Just as Mars began its closest approach in recorded history, I began to have problems with dew for the first time in my observing career. The weather has been unusually wet & humid, and I knew a dew heater was the only solution that would let me enjoy the Mars opposition. However, I couldn't justify a lot of money for something that I probably won't use very often when (or if) our weather ever returns to normal. A complete Kendrick Dew Remover setup (the standard by which all dew heaters are usually judged) would have cost me over $200. I did a little research on the internet, and found out that I could actually build my own heaters quite easily, using materials that I already had on hand. (I'm an electrical engineer in the daytime, and have a basement full of electronic components left over from hobby work.)
The first step was to build the heaters themselves. I looked at a number of plans on the internet, and decided I liked Ron Keating's design the best. When finished, they look and operate much like the commercial heaters from Kendrick. The final result is shown below on my TV-102: two black velcro straps, one around the objective lens, and one around the eyepiece. The objective heater is adjustable enough to use on my Oracle as well, while the eyepiece heater will fit any of my Televue eyepieces.
The next step was to build a controller that would allow me to adjust the heat as needed to keep dew off the lenses without overheating the scope and degrading the image quality. After a lot of research, I chose a Kendrick-like design available here. I made a couple of minor changes to Len's circuit, including the substitution of a different opto-isolated relay (to use what I had on hand), and the addition of a transient voltage suppressor diode across the load. (Len included an R-C filter to help manage transients when the load is switched off, but the TVS seemed like a really good idea to me.) The final result is a circuit which pulses the load as needed to provide variable amounts of heat, and fits in a neat little box as shown:
The controller can actually provide more than twice the power that I am presently drawing from it, so I gave it two additional outputs for future use. This gives me the option to run a second set of heaters (so that I can keep both my refractors dry at once), or to plug in a heated eyepiece case.
The heaters are powered from a sealed lead-acid battery that I got from my employer's obsolete inventory. In addition, I found a beefy "power cube" in my basement stash that puts out 12 Volts and can be used to run the heaters off AC if desired. I use it to pre-heat my scope when the dew point outside is higher than the temperature of my walk-out basement.
Please note that if you're thinking of building your own dew heater, you need to know a bit about lead-acid (and other large rechargeable) batteries! They can be dangerous if you don't know what you're doing, and they are easily damaged by discharging them too deeply. Some of the best resources for battery information are Panasonic's battery site (scroll down to the technical article links) and this excellent article . Also, while it's not the best solution (as explained in the aforementioned article), I did build a battery monitor as described here to give me some idea of my battery's condition while in the field. So far, it works exactly as advertised.
All material on this site is Copyright 2003, Neil T. Hobbs.