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The screening procedure and the reduction

As mentioned, we have discarded all magnitudes determined visually from the CLICC data set since they are too unreliable. We could not do the same for the MPC data, but there are likely very few visual observations there anyway. Total magnitudes of comet 29P/Schwassmann-Wachmann 1 were also deleted since are generally many units brighter than the large set of more than 200 nuclear estimates.

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:

In case no information is provided about the color, we then assume that they report R-magnitudes and the proper color correction is applied.

The observed apparent magnitudes V are then transformed into absolute magnitudes V(1,1,0) by:

 \begin{displaymath}V(1,1,0) = V - 5 \times \log(r \ \Delta) - \beta \ \alpha\end{displaymath} (2)

where  r  and $\Delta$ 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 30o in order to avoid gross errors in the phase correction.

For each data point we compute ephemeris to calculate the values of  r , $\Delta$ and a and apply the correction from apparent to absolute magnitudes.

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Gonzalo Tancredi