4 Alternate FITS image representations

4.1 Random groups visibility data

The random-groups extension to FITS (Greisen & Harten 1981) has been used to transmit interferometer fringe visibility sample data. It has been customary among users of this format to convey the suggested projection type as the last four characters of the random parameter types (PTYPE n) of the Fourier plane coordinates (u,v,w). Any rotation of these coordinates was carried, however, by CROTA i associated with the one-pixel array axis used to convey the field declination. Within the new convention users of random groups for visibility data should be prepared to read, carry forward, and use as needed, the new keywords PC i_ja, CD i_ja, LONPOLE a, LATPOLE a, PV i_ja, MJD-OBS, EQUINOX a, and RADESYS a. In particular, any rotation of the (u,v) coordinate system should be represented via PC i_ja or CD i_ja (but not both) attached to the two degenerate axes used to convey the celestial coordinates of the field center.

4.2 Pixel list and image array column elements

Paper I defined the coordinate keywords used to describe a multi-dimensional image array in a single element of a FITS binary table and a tabulated list of pixel coordinates in a FITS ASCII or binary table. In this section we extend this to the keywords specific to celestial coordinates.

4.2.1 Keyword naming convention

Table 3 lists the corresponding set of coordinate system keywords for use with each type of FITS image representation.

The data type of the alternate keyword matches that of the primary keyword and the allowed values are the same. The following notes apply to the naming conventions used in Table 3:

• a is a 1-character coordinate version code and may be blank (for the primary version of the coordinate description) or any single uppercase character from A through Z.
• n is an integer table column number without any leading zeros (1 - 999).

When using the BINTABLE image array format (Cotton et al. 1995), if the table only contains a single image column or if there are multiple image columns but they all have the same value for any of the keywords in Table 3 then the simpler form of the keyword name, as used for primary arrays, may be used. For example, if all the images in the table have the same epoch then one may use a single EQUINOX a keyword rather than multiple EQUI na keywords. The other keywords, however, must always be specified using the more complex keyword name with the column number suffix and the axis number prefix.

 

 

Table 3: Alternate coordinate keywords; the data type of the alternate keyword matches that of the primary keyword.

Keyword	Primary	BINTABLE	Pixel
Description	Array	Array	List
Coord Rotation	LONPOLE a	LONP na	LONP na
Coord Rotation	LATPOLE a	LATP na	LATP na
Coord Epoch	EQUINOX a	EQUI na	EQUI na
Date of Obs	MJD-OBS	MJDOB n	MJDOB n
Reference Frame	RADESYS a	RADE na	RADE na

In principle, more than one pixel list image can be stored in a single FITS table by defining more than one pair of p₁ and p₂ pixel coordinate columns. Under the convention defined here, however, all the images must share the same values for the keywords in Table 3.

Example binary table and pixel list headers are given in Sect. 7.3.2.