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Astronomical photometry is the measurement of the brightness of sources. Strictly speaking, it is not necessary to know the spatial distribution of the photons in order to make a photometric measurement - all that matters is the total number of photons received from the source. Hence it is possible to perform photometry with a single-pixel detector, such as a photomultiplier tube or an avalanche photodiode - see figure 76.

figure 76: Single pixel detectors. Left: avalanche photodiodes (APDs). Right: a photomultiplier tube.


There are a number of disadvantages, however, to single-pixel photometers:

For the above reasons, single-pixel photometers are very rare noawadays, although they did play a key role in the development of the subject many decades ago. Instead, almost all photometry is now performed using imagers, which by definition possess multi-pixel detectors. Imaging photometry has a number of major advantages:

We shall not discuss single pixel detectors again in this course, as shall assume from now on that all photometry is performed using imagers.

©Vik Dhillon, 19th November 2010