Archive Abstract
================
  This dataset contains visual magnitudes of comet C/1995 O1 (Hale-Bopp) that
  were obtained from the International Comet Quarterly and processed to provide
  a secular lightcurve from -7 AU (pre-perihelion) to +8 AU (post-perihelion).
  The original apparent magnitudes from 17 observers were corrected for
  geocentric distance and phase angle, and then combined in a systematic way
  that yielded a self-consistent consensus fit. We estimated the shifts with a
  self-consistent statistical approach, leading to a sharper light curve and
  improving the precision of the measured slopes. The dataset includes the
  original apparent magnitudes, those corrected for geocentric distance and
  phase angle, and the final shifted and weighted values. The final secular
  lightcurve is the best produced to date for comet Hale-Bopp.

Archive Package Description
===========================
  Below we explain the major data reduction steps we executed to the original
  apparent magnitudes (referred to as 'mapp,') submitted by the observers. We
  describe it in three major steps:

  1. Removing problematic entries
  2. Geocentric distance and phase angle corrections
  3. Observer shift, weighting and co-add

  Each step is explained below:

  1. Removing problematic entries

     The raw visual apparent magnitudes were obtained by multiple observers who
	 all submitted their observation data to the International Comet Quarterly
	 archive. The data were provided to us by D.W.E. Green who also provided us
	 with best practices advice. Thus, we removed values reported with:

     *  poor observing conditions
     *  only an approximate value noted
     *  no magnitude method listed
     *  under 20 degrees elevation with no extinction correction
     *  upper limits only
     *  more than one magnitude per day per observer (if so, we kept the value
	    for smallest aperture instrument)
     *  telescopes used for m=5.4 and brighter
     *  binoculars used for m=1.4 and brighter
     *  CCD or photoelectric detectors (there were very few measurements in this
        category and we deemed it inappropriate to include them along with the
	    human eye visually-determined values)

     Lastly, we removed measurements when the comparison star method used was
	 not listed as recommended by the International Comet Quarterly, published
	 in 2007. A PDF of this reference, icqref.pdf, is included as a document.

  2. Geocentric distance and phase angle corrections

     The apparent magnitudes, mapp, were corrected for geocentric distance of
	 the comet using this equation: mhelio=mapp-5log(D), where D was in
	 astronomical units. This is referred to as the heliocentric magnitude.

     The heliocentric magnitudes were corrected for phase angle of the comet
	 using this equation: mph=mhelio+2.5log10(phi), where phi=phase function for
	 a particular phase angle normalized to zero degrees. We used the composite
	 phase function derived by Schleicher et al. 2011. A text file of this
	 table, schleicherdustphasehm_table.txt, is included as a document. Thus,
	 mph values are corrected for both geocentric distance and phase angle.

  3. Observer shift, weighting and co-add

     In analyzing visual data from multiple observers, the questions inevitably
     arise of which data to reject, and under what justification, and whether
     combining data from observers, each with his or her own systematic errors,
     leads to a biased result. Without instrumental calibration, there is no
     certain answer to these questions, but such calibration is itself
	 problematic, and in any case is not available for the observations
	 discussed here.
	 
     We offer a systematic approach to combining data from multiple observers
     yielding a self-consistent consensus fit. In application to comet Hale-Bopp
     (C/1995 01), the procedure does not significantly affect the grossest
	 measure, namely the slope of magnitude vs. log distance, but, applied to
	 data already corrected to heliocentric distances and for phase, does reduce
	 the statistical error bars.
     
     We assume three categories of errors:

     1. Every observer reports the brightness of an object on a scale that is
        shifted up or down from other observers, but by the same number of
		magnitudes, dobs, independent of distance or brightness. Without
		instrumental calibration, we can best estimate dobs as that observer's
		mean deviation from a consensus fit to the data (that is, an average).

     2. Some observers may have a slope bias, underestimating the brightness of
        dimmer objects and overestimating those of brighter ones, or vice-versa.
		While it is difficult to correct for such error without calibration, the
		bias can be detected (relative to the consensus fit), and the data from
		that observer can be discarded.

     3. Finally, some observers may have a great deal of scatter in their data
	    but no bias. We can weight these observations less in fits.

     This procedure is explained in more detail with figures and tables in the
     iPoster we presented at the 49th AAS DPS meeting, and will soon be
	 submitted for publication in a refereed journal. A PDF of this method,
	 statanalysis.pdf, is included as a document.

     These final values are referred to as mshift.