Introduction to comet photometry
Measure a comet’s brightness by calibrating and stacking your images
Photometry is the science of measuring an object’s brightness and there are a number of important aspects to consider when performing photometric measurements on comets. Firstly, images need to be pre-calibrated using matching flat and dark frames. As sky brightness makes a contribution to the comet’s brightness, this too needs to be measured. Some programs,
such as MaximDL, perform this function using an aperture and an annulus. The aperture measures the comet’s brightness while the annulus is used to measure the background sky brightness.
The BAA program Comphot (see Hardware & Software, p72) can be used to perform photometry on comet images. This requires two FITS images as a source, one stacked on the background stars and the other on the comet. This is a command line program normally initiated by typing: comphot offset.fit fixed.fit x y
Here offset.fit is the image stacked on the comet, and fixed.fit is the image stacked on the stars. Both files are in the FITS (Flexible Image Transport System) file format, a technical format preferred for astrometry and photometry.
The x and y values are the coordinates of the comet’s coma centre and can be derived by using a position-measuring program such as Astrometrica. The UCAC-4 star catalogue is recommended for positional measurements. As the coma contains reflected sunlight as well as light generated by excited coma atoms and molecules, specialist photometric filters are normally used for imaging. If you’re using FITS images from a CCD without a filter, R-band magnitude values should be selected in Astrometrica. If using a green filter, V-band magnitudes should be used. An R-band filter is a red filter typically centred on 658nm, while the V-band filter is centred on the visual wavelength at 551nm.
The Comphot software is capable of outputting its measured findings in the correct format required by the International Comet Quarterly (ICQ). Like the MPC submissions mentioned in project 2, the ICQ requires that this data follows a strict submission format. Details of this format are available from www.icq. eps.harvard.edu/ICQFormat.html.
One issue when measuring a comet’s brightness in an interactive program such as MaximDL is determining an ‘aperture’ size to encompass the entire comet head
V-band (visual) 551nm +/–88nm R-band (red) 658nm +/–138nm Common photometric filters used for comets are V- and R-band filters. They help to isolate reflected sunlight from excitation radiation from within the coma
Outer annulus Object aperture Inner annulusAn object aperture should contain the comet’s head. The inner annulus is a gap, adjusted under software to ideally contain no stars