An experiment

In the late 1980s, my wife and I moved to Toronto for a limited time. During our stay there (about 3 1/2 years) I joined the RASC Toronto Centre, but I also kept my membership in the Vancouver Centre. The Toronto Centre also has a group of people who observe the sky actively. Just as here in Vancouver, light pollution problems in the city made us look for a darker sky, but within a reasonable driving distance. During fall and winter, clear nights in Ontario can be very cold, so you have to be prepared to have some related effects on your telescopes (and yourself - dress accordingly).

Occasionally, cold air's low relative humidity will fog up your telescope's lenses' external surfaces. Since wiping them by hand is always a bit chancy, some "no-touch" method is preferred. That usually requires some electric power supply to run a "gentle" warming fan or use some other warming method to clear the lenses. In general, that means fairly large, portable batteries, or a connection to your vehicle, or gasoline-driven generators. These requirements made me try another approach. 

Many telescopes come with "dew caps", meant to counter the fogging of optics. I own a C-8 telescope, whose performance and portability make it ideal for observing at various locations. My C-8 did not come with a dew cap, but I found one labelled as made by Nova Astronomy-Products in Toronto. It fit the C-8 nicely. Since my working activities involved electronics, I thought of trying a simple experiment involving standard, small sized resistors (1 to 2 watt rated) to come up with a low powered warming system to repel some more humidity. 

I decided to use 5 resistors with a heat rating of 2 Watts and a resistance of 15 Ohms each (mainly because I had them on hand) and taped this series of resistors into the dew cap so that the resistors are placed near the front of the C-8 when the dew cap is attached to the C-8. The amount of heat generated is small. If you apply 12 Volts to the resistor series (Total of 75 Ohms resistance) you'll get slightly less than 2 Watts as the total amount of heat generated. Each resistor contributes a little less than 0.4 Watts.

One caution: If you double applied voltage, you will get four times the heat, four times the voltage gives you sixteen times the heat. Remember that the resistors I used can handle only a maximum of 2 Watts each. The effects of Voltage and electrical current changes occur in the domain of the square of their original values. You can exceed ratings and do damage very quickly. Stick with your original values.

 

The black dew cap mounted on the front end of the orange C8 telescope. The 5 resistors are attached inside the dew cap, where the dew cap and telescope meet.

Inside the dew cap, one of the 15 Ohm, 2 Watt, resistors is shown in front of the C8. Held in place by a small piece of Velcro.

Note  the 5 resistors deep in the C-8 dew cap, just in front of the C-8 telescope. They are connected as a series circuit. You can see the wire which connects one to the next. The resistors do not touch the C-8 telescope.

The 2 wires for connecting the resistors to the external voltage source are a small bundle at the lower left, on top of the tripod telescope mount.

You may want to try something similar to what is described above. The "small scale" warm-up arrangement worked reasonably well (and is still, more than 30 years later) at moderate humidity levels. It requires very little electrical power, so that there is little demand on a battery used for powering other functions on your telescope.