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