Standard keywords

The FITS format specifies a number of ``standard'' keywords with well defined meaning. They can be classified in two groups namely: a) data structure definition, and b) general description. The first group is translated into special MIDAS descriptors for the data structure in question. The conversion for image files is given in Table 7.1.3. The START descriptor of a MIDAS image refers to the first pixel and is computed from the values CRVAL and CRPIX for the reference pixel as given in the FITS header. The actual reference pixel is saved in the descriptor REFPIX which is necessary for non-linear coordinate systems. Each element of the descriptor CUNIT has 16 character. Thus, the string corresponding to the FITS keyword CTYPE1 is stored in CUNIT(17:31) as it is viewed as a single string of type C*1.

 

Table 7.3: Name translation between standard FITS keywords and MIDAS descriptors. Asterisk indicates that data value is transformed (see text).

FITS keyword	MIDAS descriptor	FITS keyword	MIDAS descriptor
NAXIS	NAXIS	OBJECT	IDENT
NAXISn	NPIX(n)	DATE-OBS	O_TIME(1)
CRPIXn	REFPIX(n)	DATAMIN	LHCUTS(3)
CRVALn	START(n)*	DATAMAX	LHCUTS(4)
CDELTn	STEP(n)	EPOCH	O_POS(3)
CROTAn	ROTA(n)	EQUINOX	O_POS(3)
BUNIT	CUNIT(1)
CTYPEn	CUNIT(n+1)

 

Tables have more complicated structures which make the translation less direct. With respect to other keywords, images and tables are treated in the same way. The data types of these descriptors are checked and must correspond to the FITS standard.