alex cherney put me onto the new packs first, after previously not having much luck with the gel type warmers which didn't last long at all.
the new ones are a dry powder, quite light weight with a claimed heat time of 8-12 hours. they only cost a few dollars.. a lot of chemists have them now and i've also seen them in aussie disposals. they're not reusable though.
i find they take a very long time to activate.. about half an hour and they need a lot of shaking and flexing to work air through the mixture. sometimes i also think that when they're left on the lens they don't work as well as their intended use next to the body where they keep getting moved around which helps keep air moving through the mixture. alex seems to have had reasonable success with them though.
I have been using the 'wild country' chemical hand warmers with film lenses.
i stock up on them from rays outdoors (they only sell them in winter) they cost 1.50 each and are good for 8 hours.
i found that it is best to open them approx 15minutes before use and keep them in a pocket. then set f stop and focus, attach with rubber band to the topside of the lens, check focus then dangle a sock or something to insulate from cold air. (they dont work great on cold metal or plastic surfaces with out insulation)
The heat does not seem to affect the image or the focus during exposure.
i've attached an image, temp was 1 degree that night.
I didn't have success with the first one I used. I think the dew was just toooo heavy.
But I've only tried it once. I ordered a box of 'little hotties hand warmers' which I plan to take to the kimberley with me, but I haven't tried them yet.
wow, you should be able to get a box of 40 pair ( x2 =80 ) hand warmers for around $25 off Amazon.
I'm thinking of going this way for the secondary of the RC.
These nifty usb heated gloves, with the removable element have me thinking though. For $5.80, you get 2 powered heaters with quite fine leads.
I use the Kendrik dew heating system with the dual channel (four output) controller. This gives me four dew control heating strips to use. There are a number of heating strips available and I use two types, ones which are designed for the 2" eyepieces and larger ones which are suitable for refractor OTAs.
I find that the heating strips designed for the 2" eyepieces work fine on the lens of the camera. By wrapping it around the lens before focusing this seems to work fine. With the controller supporting four dew heating strips I have enough for the camera lens, the front of the guidescope, the finder scope and one spare. The controller takes a 12v input with the standard car cigarette lighter plug.
Link to the Kendrick site showing the dual channel controller is here:
Only cautionary note is that they do consume a reasonable amount of power so a good size battery is in order. I have a 18AH gel cell battery I use in the field and a back up in case I need it.
addition:
the Hotteeze was a used one!
I had used it 10 days ago, approximately, back then for 2 hours.
Afterwards, wrapped sock and hootteeze in a thick plastic zipper bag and suck out the air before closing.
Taking it out, yesterday, it warmed up again!
And did it's thing for 5 hours.