You're right about the time of day ... but this isn't about the time of day, it's about the light level. I want to have a photograph at the point where the light first reaches a pre-determined level. I've gone out and metered it -- the exposure would be 1 sec. @f8, EI 100. I'm going to run some tests with a photocell before deciding on a condensor lens. It turns out that the light level I'm waiting for is pretty much at the low end of the photocell's practical measuring ability. A condensor moves the effective brightness (on the photocell) into a resistance range that allow for more accurate measurements.

If this were just one photograph, I'd not be building this thing. But it's for a project that entails a photograph every two weeks, for a year, to document seasonal changes on the prairie. I simply cannot expect to be available to take the photograph on every one of those mornings, but I can expect to be able to set up the camera sometime during the day before and have it capture the scene as planned.