Photometry

Abstract

Photometry is the process of determining intensity or magnitude of or within the original object. It most commonly involves measurements of point sources or near-point sources like stars and planets, but it can also involve measuring the integrated intensities or magnitudes of more extended objects. Any of the detectors discussed in Chap. 9 can be used for photometry. The CCD and p-i-n photodiode are, however, the most straightforward because they have linear responses, and their outputs can be converted directly into magnitudes using ( 8.5). The eye and the photographic emulsion are non-linear detectors, and their responses are therefore more complex to convert into magnitudes. The visual estimation of magnitude having already been discussed in Chap. 8, we shall consider CCD photometry below. Photographic photometry can still be undertaken, but it is rather pointless to do so since the practical side is complex; measuring the photographic images requires expensive and sophisticated machinery, and the final results are of relatively low accuracy. Readers interested in the technique will, though, find it discussed in earlier editions of this book.