Coordinate systems for solar image data

A set of formal systems for describing the coordinates of solar image data is proposed. These systems build on current practice in applying coordinates to solar image data. Both heliographic and heliocentric coordinates are discussed. A distinction is also drawn between heliocentric and helioprojective coordinates, where the latter takes the observer's exact geometry into account. The extension of these coordinate systems to observations made from non-terrestial viewpoints is discussed, such as from the upcoming STEREO mission. A formal system for incorporation of these coordinates into FITS files, based on the FITS World Coordinate System, is described, together with examples.