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The determination of comet brightness has been one of the important goals of comet observers for the last century or so. The challenge of making reliable estimates has been enormous since one is dealing with extended sources in motion with respect to background stars. Most estimates of comet magnitudes have been done by visual or photographic methods. However, CCD observations have become increasingly common in recent years.

Most of the determined comet magnitudes have referred classically to the magnitude of the gaseous coma surrounding the comet nucleus. These have been known as ''total'' magnitudes. Max Beyer was one of the pioneers in trying to determine visually magnitudes of the comet nucleus during the 1930's and -40's. His nuclear magnitudes were grossly underestimated (i.e. the nucleus brightness overestimated) due to coma contamination. The following most serious attempt at determining a homogeneous data set of nuclear magnitudes was carried out by Elizabeth Roemer during a time span of more than 25 years (from 1950 to the late seventies). Roemer used photographic plates taken with the 1-m Ritchey-Chrétien reflector at the Flagstaff Station of the U.S. Naval Observatory (see, for instance, Roemer [1976]).

Near the end of the ``Roemer era'' there was still serious doubts that a comet nucleus would have been resolved in any case (e.g. Sekanina [1976]), so most researchers tended to regard ``nuclear'' magnitudes as the magnitude of the solid nucleus plus an inner coma. During the 1980's, comet 1P/Halley of course became the main target of cometary research and the great opportunity to observe for the first time a bare nucleus by means of spacecraft fly-by. This goal was successfully accomplished by the Giotto and Vega missions. The 1P/Halley nucleus turned out to be an elongated body of 14.2 km x 8.2 km x 7.5 km of very low albedo and with an active surface area no greater than about 15% of the total (Keller et al. [1987]). The knowledge of the nucleus size allowed for the first time a direct comparison between comet size and the earlier estimates of the nuclear magnitude based on ground-based CCD observations of 1P/Halley at distances greater than 8 AU. The nucleus size derived by Jewitt & Danielson ([1984]) from these distant observations turned out to be a factor of two smaller than the size derived by the space missions. In any case, the new technology of CCD cameras attached to large telescopes proved to be very promising at observing distant comets - where they are presumably little active or inactive - with the scope of deriving nuclear magnitudes and sizes.

CCD photometry of comets became of widespread use in the post-Halley era, with observers like David Jewitt, Karen Meech, Tom Gehrels and James Scotti among the pioneers in the use of this new technology for the study of comets. The much higher sensitivity of CCD detectors and the use of medium-sized to large telescopes allowed the observation of a large number of comets beyond 3 AU and the early recovery of short-period comets when they had little activity or no activity at all. Of particular relevance is the work of James Scotti with the 91-cm Spacewatch telescope. Scotti has not only observed distant comets systematically, contributing to the early recovery of a large fraction of the short-period comets, but due to the high-resolution surface photometry capacity of its CCD exposures, he has also introduced a method of coma subtraction to derive an improved magnitude of the nucleus (see Sect. 3.2).

The latest improvement on the quality of the observations has been achieved with the data taken by the Hubble Space Telescope, obtained under the leadership of Philippe Lamy. The outstanding image quality obtained by the HST allows the application of a more refined coma subtraction technique even in highly active comets (Lamy & Toth [1995] and later references, see below).

In summary, the last decade of the century has seen a growing activity in photometric observations of distant comets. However, overall only a wealth of photometric data has been produced without detailed analysis of its physical meaning. There are just a few exceptions for some particular comets. We now deem that the time is ripe to undertake a broad analysis of the observed nuclear magnitudes. We restrict our sample to the comets of the Jupiter family (JF) that we define following Valsecchi ([1992]) as those with Tisserand constants T > 2 and periods P < 20 yr (there are so far only four comets with P < 20 yr that have T < 2). We choose this population for two reasons: (1) a large fraction of the JF population has been extensively observed photometrically, whereas long-period comets and Halley-type comets show only scattered data; and (2) we would like to analyze a homogeneous population, presumably coming from the same source region (in this case the Edgeworth-Kuiper belt). Admittedly, we do not know to what extent this presumption is correct, since the JF population may be contaminated with comets coming from other sources as, for instance, the Oort cloud (Bailey [1986]) or the Trojans (Rabe [1972]), it is likely that such contaminations represent only a minority of the whole population.

The present catalog is a continuation of a project started several years ago (see Fernández et al. [1992]), that includes our own observational program (Licandro et al. [1999a]). A first version of this catalog was presented by Tancredi et al. (2000). A theoretical analysis based on the information described in the first version was presented by Fernández et al. [1999]. A re

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Gonzalo Tancredi