8.2. WORLD COORDINATE SYSTEM REPRESENTATIONS

77

Table 8.2.

Reserved WCS Keywords

Keyword

Primary

BINTABLE vector

Pixel List

Description

Array

primary  alternative

primary  alternative

Coordinate dimensionality

...

WCSAXESa

WCAXna

Axis type

CTYPEi a

i CTYPn

i CTYna

TCTYPn

TCTYna

Axis units

CUNITi a

i CUNIn

i CUNna

TCUNIn

TCUNna

Reference value

CRVALi a

i CRVLn

i CRVna

TCRVLn

TCRVna

Coordinate increment

CDELTi a

i CDLTn

i CDEna

TCDLTn

TCDEna

Reference point

CRPIXj a

j CRPXn

j CRPna

TCRPXn

TCRPna

Coordinate rotation1

CROTAi

i CROTn

TCROTn

Transformation matrix2

TPCn_k a or TPn_k a

PCi_j a

ij PCna

Transformation matrix2

TCDn_k a or TCn_k a

CDi_j a

ij CDna

Coordinate parameter

i PVn_ma or i Vn_ma

TPVn_ma or TVn_ma

PVi_ma

Coordinate parameter array

···

···

i Vn Xa

i PSn_ma or i Sn_ma

TPSn_ma or TSn_ma

Coordinate parameter

PSi_ma

Coordinate name

WCSna or TWCSna

WCSNAMEa

WCSNna

Coordinate axis name

CNAMEi a

i CNAna

TCNAna

Random error

CRDERi a

i CRDna

TCRDna

Systematic error

CSYERi a

i CSYna

TCSYna

WCS cross-ref. target

···

···

WCSTna

WCS cross reference

···

···

WCSXna

Coordinate rotation

LONPOLEa

LONPna

LONPna

Coordinate rotation

LATPOLEa

LATPna

LATPna

Coordinate epoch

EQUINOXa

EQUIna

EQUIna

Coordinate epoch3

EPOCH

EPOCH

EPOCH

Date of observation

MJDOBn

MJDOBn

MJD-OBS

Average date of obs

MJDAn

MJDAn

MJD-AVG

Date/time of observation

DOBSn

DOBSn

DATE-OBS

Average date/time of obs

DAVGn

DAVGn

DATE-AVG

Reference frame

RADESYSa or

RADEna

RADEna

RADECSYS4

Line rest frequency (Hz)

RESTFRQa or

RFRQna

RFRQna

RESTFREQ4

···

···

Line rest vac wavelength (m)

RESTWAVa

RWAVna

RWAVna

Spectral reference frame

SPECSYSa

SPECna

SPECna

Spectral reference frame

SSYSOBSa

SOBSna

SOBSna

Spectral reference frame

SSYSSRCa

SSRCna

SSRCna

Observation X (m)

OBSGXn

OBSGXn

OBSGEO-X

Observation Y (m)

OBSGYn

OBSGYn

OBSGEO-Y

Observation Z (m)

OBSGZn

OBSGZn

OBSGEO-Z

Radial velocity (m s-1)

VELOSYSa

VSYSna

VSYSna

Redshift of source

ZSOURCEa

ZSOUna

ZSOUna

Angle of true velocity

VELANGLa

VANGna

VANGna

1CROTAi

form is deprecated but still in use. It must not be used with PC i_j, PV i_m, and PS i_m.

2PCi_j

and CDi_j forms of the transformation matrix are mutually exclusive, and must not appear

together in the same HDU.

3EPOCH

is deprecated. Use EQUINOX instead.

4These

8-character keywords are deprecated; the 7-character forms, which can include an alter-

nate version code letter at the end, should be used instead.

Note. -- The indexes j and i are pixel and intermediate world coordinate axis numbers, re-

spectively. Within a table, the index n refers to a column number, and m refers to a coordinate

parameter number. The index k also refers to a column number. The indicator a is either blank

(for the primary coordinate description) or a character A through Z that specifies the coordinate

version. See text.

FITS Standard