Posted by Richard Miles at 12:02 on 2012 Jun 30
Good-day Mike – Nice to see the interest in photometry of comets – Martin’s book although not dealing with the intricacies of photometry is nonetheless well worth having. I’m sure you’ll find it a very stimulating read.Since asteroids and comets both wander the skies, their photometry share some common issues most notably selection of comparison stars having accurate magnitudes and known colours. Here we are at a watershed in that the AAVSO APASS survey is now reaching fruition with an excellent online source of data at:http://www.aavso.org/download-apass-dataSoon these data will be added to the catalogues used by astrometry / photometry software.There are a few key differences between comets and asteroids. The main one stems from one being essentially a point source of light whereas the other is an extended object. However, an important issue with comets is that there is a whole zoo of comet types out there for which no single photometric approach suits all types of object. You can look at a comet as comprising 3 components; (a) the nucleus acting as a point source and reflecting sunlight directly; (b) the dust coma also reflecting sunlight but which has a diffuse distribution, more concentrated towards the nucleus but which is asymmetric once a tail appears; and (c) a gas coma / ion tail which emits light by chemiluminescence towards the blue-green end of the spectrum. Even the same type of comet will behave differently depending on its heliocentric distance and so observing the same object far from perihelion may require different observing methodology than if it is at say 1 AU from the Sun. Having dedicated observing campaigns on particular objects is a good way forward.I should also mention that a joint BAA Section meeting is taking place on Saturday 6 October at The Open University, Milton Keynes on the subject of Comet-Asteroid Connections during which several talks will refer to the topic of photometry comparing and contrasting the two types of object. We also have Luca Buzzi planning giving a talk on the T3 project, which aims to identify comet-like asteroids: multi-aperture photometry is one tool that can be used to confirm cometary nature, for example.Richard Miles