I had not looked into the temperature dependence. Good point. I think I'm going to use the TSL230R chip suggested above. It's temperature stable and can be configured for optimal sensitivity. Yes, I plan to have the shutter in B mode, since the exposure will be on the order of 8 sec. @ f22. Someone mentioned a servo for the cable release -- I agree that a stepper motor of some kind would work well. I like the idea of being able to move it incrementally, so as to minimize vibration. I'm thinking the flash sync would be a good way to detect when the shutter has actually opened; at that point stop pushing and start timing, and then release after 8 sec.