Unweighted Mean Positions

These files are intended to provide a handy source for fairly accurate 
positions for the NGC/IC objects.  They have only one entry per object.  An 
unweighted mean position follows the NGC/IC name as I've adopted it in the
position data files.  Next comes the number of positions used in calculating 
that mean, the calculated standard deviations in RA and Dec, and the sources 
used.  The precision to which the position is given reflects the precision of 
the most precise position in the full list.  

Note that precision does not necessarily reflect accuracy! especially with 
unweighted means.  However, I've rejected obvious blunders, and many objects 
have enough accurately-measured positions now that overall accuracy will not 
suffer much from poor positions out on the wings of the "bell curve".

Two sets of files are available, identical except for the equinoxes:  B1950.0 
and J2000.0.  The equinox B1950.0 file is the "root" file, with the equinox 
J2000.0 file being derived from it.  The data in the individual files (of one
thousand NGC/IC numbers each) are identical to the data in the combined files;
download whichever set you want.

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Please note that J2000 positions produced between mid-2001 and 28 October 2005 
may have errors of over an arcminute in them due to a bug in GCC 2.96.  Most 
of the precessed positions are all right, but a few are not (see e.g. NGC 1593 
where the RA seconds for the incorrect position are listed as 16.333; the 
correct RA seconds is 06.126).  The B1950 positions are not affected.  My 
apologies for any problems arising from this.
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Some Picky Details:

I've used standard deviations rather than mean errors as these more accurately 
reflect the uncertainties in the unweighted mean positions.  Errors in 
published positions (see the Sources file) are almost always underestimated, 
so quoting the standard deviation increases the qualitative perception of the
errors to about what they should be in an ideal world.  Once I have calculated 
weighted means, the mean errors will have more relevance.  In the meantime, if 
you want them, mean errors can be easily calculated by dividing the standard 
deviations by the square root of the number of positions used.

The precession routine that I used came from NED, which I am pleased to 
acknowledge:  

  This research has made use of the NASA/IPAC Extragalactic Database (NED) 
  which is operated by the Jet Propulsion Laboratory, California Institute of 
  Technology, under contract with the National Aeronautics and Space 
  Administration.  See http://nedwww.ipac.caltech.edu/help/calc_help.html for 
  more details about the precession.  

Finally, these positions are strictly on the FK5 system -- they have not
been reduced to the more recently adopted International Celestial Reference 
Frame (ICRF).  However, the FK5 optical system is consistent with ICRF to 
within the known systematic errors of the older system (see Ma et al, 
Astronomical Journal, Vol. 116, p. 516, 1998 and references therein for 
more information about ICRF), so given the relatively large accidental 
errors in the present data, you may also take these positions as being 
consistent with the ICRF.

21 November 2008