Spanish Monitoring of Comets: Making Sense of Amateur Photometric Data

Amateur astronomers have always represented an important observing group in cometary astronomy. Much of our knowledge of cometary light curves has come from amateur data, initially in the form of total visual magnitude estimates and now increasingly in the form of CCD observations. The increasingly widespread use by amateur astronomers of CCD cameras of excellent sensitivity and good cosmetic quality has revolutionised astrometry, allowing far more intensive astrometric monitoring of comets to be carried out down to magnitude 18 and fainter, with a corresponding enormous increase in the quality of calculated orbits. Although amateur CCD photometry is extensively available in the Internet, its use has been less widespread. The reason is the lack of standardisation in the way that this data is taken that leads to amateur CCD light curves having enormous dispersion. All amateur CCD photometry is aperture photometry, but it is seen that neither does it represent well the equivalent of m1 (total visual magnitude), even with a large aperture, nor is it close to the definition of m₂ (the nucleus magnitude). The problem is examined using data from the Spanish Comet Observers Group archives to show that by careful standardisation of data acquisition amateur CCD data can produce high quality, well-sampled and physically meaningful light curves. Examples are presented of the results for recent comets including 19P/Borrelly, 51P/Harrington, C/2001 TU80 (LINEAR-NEAT), C/2000 WM1 (LINEAR) & C/2001 A2 (LINEAR).