Online help for the IRAF SPECTIME package
Set the integration time to INDEF or leave it blank if you wish to compute the total exposure time needed to achieve the desired signal-to-noise per pixel at the wavelength specified below. If you set the integration time to some value a new signal-to-noise per pixel will be computed for that time, ignoring what is in the signal-to-noise ratio box.
This calculator assumes clear photometric conditions. Proposals for poor-weather conditions must adjust the results for the requested observing constraints.