The FITS header specifies both the format and size of the data records
following. The data representation is defined by the value of the
BITPIX keyword and can have the values given in
Table 
. The MIDAS data format closest matching it is
chosen by default. Single precision real values are used when the
explicit scaling is given in the FITS file ( i.e. by the keywords
BSCALE and BZERO).
Table: Relation between FITS and MIDAS data types
When writing FITS files, their data type will also be the one closest to the internal representation except if basic FITS formats are explicitly requested by an option. In that case, BITPIX will be one of the originally allowed values ( i.e. 8, 16 or 32) or for tables the ASCII format.
Each HDU in a FITS file will normally correspond to a single MIDAS
file. HDU's with no associated data are not stored. Besides the prime
HDU which either is a simple data matrix or a random group structure
a number of extension are defined. Currently, the extensions listed
in Table are translated by MIDAS whereas other
'unknown' extensions are skipped.
Text and MIDAS- fit files can also be stored in FITS. The content of these file types is stored as FITS headers using a special MIDAS conversion. Thus, other FITS readers may not be able to retrieve the full information.
Table: Relation between FITS Extensions and MIDAS frame types