Detection limit for the satellite apparent magnitude (coloured lines) as a function of the limiting magnitude of the instrument (both are at the same S/N). Panel a: results for 60 s exposures with an imager, while (b): prediction for 1200 s exposures with a spectrograph. The trailing of a satellite below the coloured line will make its effective magnitude too faint for detection. The shaded areas highlight the range where constellation satellites are likely to be detected. Two elevations (zenith and airmass 2) are considered, with the corresponding typical satellite visual magnitudes and angular velocities.