Number of photons per sq.cm. per sec in V-band from a V=7.6 star is 10^6 photons -0.4*7.6 N(star) = --------------- * 10 sec * cm^2 photons = 912 -------------- sec * cm^2 Number of photons collected per second in V-band is 912 photons N(coll) = ------------- * pi * (180 cm)^2 sec * cm^2 ~ 10^8 photons / sec In order to see a feature of depth 10^(-4), one must exceed the Poisson noise in the starlight. 1 starlight noise = ------------------------ sqrt(number of photons) Therefore, the number of photons collected must be N(coll) > (starlight noise)^(-1/2) > [ 10^(-4) ]^(-1/2) > 10^(8) How long should this take? 10^(8) photons exptime = ----------------------- 10^(8) photons/sec = 1 sec How long _did_ they actually expose? Three nights, several hours per night Why the difference? They broke up the light into spectra, instead of adding up all the light within a broadband filter. V-band filter bandpass width approx 900 Angstroms spectroscopy "bandpass" width approx 0.5 Angstroms