We have corrected all the photographic magnitudes (P) introducing the color correction: P - V = 0.6. For the magnitudes we apply a standard color correction V - R = 0.5.
From comparison of MPC reports and IAUC (where more information is given on the magnitude estimates), we have seen that the following observers always report magnitudes in R:
The observed apparent magnitudes V are then transformed into absolute magnitudes V(1,1,0) by:
where r and are the heliocentric and geocentric distances at the instant of the observation and is the phase angle. is the phase coefficient, for which we have assumed a common value of 0.04 mag/degree for all the comets (Jewitt & Luu ). Since the phase coefficient might depart significantly from the adopted value in individual cases (Scotti, private communication), we have discarded magnitudes measured at phase angles greater than 30o in order to avoid gross errors in the phase correction.
For each data point we compute ephemeris to calculate the values of r , and a and apply the correction from apparent to absolute magnitudes.