DSLR lens focusing problem solved (I think)

[michaelmorris]

I've been struggling with an astrophotography problem for over a year now.  At star parties I like to use both my DSLRs simultaneously (Canon 1000D and Canon 1100D) .  One is connected to the scope (controlled by APT) and the second with a camera lens on my Astrotrac.   For this second camera I like to use a simple intervalometer for exposure control after first using DSLRcontroller software on my Android tablet for focussing and framing. (I don't want to leave the tablet out unattended).

The lens has a dew heater attached. When switching between the tablet and intervalometer I have to turn off the AF on the lens. This means moving the dew heater, which frequently knocks the focus - aaaargh!  Additionally, on occasions, dew heater cable dragging has caused the focus to move - double aaargh!

I think I've now come up with the (hopefully) perfect solution
 

After making some enquires on the Canon DSLR forum on Flickr, I was led to the following brilliant article on using the camera's custom settings for how the shutter release button works.
http://www.learn.usa.canon.com/resources/articles/2011/backbutton_af_article.shtml

Here's how it works

First make sure the camera is set to manual exposure. Now set the 'Shutter/AE lock button' custom function to 'AE lock/AF'.  Now I can control the focus using DSLR controller on my Android tablet, but when I operate a shutter release/intervalometer it doesn't activate the auto focus.
This means I can simply alter the custom setting at the beginning of a sessions, connect DSLR controller and the intervalometer, use DSLR controller to focus and do test shots, then unplug the tablet and run the imaging session from the intervalometer - all without having to touch the camera or lens.  What's more, the lens is now locked in position, so cable drag on the dew heater is unlikely to move the focus position.

BRILLIANT!