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Description
The files icpos.dat and ngcpos.dat are intended to provide a handy source for fairly accurate positions for the NGC/IC objects. They have only one entry per object, giving an unweighted mean position followed by 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, obvious blunders have been rejected, 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". Note also that the standard deviations reflect more accurately the uncertainties in the unweighted mean positions rather than the mean errors. Errors in published positions (see the "refs.dat" 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. Mean errors can be easily calculated by dividing the standard deviations by the square root of the number of positions used. Finally, these positions are on the FK5 system, and are not reduced to the International Celestial Reference Frame (ICRF). However, the FK5 optical system is consistent with ICRF to within the known errors of the older system (see Ma et al, 1998AJ....116..516M and references therein for more information about ICRF). The individual positions are reported in the files icdata.dat and ngcdata.dat. The files icnotes.txt and ngcnotes.txt contain discussions about ambiguous, non-existent or other mysterious NGC/IC objects. A detailed introduction to the contents of these discussion files can be found in the "ninotes.txt" file.
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