nextupprevious
Next:The screening procedure andUp:The dataPrevious:The sources
 


Main categories of observers

There are five data sets that deserve a detailed analysis:
  1. long-focus photographic observations by Elizabeth Roemer;
  2. James Scotti's Spacewatch observations;
  3. recent data taken by other professional astronomers using CCD cameras attached to medium- and large-size telescopes;
  4. amateur observations with small-size telescopes.
  5. observations of cometary nuclei using the Hubble Space Telescope.
Elizabeth Roemer started her observations in 1950. Almost every comet to appear in the following 25 years was photographed by her. She used the 1-m Ritchey-Chrétien reflector at the Flagstaff Station of the U.S. Naval Observatory. The trend at that time was to estimate the nuclear magnitude by visual inspection of the plates, where the sharp concentration of the inner coma was actually measured. Roemer argued that long-focus instruments and photographic techniques were best suited for the observation of quasi-stellar comets, where little or no traces of the coma were recorded.
Contributing 18% of the entries in our data base, Roemer's material is obviously of great interest. During the Roemer era, her observations were generally the faintest, indicating that she indeed came closest to measuring the actual nuclei, whereas other observers were more influenced by coma contamination. However, when viewed from a present-day perspective, it is clear that there is no guarantee that her stellar sources would represent the bare nuclei, and generally speaking, CCD frames offer much better opportunities. This is especially true in the many cases when they have reached 2-3 magnitudes deeper than Roemer's plates and caught the comets closer to their aphelia.

The second large data set from a single observer corresponds to James Scotti's observations with the Spacewatch telescope. This is a 91-cm Schmidt telescope at Kitt Peak dedicated to the discovery and follow-up of Near-Earth Objects. Scotti started his cometary observations with T. Gehrels, but since the late 80's he has conducted his own program for cometary photometry. 24% of the entries in our data base corresponds to his observations (we do not take into account the reports of total magnitudes at r < 3 AU). During the 90's he has recovered many comets at large heliocentric distances (in the sense of the first observation after aphelion passage).

Scotti has developed a tentative method to face the problem of nuclear magnitude determination in active comets. Though a detailed description of the method has yet to be published, we asked Scotti for a brief sketch of it. As described in Sect. 1, the brightness profile of an active comet is the addition of the brightness profile of the coma plus a stellar-like nucleus. Assuming an optically thin coma, the typical width of the nucleus contribution is on the order of two times the Full Width at Half Maximum of the PSF (the seeing). At a distance a few times the radius of the seeing disk from the photometric centre of the comet, the contribution to the profile mainly comes from the coma. Scotti then takes a thin annulus of this radius and computes the mean surface brightness ($\sigma$). He assumes a constant coma surface brightness inward of that annulus. From the total flux of a disk centered on the brightest pixel he subtracts a coma flux corresponding to: $\sigma \times A$  (A - the disk area). The remaining flux supposedly corresponds to the contribution of the nucleus. A nuclear magnitude can thus be computed.

Since 10% of our data correspond to coma-corrected observations thus derived by Scotti, let us scrutinize these a little closer. Several drawbacks may question the validity of this coma-correction method:

i) The assumption of an optically thin coma

ii) The assumption of a constant coma surface brightness ($\sigma$) inside the annulus.

iii) The contamination of the background ``sky'' by fain objects.

iv) The difference between the total and coma fluxes inside the small disc may be close to the noise level.

v)The color of the magnitude. Scotti's CCD frames are taken without filters.

From the previous discussion we can conclude that Scotti's coma subtraction method may be rather uncertain, at least in the cases of very active comets, as shown by the wide spread of the estimated nuclear magnitudes in some cases; for low-active comets it may give more useful estimates of the nuclear magnitudes.

The majority of Scotti's reports are taken from the MPCs. We have no information if a total magnitude corresponds to a comet with stellar appearance, or if the coma subtraction method was applied to the reported nuclear magnitude. In many cases, for similar observing days, Scotti reported total as well as nuclear estimates. We decided to include all his nuclear as well as total reports (even those at r < 3 AU); in that way we were able to analyse the relative contribution of the nucleus and the coma to the total magnitude and the validity of the coma subtraction method.

Note that almost half of the observations in our data base correspond to Roemer or Scotti.

Professional astronomers using medium and large telescopes (say, larger than 1.5 m aperture) with CCDs constitute another important group of observers to be discussed. The pioneer in this field was David Jewitt, who started in 1984 a photometric program of distant comets, first with Karen Meech and later with Jane Luu. During the 90's several other groups have engaged in the difficult task of detecting comets close to their aphelia; e.g., Larson & Hergenrother, Meech, Hainaut & West, Mueller, Fitzsimmons & Williams and our group (Licandro et al. [1999a]). These groups use high quality CCDs attached to large telescopes in sites with good seeing conditions; the detection of even a weak coma is much easier and the distinction between nuclear and total magnitudes more clear. The members of these groups define the nuclear magnitude as the total magnitude of the comet when it has a stellar appearance. If there is no coma, this magnitude would correspond to the definition of nuclear magnitude we have adopted in Sect. 2.

We then have the contribution of many amateur astronomers. The amateur data is very inhomogenous. Some consider a magnitude as nuclear only if they see the comet inactive (no detectable coma), but most of them define the nuclear magnitude as the magnitude of the central brightness. In the past, they may even have tried some methods for ``better'' estimates of nuclear magnitudes from visual observations, such as different defocussing methods (see e.g. Kamél [1991]). Though we have taken away all the visual observations from the CLICC data set, it was not possible to do the same for MPC data due to the lack of information. Nevertheless, the number of visual reports of nuclear magnitudes has been very low in recent years.

Though the amateur contribution has been of great value in the analysis of total magnitudes, in particular perihelion comet lightcurves, the quality of the amateur nuclear magnitudes is very poor, which makes their reports of little use, unless the comet is far from the Sun and inactive, but ``professional-like'' amateurs with large telescopes and CCDs, like William Offutt, are then required.

The data taken from the HST observations included in our catalog correspond to only 4 comets: 4P/Faye, 22P/Kopff, 9P/Tempel 1 and 46P/Wirtanen. There are observations of 19P/Borrelly and 45P/Honda-Mrkos- Pajdusáková, but they were taken at large phase angles (38o and 90o, respectively). As explained below, these large phase angle observations are discarded from our catalog.


nextupprevious
Next:The screening procedure andUp:The dataPrevious:The sources
Gonzalo Tancredi

2000-03-07