History and Accurate Positions for the NGC/IC Objects

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.