I am thinking of using an LED as a light detector to detect illumination of another LED, which indicates operation of an astro CCD camera in three states.
LED off - no operation
LED blinks - operating (taking a picture)
LED steady downloading image to PC - not taking picture.
The states that I need to differentiate are blinking and steady; that is
LED blinking Arduino do nothing.
LED steady, Arduino operate ancillary equipment.
That is, differentiating fluctuating and steady analog input.
Is this a feasible approach? Would blink rate - sampling the analog input be a restriction.
The project is feasible, given the right hardware. I don't think that using an LED as the detector is using the right hardware. There are light detecting resistors and other sensors that are more sensitive and faster reacting.
Would blink rate - sampling the analog input be a restriction.
Yes. But, if you can count the number of blinks in some period of time, so can the Arduino. If it's flashing too fast for you to count, it still may be glacial speed to the Arduino.
Recording when the on and off states are detected will let you distinguish between steady on, steady off, and blinking.
The LED blinking produces a lower average analog input, with steady and off, the high and low. I think it's just a matter of writing a sketch to calculate the average voltage of the flashing LED, giving 3 states to work with for hardware control.
LED during capture is on/off per second, which is quite slow. Sampling sensor inputs over several seconds, adding and averaging should establish the 3 states of operation. No doubt there will be slight variation in sensor values. Perhaps calibration or a range for high and mean values.