Paul, Because the heater only runs @ around 2 watts, you could use a simple pot, ie; potentiometer, to adjust the temp. BUT you must buy a 'WIRE WOUND' pot, rated @ a minimum 3 Watts. The normal carbon track ones are not suitable. They are only rated in mW., ie; the carbon track will cook.
A suitable value would be a 100 Ohm 3 Watt Wire wound pot.(Readily avail.). This will give a very cool to warm range. Max current with a length of 300 mm of electric blanket heater is 170 mA with 12 volts applied. This equates to around 2 Watts.(Resistance = about 70 Ohms). Should be more than enough to heat your finderscope objective lens. (same as mine).
I used a simpler idea, in that I 'tapped' into the heater wire about 60 mm from one end of the 300 mm length, & ran another wire to it. This gave me, LOW HEAT = FULL LENGTH, HIGH HEAT = @ SHORTENED LENGTH. I hardly ever use the HIGH heat tho! The heater, being in contact,ie; 2 turns around the finder, maintains a fairly constant temp. all night. ( warm mass). A single pole changeover switch is all that's required to go from H to L.
I posted a pic somewhere. Will try to find if req'd.
HTH.
L.
ps.
http://www.iceinspace.com.au/forum/a...achmentid=1484
Details are slightly diff. but close enough.....
L.