84

SECTION 8. WORLD COORDINATE SYSTEMS

Reserved Spectral Coordinate Type Codes1

Table 8.5.

Code

Type

Symbol

Assoc. Variable

Default Units

Frequency

ν

ν

Hz

FREQ

Energy

E

ν

J

ENER

m-1

Wavenumber

κ

ν

WAVN

Radio velocity2

m s-1

V

ν

VRAD

Vacuum wavelength

λ

λ

m

WAVE

Optical velocity2

m s-1

Z

λ

VOPT

Redshift

z

λ

···

ZOPT

Air wavelength

λa

λa

m

AWAV

m s-1

Apparent radial velocity

v

v

VELO

Beta factor (v/c)

β

v

···

BETA

1Characters

1 through 4 of the value of the keyword CTYPEi a.

2By

convention, the "radio" velocity is given by c(ν0 - ν )/ν0 and the "optical"

velocity is given by c(λ - λ0)/λ0 .

Following the convention for the CTYPEi a keyword, when i is the spectral axis the first

four characters must specify a code for the coordinate type; for non-linear algorithms

the fifth character must be a hyphen, and the next three characters must specify a

predefined algorithm for computing the world coordinates from the intermediate physical

coordinates. The coordinate type must be one of those specified in Table 8.5. When

the algorithm is linear, the remainder of the CTYPEi a keyword must be blank. When

the algorithm is non-linear, the 3-letter algorithm code must be one of those specified

in Table 8.6. The relationships between the basic physical quantities ν, λ, and v, as well

as the relationships between various derived quantities are given in reference [14].

The generality of the algorithm for specifying the spectral coordinate system and

its representation suggests that some additional description of the coordinate may be

helpful beyond what can be encoded in the first four characters of the CTYPEi a keyword;

CNAMEi a is reserved for this purpose. Note that this keyword provides a name for an

axis in a particular WCS, while the WCSNAMEa keyword names the particular WCS as a

whole. In order to convert between some form of radial velocity and either frequency

or wavelength, the keywords RESTFRQa and RESTWAVa, respectively, are reserved.

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

Spectral coordinate description which must not exceed 68 characters in length.

FITS Standard