just a note, when making estimations of a comets brightness, care should be taken to to try to use objects/stars at around the same elevation as the comet's head/coma, (atmospheric extinction) evry degree of elevation makes a big diff of course.
also dont use variables or stars with a BV value over (+) 0.5 (altho you can use up to 1.00 in a pinch i spose - the higher the BV the more towards red - ie Antares is B-V + 1.86, comets heads tend to be green/blue of course
