Hey PF, I'm just being stupid today, because I can't figure out the way HR diagram is constructed, hope you guys can help. So suppose that you complete an observation in several different filters, and you need to work out the luminosity and temperature for each object in your field of view. Now for the temperature you could just plot an instrumental magnitude against filter wavelength and fit a black-body profile to it, the peak of the curve showing the effective temperature of the star. But what about the luminosity? Is it reasonable to take an area of one pixel and, knowing average photon count rate, integrate it over 4π steradians? Then, after multiplying by a typical photon energy (assuming the filters are quite narrow), I'd get a luminosity from a particular source? Something seems wrong here, but I can't figure out what. Is it because the atmospheric absorption is not considered? If so, how to do it properly?