If you're competent with a soldering iron and simple electrical components, you can make your own antidew device for a fraction of the price of a commercial unit.
Electrical resistors, available for small change at any electronic supply shop, make excellent customized warming devices for your telescope's dewcap, eyepiece holder, finder, and/or reflex sight.
First decide how much heat you need. The usual suggestion is 3 watts for an 8-inch Schmidt-Cassegrain corrector plate and 1½ watts for a finderscope objective or eyepiece. If your dew problems are severe you may need more. Fortunately, resistors are so cheap that you can experiment to find the minimum power consumption that works for a given application.
Electrical resistance is measured in ohms. To get a desired heat output in watts, the resistance you need is given by the formula
Ohms = Volts squared / Watts
where "Volts" refers to the voltage of the power source.
For instance, if you have a 12-volt battery and want 3 watts, you need 48 ohms of resistance. Eight resistors of 6 ohms each, wired in series, will do it. Resistors come in a limited variety of values, so you may have to settle for a little more or less than you want.
The resistors should be rated to handle the load you'll put on them. With eight identical resistors delivering a total of 3 watts, each puts out 3/8 watt of heat. So ½-watt-rated resistors should be good enough, though 1-watt resistors would provide a wider margin of safety.
The resistors can be taped into place with black rubber electrical tape. Get them as near the glass as possible. But be careful to ensure that no bare wire can touch metal; you wouldn't want a short circuit, much less an electrical fire.
How fast will the heater drain your battery? To find how much current it draws, use the formula
Amps = Watts / Volts
For example, a 3-watt heater running on a 12-volt battery draws ¼ amp. So a battery rated at 1 amp-hour will run the heater for 4 hours before needing a recharge.
If 120-volt power is available, obtain 6 or 12 volts with a small step-down transformer to run all the heaters you want indefinitely. WARNING: It would be extremely dangerous to design a resistor heater, with its exposed wires, to run at 120 volts directly. Of course, you shouldn't mess with 120-volt power at all unless you're qualified. That includes knowing that anything using house current outdoors must be plugged into a ground-fault interrupt (GFI) circuit for safety — especially if you'll be touching a metal telescope while standing in wet grass! Plug-in GFI adapters are sold in hardware stores. And any power supply that plugs into a 120-volt outlet must be completely weatherproofed against dew.