International Comet Quarterly

Brief Introduction to Comet Photometry

Comet photometry involves measuring the brightness of part or all of the visible comet. In practice, this is much more difficult than measuring the brightness of a star, because a comet is usually extended (usually a nuclear condensation and/or a surrounding coma and/or one or more tail or jets extending outward from the nuclear condensation) and because the comet moves over time (so that different reference stars are usually needed every night).

The printed ICQ has, since its formal beginnings under that title in 1978, aimed toward standardizing useful procedures for obtaining (and reducing and analyzing) photometry of comets. As such, this ICQ website has, over time, built up much useful information for observers, to complement that found in the 25 years of ICQ printed journals and supplements. The only really useful photometric work that visual observers can undertake is the measurement of the total brightness (total visual magnitude) of a comet -- meaning generally the entire visible comet (including tail, which will usually have such low surface brightness with respect to the coma as to be negligible), and this means defocussing the comparison stars (and sometimes also the comet) in a proper way so as to have a meaningful way to relate the brightness of pointlike stars to that of extended comets (or galaxies or nebulae).

Because very few observers use proper techniques for CCD photometry of bright comets (except for photometry of the inner coma, which has little value outside of narrowband spectrophotometry), total visual magnitude estimates remain extremely important as the only way to get a good handle on the overall brightness activity of such comets. Visual comet photometry also has great utility and importance in connecting comet information of the historical past to the present, as there is little useful electronic comet photometry from before the 1980s and the extant photographic magnitudes prior to that time are horribly limited in their usefulness. However, CCD photometry can be highly useful if proper procedures are followed involving the use of proper filters: (1) the measurement of broadband `total' magnitudes of the entire coma are encouraged in this way, and (2) those with more expensive (and difficult-to-obtain) narrowband filters can also do useful work, both via photometry of the entire coma and photometry of a specified aperture centered on the central condensation. So-called `nuclear' magnitudes are very problematical because of coma contamination close to the nuclei of nearly all comets at nearly all times of observation; even for faint comets that show little or no apparent coma, it is nearly impossible to know whether or not the measured light contains only nucleus contribution or also light from the inner coma. For this reason, general ground-based photometry of the central condensations of comets is not encouraged (though a clearly-thought-out program of such observation might have some useful application).

Photographic photometry is so plagued by problems (due to the nonlinear aspect of emulsions, etc.) as to be not recommendable. CCD observers can make important contributions to comet photometry, as this is the only really practical way to obtain magnitudes of comets below the visual threshold. CCD photometrists are urged to use standard filters, whether broadband or narrowband, and to report a large range of information to make their data of most use to those analyzing comet magnitudes. The ICQ thus has established recommended procedures, comparison-star sources for magnitudes, and reporting formats, as an extension of the same standardized procedures for visual observations. Observers obtained comet magnitudes with unfiltered CCDs need to be especially careful to use magnitudes from *proper* catalogues that have bandpasses close to the peak response of the CCD in use (formal R magnitudes for CCDs peaking in the red, formal V magnitudes for CCDs peaking close to the visual), and for each single unfiltered comet magnitude, a single comparison star should be used with the color (B-V) or spectral type of the star sent in descriptive form.

Photometry of comets sent in the proper format are published in the ICQ and archived in electronic form for use by cometary researchers. Preliminary magnitude information sent in a second format (when contributed by those also sending tabulated data) are posted here. Researchers can receive e-mailed files of ICQ-formatted data on a comet-by-comet basis via request to the Editor (icq@cfa.harvard.edu). Serious users of photometric comet data are strongly encouraged not to use the preliminary, undetailed information typically posted at websites, but rather to use the formal, tabulated ICQ data. Even then, one must generally be careful to do the following: (1) use only observations by experienced, trusted observers (preferably those who have observed dozens of comets over many years); (2) be careful only to use one observation per observer per night (ICQ contributors frequently make multiple observations on a single night with different instrumentation, methodology, and/or reference stars) -- and then selecting generally those magnitudes obtained using the smallest instrumental objective aperture and magnification needed to easily see the comet (to minimize aperture effects); (3) select observations using proper methodology (for visual observations, this means the VBM method when comets are very condensed and small, and the VSS method when comets are large and diffuse, and the "Modified-Out" method when comets are in between; for CCD data, this means appropriate-aperture, filtered data); (4) select observations made using proper comparison-star sources (i.e., good V magnitudes for visual observations; and proper bandpass magnitudes for CCD data); and (5) be careful about selecting observations by single observers when a comet is not reported by other experienced observers (i.e., there are surely erroneous data in the ICQ archive).

Index to the CBAT/MPC/ICQ pages.