4.4. KEYWORDS

35

through 80 may contain any of the restricted set of ASCII text characters, decimal 32

through 126 (hexadecimal 20 through 7E).

COMMENT Keyword

This keyword may be used to supply any comments regarding the

FITS file.

HISTORY Keyword  This keyword should be used to describe the history of steps and

procedures associated with the processing of the associated data.

Keyword Field is blank  This keyword may be used to supply any comments re-

garding the FITS file. It is frequently used for aesthetic purposes to provide a break

between groups of related keywords in the header.

4.4.2.5

Keywords that Describe Arrays

These keywords are used to describe the contents of an array, either in the primary

array, in an IMAGE extension (§7.1), or in a series of random groups (§6). They are

optional, but if they appear in the header describing an array or groups, they must

be used as defined in this section of this standard. They shall not be used in headers

describing other structures unless the meaning is the same as defined here.

BSCALE Keyword  This keyword shall be used, along with the BZERO keyword, to

linearly scale the array pixel values (i.e., the actual values stored in the FITS file) to

transform them into the physical values that they represent using Eq. 4.3.

physical_value = BZERO + BSCALE × array_value

(4.3)

The value field shall contain a floating-point number representing the coefficient of the

linear term in the scaling equation, the ratio of physical value to array value at zero

offset. The default value for this keyword is 1.0. Before support for IEEE floating-point

data types was added to FITS [7], this technique of linearly scaling integer values was

the only way to represent the full range of floating-point values in a FITS array. This

linear scaling technique is still commonly used to reduce the size of the data array by a

factor of 2 by representing 32-bit floating-point physical values as 16-bit scaled integers.

BZERO Keyword This keyword shall be used, along with the BSCALE keyword, to

linearly scale the array pixel values (i.e., the actual values stored in the FITS file) to

transform them into the physical values that they represent using Eq. 4.3. The value

field shall contain a floating-point number representing the physical value corresponding

to an array value of zero. The default value for this keyword is 0.0.

FITS Standard