Mike Blanton (NYU) and I agree and disagree on almost everything about fundamental astronomy. As my loyal reader knows, I am writing something on how apparent magnitudes work, and also absolute, bolometric, reddening-corrected, and so on. The apparent magnitude of star depends (among other things) on a filter bandpass or a transmission function. There are two possible definitions of this. One is the fraction of light (as a function of wavelength or frequency) that makes it through the system, from the top of the atmosphere to stimulating the detector. The other is the mean contribution to the total counts read out by the detector of a photon (of a particular wavelength or frequency) impinging on the top of the atmosphere. These might sound similar—they are identical when your detector is a photon counter. But they aren't identical if your detector is, say, a bolometer. I struggled with how to simply communicate all this today.
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