Without a dew heater my lenses fog up after 30 minutes. My latest dew heater is USB heaters. From the beginning too hig power, but I connected two of them in serial and get more normal temperature. Very cheap solution, bought from Amazon, a package of 5 inclusive 5 power regulators, maybe I payed 60 Euro for all of them.
Here is my project dew heater:
http://www.astrofriend.eu/astronomy/...ting-band.html
Here in the Northern Europe (Sweden) we have our astronomy season from October to April. Temperatures from 0 C degree to -20 C Degree. I have tested my equipment at -20 C degree and to my surprise it worked, but not me.
A lot of updates in my ongoing project Nikon D800/Pentax 645 FA 300mm f/4 ED lens:
http://www.astrofriend.eu/astronomy/...ikon-d800.html
Lars