## Specifying a Particular Celestial Coordinate System

For many purposes, the ICRS coordinate system is perfectly adequate. In order to support conversion between a variety of celestial coordinate systems, however, you can create SkyFrames that represent any of these.

Selection of a particular coordinate system is performed simply by setting a value for the SkyFrameSkyFrame's (character string) SystemSystem attribute. This setting is most conveniently done when the SkyFrame is created. For example, a SkyFrame representing the old FK4 (B1950.0) coordinate system would be created by:

153#153

Note that specifying “System154#154FK4” also changes the associated equinox (from J2000.0 to B1950.0). This is because the default value of the SkyFrame's EquinoxEquinox attribute (§8.4) depends on the System attribute setting.

You may change the System value at any time, although this is not usually needed. The values supported are set out in the attribute's description in Appendix C and include a variety of equatorial coordinate systems, together with ecliptic and galactic coordinates.

General spherical coordinates are supported by specifying “System154#154unknown”. You should note, though, that no MappingMapping can be created to convert between “unknown” coordinates and any of the other celestial coordinate systems (see §12 ).