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Peter,
I've found that the disposable heat packs used for warming hands or relieving muscle aches work quite well on camera lenses. Open up the heat pack, tape it to the lens or hood, put some insulation around it (e.g. a thick sock) and you'll be dew free for hours. There have been a few threads on it here - worth a quick search.
Focusing camera lenses is tricky at the best of times, but it's even trickier with zoom lens that have a small/slower maximum aperture. I generally point the centre of the camera on a bright star (Alpha Cent, Sirius, Canopus, Achernar, etc), use LiveView with the camera set to Manual mode, ISO at the highest possible setting (e.g. H1 with the ISO Expansion custom function enabled), shutter speed at the slowest setting (e.g. 1/30 sec), and the lens at the widest aperture.
As you turn the focus ring, watch the star carefully - aside from becoming smaller and brighter as it comes into focus, it may also change colour (red/green chromatic aberration in front or behind the focus plane) or shape (from circular to egg shaped). Using a combination of these cues, I can usually focus the lenses I use.
Alternatively, you can draw/stick something onto the focus ring and lens barrel to help you keep track. Just do a bunch of test exposures at different focus settings, check the exposures, and you'll know where the true focus point roughly lies.
If you have a computer available, it's even easier - software such as Backyard EOS can automatically take a series of exposures at different focus points and analyse the exposures to tell you where the optimum focus is.
Good luck!
Cheers,
Dave
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