Reserved Domain Names

New co-ordinates Frames can be created and added into an NDF using WCSADD. When you create a new Frame you should give it a meaningful Domain name which indicates the sort of co-ordinates that it represents. You are free to choose any name you like (white space is removed, and lower case characters are converted to upper case before using the supplied Domain string), but you should usually avoid the following reserved Domain names:

GRID

-- Reserved to represent grid co-ordinates (in units of pixels).

PIXEL

-- Reserved to represent pixel co-ordinates (in units of pixels).

AXIS

-- Reserved to represent AXIS co-ordinates (in arbitrary units).

FRACTION

-- Reserved to represent normalised pixel/grid co-ordinates from zero to one (unitless).

SKY

-- Reserved to represent celestial longitude and latitude (in any suitable system, but always stored internally in units of radians).

SPECTRUM

-- Reserved to represent position within an electro-magnetic spectrum (various spectral systems and units are supported).

GRAPHICS

-- Reserved to represent positions on a graphics device in units of millimetres, measured from the bottom-left corner of the device.

BASEPIC

-- Reserved to represent positions on a graphics device in normalised units, in which the shorter axis of the graphics device has length 1.0.

NDC

-- Reserved to represent positions on a graphics device in normalised units, in which the both axes of the graphics device have length 1.0.

CURPIC

-- Reserved to represent positions in normalised units within each individual graphics database picture. The shorter axis of each picture has length 1.0 in this Frame.

When two Frames are joined together to form a compound Frame describing a co-ordinate space of higher dimensionality, the default Domain name for the compound Frame is formed from the individual Domain names, separated by a hyphen. For instance, the WCS FrameSet associated with a spectral cube will often contain a (compound) Frame with the Domain name ``SKY-SPECTRUM''. Thus you should also avoid Domain names that contain a hyphen, particularly if they also contain any of the reserved Domain names listed above.