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:
(2) |
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 [1989]). 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 30^{o} 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.