Koma (komet)

With basic Earth-surface based telescope and some technique, the size of the Coma can be calculated. Called the drift method, one locks the telescope in position and measures the time for the visible disc pass through the field of view. That time multiplied by the cosine of comet's declination, times .25 should equal the coma's diameter in arcminutes. If the distance to the comet is known, then the apparent size of the coma can be determined.