My DIY Dew Heaters for just a few £'s

I thought I would share my Dew Heater design as it was so easy and cheap to make.
Without the battery these heaters cost me less than £10.

Dew heaters can be expensive and often need expensive controllers to adjust the temperature output of each heater. The idea is to heat the corrector plate end of the telescope just enough to replace the heat lost to space thus stopping dew from forming on the lens. Much like the dew on your car windscreen collects overnight.

My aim was to design a simple and cheap 12 volt system without the fancy heat controllers.This would save me using a 240 volt hair dryer with is obvious dangers outside in the damp night conditions.

I worked out from research on the net that I needed somewhere between 0.5 watts and 1.5 watts of heat on my finder scope and between 1 watt and 2.75 watts on my 105mm dia Meade etx.

If you look at the diagram this can be achieved by wiring both dew heaters together across a 12 volt battery (mines an old car battery). To adjust the heat output I simply use a direct connection across the battery for maximum heat or add either one or two 10 ohm resistors in line next to the battery as shown in the diagram below. SIMPLE.

The wiring is looped back around the heater band so that it can be easily attached.
The parts list shows roughly what I needed.

The resistors are really cheap. They come in bags of 20 and you only need 11 x 4.7 ohm resistors
and 13 x 10 ohm resistors for the whole job.
Just solder the resistors together exactly like the diagram and stick them on the inside of the foam using duck tape. Connect them to the battery car with speaker wire and watch out for any short circuits.
A cheap foam camping matt was used as the band to carry the resistors. It would make loads of bands.

I use self adhesive Velcro on each end to easily attach and detach the heater bands.

Last night I used the heaters connected to a car battery continuously for 5 hours at between +3 and -2 degrees with no dew on my lenses. I did not see any reduction in image quality due to the heat.

Please note that I am not an electrician and if you choose to copy this design you do so at your own risk .If wired incorrectly the resistors or wiring could overheat.