I: "Representations of world coordinates in FITS", Greisen, E.W. & Calabretta, M.R. (2002), Astronomy & Astrophysics, 395, 1061-1075.  

Also a supplement to Paper I entitled Concatenation of FITS World Coordinate Systems by Steve Allen and Doug Mink.

II: "Representations of celestial coordinates in FITS", Calabretta, M.R., & Greisen, E.W., (2002), Astronomy & Astrophysics, 395, 1077-1122.  

III: "Representations of spectral coordinates in FITS", Greisen, E.W., Calabretta, M.R. Valdes, F.G., & Allen, S.L., (2006), Astronomy & Astrophysics, 446, 747-771.  

IV: "Representations of distortions in FITS world coordinate systems", Calabretta et al., draft dated 2004/04/22.

Miscellaneous notes on the derivation of some formulae and special conditions in Paper II, dated 2004/02/11.  

Errata dated 2013/09/21 for Papers I, II & III.

Introduction to celestial coordinate systems; slides (in HTML) from a talk explaining FITS world coordinate systems.

This diagram illustrating Bonne's projection with conformal latitude +35° was produced by WCSLIB, a C implementation of the coordinate transformations defined in the FITS WCS papers. The non-oblique native graticule is in dark green.

The oblique 15° celestial graticule centred on celestial coordinates (0°, +45°) with north celestial pole at native coordinates (45°, 0°) is colour-coded blue, red and white (in increasing order) with the equator and prime meridian in yellow.

"Mapping on the HEALPix grid", Calabretta & Roukema (2007), as accepted by Monthly Notices of the Royal Astronomical Society (MNRAS) on 2007/07/25, describes the HEALPix projection and introduces the HPX projection type in FITS.

The official refereed version of the paper was published as MNRAS 381 865, and can be accessed from ADS .

"Representing the 'Butterfly' Projection in FITS — Projection Code XPH", Calabretta & Lowe (2013), as published with open access in Publications of the Astronomical Society of Australia (PASA) (copyright Cambridge University Press).