I did see a circuit somewhere with dual temperature sensors which is designed to keep an object just a degree or two above ambient.
Such a system is superior to those which just pump the heat in and hope for the best, wasting battery power and causing thermal image degredation if set too high.
All my images are taken with a Kendrick dew controller and the Kendrick heater strips fitted to either my Vixen 4" refractor or the Celestron C9.25 SCT. I leave the power switch set to the lowest setting, except for those really dew laden Qld mornings.
The Kendrick control unit cycles power on and off rather than supplying it continuously.
I just go my new dew shield the other day from Astrozap.
Have a look at http://www.astrozap.com/
It is a heated flexi shield with 12 v adaptor. It fits my scope perfect.
I have delt with Joe at Astrozap before very helpfull and had no problems ordering online. Joe seems to have a dew shield for most scopes.
I use to run a Orion dewzapper on my old 10" SCT and dew shield with it. This time I thought the lighter Flexi shield would be better on the Mak.
By the way I only waited 6 days to get my dewshield from the US, weekend also in there.
I use an Orion flexishield, does the trick most nights. I have, in reserve, a dew strap from scopestuff, and a homebuilt PWM controller, both tested but yet to be used in anger. I also have a 12Volt hair dryer from the local boating store (at $19 excellent value) for defogging EP's etc.
It's funny.. I've been looking into this today also, I thought the "Dew-Not" (http://www.dew-not.com/) heaters looked good, much cheaper than Kendrick and apparently use a lot less current for the same effect.. I'm thinking of getting an eyepiece strap and buying or making a controller.. fogged-up eyepieces are the biggest PITA for me so far.
According to the, "Dew-Not" specs, the current is exactly the same as calculated by 'Ohms Law'. ie; I = E/R....W = E x I....I = W/E.....etc etc..... Can't be changed, northern or southern hemispheres or elsewhere... afaik. L.
ps. A very simple PWM cotroller will be suitable for all these...
According to the, "Dew-Not" specs, the current is exactly the same as calculated by 'Ohms Law'
I think it has more to do with efficiency of heat transfer - I was just going by this quote from the AstroMart review linked to from their site: "In simple terms the Kendrick design has a large amount of rubber involved as well as the fabric overlay. The result is that you must heat the rubber and the fabric before any heat is transferred to your OTA or device. Thus, as per my example stated above, a 14/16” Kendrick heater draws a whopping 4.75 amps at 100% power. By comparison, the Dew-Not 14” strip with it’s unique design only draws 1.92 amps at 100% power."
so I am fitting a home made anti dew device right now to the telescope
Hi Ed, you've inspired me.. your eyepiece heater looks very professional.. I'd love to know more details on how you put it together? What are you using the resister heat pad for?
For my C9.25 I use an Astrozap Dewshield with in-built heat strip and feed this from a Kenrick 12v hand controller. Both I ordered online from the US and Canada respectivey and received swiftly with no bother.
The astrozap dewshield does the trick soe nights by itself and if I do use the heater either leave it on low or turn up full blast to de-dew and then turn off again.
One thing that confuses me about dew control is the apparent conflict between mirror cooldown and dew heaters..does using a dew heater not heat the mirror to above ambient temperature?
FWIW. If the resistances & applied voltages are the same, then the current thru each will be the same. A larger mass will take longer to heat and will consume more 'power' (W = E x I), than the lower mass heater, ie; to reach the same temperature, but the lower mass heater will not use less current. Not possible..... (refer to Ohms Law)....... L.
ps. The higher mass heater will retain it's temp for longer than the lower mass one, & so will stop the dew from forming for longer. It will also be more efficient in overall power consumption over a longer time period.
pps. Use a PWM control....
I have a dew shield for my C8, and a 12V hair drier, however, that just isn't enough or me. Sometimes I'll only get 30 minutes or so from drying the lens till it's dewed up again (and of course, other nights it isn't a problem!). So I have bought an Astro Heat Strap from Astro Optical, and I'm in the process of building a pulse width modulated controller... I'm planning to build in 3 outputs one each for OTA heater, piggy-back lens/guide-scope heater, and an eyepiece heater. (I too have experienced dewed up / frosted up eyepieces).
I have hade up a number of eyepiece heaters (2" and 1.25") and an objective heater for my 70mm refractor, using a "ladder" of 470 ohm resistors, adhesive backed closed cell foam double sided mounting tape and velcro. Cost about $3 each tops including the RCA jack. I have also made heaters for my Telrad and Quikfinder units using several resistors encased in heatshring tubing for same cost.
In my solid tube dob there is no variable heat control, that is when pluged in they are on. In my string truss dob I have used a PWM unit to provide vairable heat output. I have to confess I think I went a little overboard with the stringscope electrical panel .
I should have also included reference to extended dew shields for both the dobs and the refractor. Since installing all this "stuff" I haven't had a dew problem.
L. 
