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SECTION 8. WORLD COORDINATE SYSTEMS

Table 8.7.

Spectral Reference Systems

Value

Definition

Topocentric

TOPOCENT

Geocentric

GEOCENTR

Barycentric

BARYCENT

Heliocentric

HELIOCEN

Local standard of rest (kinematic)

LSRK

Local standard of rest (dynamic)

LSRD

Galactocentric

GALACTOC

Local Group

LOCALGRP

Cosmic microwave background dipole

CMBDIPOL

Source rest frame

SOURCE

Note.  -- These are the allowed values of the

SPECSYSa, SSYSOBSa, and SSYSSRCa keywords.

CUNITi a keyword is not used for this purpose since the WCS version a might not

be expressed in velocity units.

ZSOURCEa -- [floating-point; default: none]

Radial velocity with respect to an alternative frame of rest, expressed as a unitless

redshift (i.e., velocity as a fraction of the speed of light in vacuum). Used in

conjunction with SSYSSRCa to document the systemic velocity of the observed

source.

VELANGLa -- [floating-point; default:+90.]

In the case of relativistic velocities (e.g., a beamed astrophysical jet) the trans-

verse velocity component is important. This keyword may be used to express the

orientation of the space velocity vector with respect to the plane of the sky. See

appendix A of reference [14] for further details.

8.5

Conventional Coordinate Types

The first FITS paper [2] listed a number of "suggested values" for the CTYPEi keyword.

Two of these have the attribute the associated world coordinates can assume only integer

FITS Standard