4.4. KEYWORDS

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FITS data array that is used to represent an undefined pixel and not the corresponding

physical value (computed from Eq. 4.3) . To cite a specific, common example, unsigned

16-bit integers are represented in a signed integer FITS array (with BITPIX = 16) by

setting BZERO = 32768 and BSCALE = 1. If it is desired to use pixels that have an

unsigned value (i.e., the physical value) equal to 0 to represent undefined pixels in the

array, then the BLANK keyword must be set to the value -32768 because that is the

actual value of the undefined pixels in the FITS array.

DATAMAX Keyword  The value field shall always contain a floating-point number, re-

gardless of the value of BITPIX. This number shall give the maximum valid physical

value represented by the array (from Eq. 4.3), exclusive of any IEEE special values.

DATAMIN Keyword  The value field shall always contain a floating-point number, re-

gardless of the value of BITPIX. This number shall give the minimum valid physical

value represented by the array (from Eq. 4.3), exclusive of any IEEE special values.

WCS Keywords  An extensive set of keywords have been defined to describe the

world coordinates associated with an array. These keywords are discussed separately in

§8.

4.4.2.6

Extension Keywords

Although these keywords were originally defined for use within the header of a conform-

ing extension, they also may appear in the primary header with an analogous meaning.

If these keywords are present, it is recommended that they have a unique combination

of values in each HDU of the FITS file.

EXTNAME Keyword  The value field shall contain a character string to be used to

distinguish among different extensions of the same type, i.e., with the same value of

XTENSION, in a FITS file. Within this context, the primary array should be considered

as equivalent to an IMAGE extension.

EXTVER Keyword  The value field shall contain an integer to be used to distinguish

among different extensions in a FITS file with the same type and name, i.e., the same

values for XTENSION and EXTNAME. The values need not start with 1 for the first extension

with a particular value of EXTNAME and need not be in sequence for subsequent values.

If the EXTVER keyword is absent, the file should be treated as if the value were 1.

EXTLEVEL Keyword  The value field shall contain an integer specifying the level in a

hierarchy of extension levels of the extension header containing it. The value shall be 1

FITS Standard