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SECTION 8. WORLD COORDINATE SYSTEMS

8.2.1

Alternative WCS Axis Descriptions

In some cases it is useful to describe an image with more than one coordinate type  1.

Alternative WCS descriptions may be added to the header by adding the appropriate

sets of WCS keywords, and appending to all keywords in each set an alphabetic code in

the range A through Z. Keywords that may be used in this way to specify a coordinate

system version are indicated in Table 8.2 with the suffix a. All implied keywords with

this encoding are reserved keywords, and must only be used in FITS HDUs as specified in

this Standard. The axis numbers must lie in the range 1 through 99, and the coordinate

parameter m must lie in the range 0 through 99, both with no leading zeros.

The primary version of the WCS description is that specified with a as the blank

character2 . Alternative axis descriptions are optional, but must not be specified unless

the primary WCS description is also specified. If an alternative WCS description is

specified, all coordinate keywords for that version must be given even if the values do

not differ from those of the primary version. Rules for the default values of alternative

coordinate descriptions are the same as those for the primary description. The alter-

native descriptions are computed in the same fashion as the primary coordinates. The

type of coordinate depends on the value of CTYPEi a, and may be linear in one of the

alternative descriptions and non-linear in another.

The alternative version codes are selected by the FITS writer; there is no require-

ment that the codes be used in alphabetic sequence, nor that one coordinate version

differ in its parameter values from another. An optional keyword WCSNAMEa is also

defined to name, and otherwise document, the various versions of WCS descriptions:

WCSNAMEa -- [character; default for a: ` ' (i.e., blank, for the primary WCS, else a

character A through Z that specifies the coordinate version]

Name of the world coordinate system represented by the WCS keywords with the

suffix a. Its primary function is to provide a means by which to specify a particular

WCS if multiple versions are defined in the HDU.

8.3

Celestial Coordinate System Representations

The conversion from intermediate world coordinates (x, y) in the plane of projection to

celestial coordinates involves two steps: a spherical projection to native longitude and

latitude (φ, θ), defined in terms of a convenient coordinate system (i.e., native spherical

coordinates ), followed by a spherical rotation of these native coordinates to the required

1

Examples include the frequency, velocity, and wavelength along a spectral axis (only one of which,

of course, could be linear), or the position along an imaging detector in both meters and degrees on the

sky.

2

There are a number of keywords (e.g. ij PCn a ) where the a could be pushed off the 8-char keyword

name for plausible values of i, j, k, n, and m. In such cases a is still said to be "blank" although it is

not the blank character.

FITS Standard