That sounds like a decent idea, though what is the actual range of the IR detectors, because if they are only say 100 meters, then they might be giving you a errornous reading but if they are giving you readings that will change that much then your in, its just a matter of as you said reading like a ADC or DAC system, if it falls low, then output this, which will close your obs up but when it is high then your obs opens and happy days LET THE IMAGING COMENCE (in a godley tone)
