the corrector plate on my telescope but to replace the heat lost due to radiant heating of the glass to the night air. A large glass corrector will quickly radiate its heat away causing it to drop below the current air temperature. When the heat loss is great enough that the temperature of the glass reaches the dew point, dew will form on it, and will have to be removed by some means. One way is to use a hair dryer but this can cause hot spots on the corrector plate and distort the image. It also means having to keep an eye on the corrector plate to determine when the dewing was taking place. A dew cap offers limited protection by slowing the heat loss of the glass but will not prevent it entirely. A dew heaters job is to actively replace the heat that is being radiated away, keeping the glass at a constant temperature above the dew point.
In producing a dew heater, there is a delicate balance to be struck so as not to introduce too much heat in the system which will cause optical distortion. The scope should not feel warm to the touch but should simply not be as cold as it would have been without the heater.
The most effective solution would be a dew heater and dew cap combination. The dew cap keeps the corrector plate away from the night air and reduces the amount of heat needed from the active dew heater system. My dew cap is made from a sheet of polystyrene, about 8 mm thick, painted black and formed into a tube which fits over the end of the scope.
I dismantled the old toaster and unwound the heating element from one side. I then measured the resistance of the element and found it to be 24 ohms. I came across a length of self adhesive draught excluder strip in my garage and measured it carefully so that it would fit around the metal rim of the corrector plate on my scope. With the strip lying sticky side up on the table, I carefully laid the heating element on to the strip so that the element stuck to the strip. After this the combined element and strip was stuck to the metal rim of the corrector plate as can be seen in the photo. the ends of the heating element were then connected to a length of twin core wire, the other end of which I connected to a 12 volt power supply.
Using the simple power equation:- W=V^2/R (W equals V squared over R) where W is the power in Watts, V is the voltage and R is the resistance, we get W=6 watts. The current drain was half an amp which was easily supplied by my little power supply.
I found the dew heater to work very effectively and, in fact, have recently produced a similar design for my refractor guidescope. In this case the element is simply wound around the outside of the refractor close to where the objective lens is positioned.
This project was completed in about 4 hours.