On Saturday 11 April 2015 at the BAA Winchester Weekend, I gave a short demonstration of the processes involved in measuring the magnitude of comets. The information provided in this section of the website includes the files that I used on the day in order that others may practice at home.I hope that by practicing using the images that I used in Winchester, others will adopt the techniques and submit data extracted from their own images to the relevant databases.I started using these techniques a few years ago following a BAA Comet Section meeting at which we were encouraged to use the approach explained by Roger Dymock on the BAA website at the following link: http://www.britastro.org/projectalcock/CCD%20Astrometry%20and%20Photometry.htmThe techniques explained by Roger go beyond what I demonstrated in Winchester, so pleasedo make use of the guidance provided by him on the BAA website.At Winchester I broke the technique into five steps.1 - Working with Astrometrica software2 - Working with FoCAs software 3 - Working with kPhot software4 - Measuring the diameter of a comets coma5 - Producing the text files for submissionOn the menu bar on the left hand side under “BAA WINCHESTER” you will find links to the five steps and withiin each section there are links the files that I used in Winchester.At first it can seem a bit daunting, but with practice it becomes second nature. The process is necessary because unlike stars, which are point sources, comets are diffuse objects and so it’s difficult to create a system whereby two observers would arrive at the same result. In summary...First, Astrometrica is used to create the initial measurements of position and magniutude using two different star catalogues, UCAC 4 (good for positions - astrometry) and CMC-15 (good for magnitudes - photometry).Second,FoCAs is used to produce multibox measurements of magnitude, which are estimates of brighness using six different measuring “boxes” of the following sizes: - 10 x 10 seconds of arc - 20 x 20 seconds of arc - 30 x 30 seconds of arc - 40 x 40 seconds of arc - 50 x 50 seconds of arc - 60 x 60 seconds of arcFoCAs also outputs data in the correct format for reporting to the Minor Planet Center (MPC) and Cometas (more about these later).Third, Kphot is used to estimate what a visual observer would report if they were to observe the comet with the naked eye. These “visual equivalent magnitudes” are helpful because they allow data obtained from CCD observations to be analysed alongside visual observations.Fourth, the measurement of the diameter of the comet’s coma is done by eye (using software to help of course).Fith, the informtion is summarised in simple text files in the format required by the MPC, Cometas and COBS and submitted for analysis with the observations of many other.Good luck!
Hills Observatory: 1 January 2013 to 22 March 2020