Up: A catalog of observed comets
Several criteria have been taken into account when assigning
values
to the JF comets. Furthermore, we have classified the adopted nuclear
magnitudes into four quality classes. We now present some general
considerations that were used for
assignment and quality classification.
- We require consistency of several observations taken at somewhat
different heliocentric distances. In almost all cases, CCD data taken from
the Spacewatch or larger telescopes are involved. The cases where such data
are absent are confined to the worst quality class or eliminated altogether;
- In many cases, different observers give discordant magnitudes.
Typically, a set of corrected nuclear magnitudes by Scotti is
fainter than another set of data. The weight given to the Scotti data then
depends on the number of his observations, their spacing in r, and their
internal consistency. A single observation is usually discarded, a couple of
closely spaced observations is given low weight, while if there are
three or more data points well spaced in r and in good mutual agreement we
give to their mean a strong weight in the derived magnitude;
- We take special care of comets that show a strong trend for the
nuclear magnitudes to get fainter with increasing r. If there is not a
clear indication of a leveling off and if the observations do not approach
the aphelion distance, the quality class assigned is usually the worst. The
adopted magnitude is a mean of the faintest, more distant observations, and
this should be taken as an upper limit to the true magnitude of the comet
nucleus;
- Total magnitudes at r > 3 AU are included for purposes of comparison
with nuclear magnitudes measured at similar distances. If the total
magnitudes
are much brighter than the nuclear ones, this is an indication that the comet
keeps active all along the orbit. On the other hand, if the total magnitudes
are similar to the nuclear ones, the comet may have little or no activity;
- In line with our choice of not placing much weight on single
observations or close pairs, whether discordant or consistent, even if these
data are of the CCD kind, we have discarded almost all comets where the only
data were such. The only comet included in this category was
97P/Metcalf-Brewington, where the observations were taken at far heliocentric
distances and the comet was inactive (Licandro et al. 1999a).
Our definition of the quality classes (QC) is as follows:
QC 1: These are our best nuclear magnitudes for which we have many
observations spread along a wide range of heliocentric distances and from
more than one observer. The uncertainty in the adopted nuclear magnitude is
less than or about
mag.
QC 2: These are fairly good estimates based on several observations
spread throughout a wide range of r, or few observations but made at very
large r. The uncertainty is estimated to be between
and
mag.
QC 3: These are estimates made at different r with a somewhat larger scatter
than for the previous class. The uncertainty should be between
and
mag.
QC 4: These are poor estimates, either because they rely upon old data, or
upon very few observations, or because the observations show a large scatter.
Some of
them can only be considered as a lower limit to the nuclear magnitude (i.e.,
the true magnitude of the comet nucleus is fainter than the quoted value).
The uncertainty is generally well above
mag.
Up: A catalog of observed comets
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