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SECTION 8. WORLD COORDINATE SYSTEMS

keywords are missing, default values must be assumed, as specified below.

WCSAXES -- [integer; default: NAXIS, or largest of WCS indexes i or j]

Number of axes in the WCS description. This keyword, if present, must precede

all WCS keywords except NAXIS in the HDU. The value of WCSAXES may exceed

the number of pixel axes for the HDU.

CTYPEi -- [character; indexed; default: ` ' (i.e. a linear, undefined axis)]

Type for the intermediate coordinate axis i. Any coordinate type that is not

covered by this standard or an officially recognized FITS convention shall be taken

to be linear. All non-linear coordinate system names must be expressed in "4­3"

form: the first four characters specify the coordinate type, the fifth character is

a hyphen (`-'), and the remaining three characters specify an algorithm code for

computing the world coordinate value. Coordinate types with names of less than

four characters are padded on the right with hyphens, and algorithm codes with

less than three characters are padded on the right with blanks1. Algorithm codes

should be three characters.

CUNITi -- [character; indexed; default: ` ' (i.e., undefined)]

Physical units of CRVAL and CDELT for axis i. Note that units should always be

specified (see §4.3). Units for celestial coordinate systems defined in this Standard

must be degrees.

CRPIXj -- [floating-point; indexed; default: 0.0]

Location of the reference point in the image for axis j corresponding to rj in

Eq. (8.1). Note that the reference point may lie outside the image and that the

first pixel in the image has pixel coordinates (1.0, 1.0, . . .).

CRVALi -- [floating-point; indexed; default: 0.0]

World Coordinate value at the reference point of axis i.

CDELTi -- [floating-point; indexed; default: 1.0]

Increment of the world coordinate at the reference point for axis i. The value

must not be zero.

CROTAi -- [floating-point; indexed; default: 0.0]

The amount of rotation from the standard coordinate system to a different coor-

dinate system. Further use of this of this keyword is deprecated, in favor of the

newer formalisms that use the CDi_j or PCi_j keywords to define the rotation.

PCi_j -- [floating-point; defaults: 1.0 when i = j, 0.0 otherwise]

Linear transformation matrix between pixel axes j and intermediate coordinate

axes i. The PCi_j matrix must not be singular.

1

Example: `RA---UV '.

FITS Standard