Defining minimum detection limit as that Lux level at 550nm where S/N of a signal limited image = 1, we can compute the the lux level where that occurs.
The parameters used:
Telescope: 2500mm f/7.4
Telescope optical transmission: 90%
Filter optical transmission: 85%
Camera window optical transmission: 95%
QE: 65%
Exposure time: 1 second
