Any amateur astronomer who takes images of comets will wonder, at some point, if they can be used for a scientific purpose. We have already seen that astrometry, the measurement of an object’s precise position relative to the background stars, is a very useful contribution, especially for newly discovered comets and NEOs. However, comet photometry, the measurement of the brightness of comets, is also valuable. The problem is that while comet astrometry is a straightforward and very precise science, with little chance of major errors once you have mastered the technique, comet photometry is not. I have seen respected astronomers becoming very hot under the collar about comet photometry on a number of occasions, largely because if they are wrong in their approach to the subject years of work may be regarded as distinctly dubious. The bottom line is that comets are fuzzy objects that fade away into the light polluted background sky and whose images are peppered with background stars. Measuring their precise brightness is incredibly difficult and potentially subject to huge errors. In many ways the visual observers have a similar set of problems to face, but I think most of them would accept that their estimates are, at best, just that: estimates. Conversely, CCD photometry is supposed to be a precise science. However, at the present time it is more like the proverbial can of worms! Before we can even start to consider comet photometry we should understand the basics of stellar photometry, in other words, measuring the magnitudes of stars.