FITS data-types and extensions

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 7.1.2. 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 7.1: Relation between FITS and MIDAS data types

BITPIX value	Data representation	Scaling	MIDAS data type
8	8-bit unsigned integer (ASCII)	No	D_I1_FORMAT
		Yes	D_R4_FORMAT
16	16-bit twos-complement integer	No	D_I2_FORMAT
		Yes	D_R4_FORMAT
32	32-bit twos-complement integer	No	D_I4_FORMAT
		Yes	D_R4_FORMAT
-32	32-bit IEEE floating point	-	D_R4_FORMAT
-64	64-bit IEEE floating point	-	D_R8_FORMAT

 

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 7.1.2 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 7.2: Relation between FITS Extensions and MIDAS frame types

FITS Extension type	$\rightarrow$	MIDAS frame
prime		image ( .bdf)
random groups		image ( .bdf) + table ( .tbl)
TABLE		table ( .tbl)
BINTABLE		table ( .tbl)
IMAGE		image ( .bdf)