Observing comets is much like observing deep sky objects. Star clusters at the limit of visibility, nebulae, galaxies, and comets all look pretty much alike. Charles Messier was a comet hunter who assembled a list of deep sky objects that could be mistaken for comets. This list is now a list of things to look for, not to avoid, as he intended.
The visible coma is the part we can see the most often. If it brightens toward the center, you are seeing the apparent nucleus (the real nucleus is usually too small to see). The dust tail, if visible, will be behind the coma, and is shaped by the path the comet is traveling, and the pressure of the solar wind. The gas tail is an ion stream of material torn from the nucleus by the solar radiation. It is usually very difficult to see by eye, but may show up in photographs.
According to David Levy1, a 6-inch telescope should catch a 9th magnitude comet; 8-inch will see to 10th magnitude, and 16th inch should reach to 13th magnitude. These magnitudes are much dimmer than the magnitudes would suggest due to the way magnitude is computed for large objects. Comet magnitudes (also works for nebulae, galaxies,) can be estimated using a star of known magnitude and one of the following methods:
| Beyer | Set both the comet and the star more and more out of focus until they disappear and compare their brightness according to the order in which they disappear. |
| Bobrovnikoff | Place both the comet and the star out of focus until they are the same size, and compare their brightness. |
| Morris | Memorize the brightness of the slightly out-of-focus comet, then compare it to stars that are defocused to the same size. |
| Sidgwick | Memorize the appearance of the in-focus comet and compare it to stars that have been defocused to the same size. |
Schaaf indicates the Bobrovinkoff and Morris methods are used most; Levy appears to prefer Sidgwick. For all the methods, you should use the lowest magnification possible.
The morning scan is performed in the same manner, except that you start high, and work towards the horizon. Though no reason is known, statistics indicate that three times as many comets are discovered in the morning. One speculation is that there is less air-borne dust and particulate pollution from human activity when viewing in the morning.
If you locate a candidate "fuzzy," check the region against a detailed star atlas such as Sky Atlas 2000 or Uranometria, or a computerized star atlas such as MegaStar, Guide, SkyMap or The-Sky. Online resources that should be consulted:
If it is still there, but has moved somewhat, check with the "Skyline" service of Sky and Telescope Magazine at (617) 497-4168. This service provides a few minutes worth of late breaking news, updated every Friday afternoon, or more often if something big breaks. If you still think you have something new, send a telegram or telex to the Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory (SAO) in Cambridge, MA. The Telex number is (710)320-6842, answerback ASTROGRAM CAM. Give your name and address, the date and time (Universal Time) in decimals of a day, the right ascension and declination of the suspected comet, the direction of motion, and magnitude. After confirmation, and if you are no later than third to report the fording, your name may go down in history as a comet discoverer.
Harvard's International Comet Quarterly, at Descriptive Information on Recent Comets, has some historic descriptions of comet observations that give you an idea of the type of reports that are submitted.
1 Bishop, Roy L., ed., Observer's Guide: 1993, The Royal Astronomical Society of Canada, "Observing Comets," Levy, David H., p. 174.
2 Harrington, Phillip S., Touring the Universe through Binoculars, Wiley & Sons, 1990.
3 Levy, David H., The Sky: A User's Guide, Cambridge University Press, 1991.
4 Liner, William, The Cambridge Guide to Astronomical Discovery, Cambridge University Press, 1992.