As noted, a variety of input co-ordinate systems are available through GO. Not all of these are supported in Version 1.1 of the GO FITS file. The mapping between the labels in Figure 1 and the GO Main Screen pull-down menu is as follows (this should be checked):
The keyword COORDSYS will be used to define the type of co-ordinate system in use. The possible values in FITS version 1.1. are ``GALACTIC'', ``RADEC'', ``HADEC'', ``AZEL'', or ``OTHER''. OTHER will be used for any situation where the precise specification of the co-ordinate system has not yet been decided; for FITS versions 1.1 this includes user defined co-ordinates and solar system objects. Later versions of the FITS definition may include a representation of these co-ordinates, in which case a new value will be provided for COORDSYS.
For COORDSYS = ``RADEC'', the standard (WCS) keyword RADESYS (note missing ``C'') will be used to define the specific type of system in use.
Note that the standard FITS keyword EQUINOX is used to provide the epoch (i.e. instant of time) of the mean equator and equinox. Prior to version 1 of the FITS standard, the keyword EPOCH was used for this quantity; it is illegal to use the keyword EPOCH in new FITS files. For GAPPT, WCS paper III recommends use of the start time of the observation (via DATE-OBS or MJD-OBS) to provide the epoch of the equator and equinox.
The actual co-ordinate value used will be provided by the appropriate (long, lat) pair, depending on the value of COORDSYS.
The required FITS keywords for each of the supported input co-ordinate systems is summarised in table 1 below:
co-ordinate COORDSYS RADESYS EQUINOX major axis minor axis system keyword keyword GALACTIC GALACTIC absent absent GLON GLAT J2OOO RADEC FK5 2000.0 RA DEC B1950 RADEC FK4 1950.0 RA DEC JMEAN RADEC FK5 nnnn.n RA DEC GAPPT RADEC GAPPT absent (1) RA DEC HADEC HADEC absent absent (1) HA DEC AZEL AZEL absent absent AZ EL USER OTHER (2) absent absent absent absent SOLAR SYSTEM OTHER (2) absent absent absent absent
(1) As proposed by WCS paper III, for these co-ordinate systems, the epoch of the equator and equinox will be taken as the time of the start of the observation, given by the standard FITS header keyword DATE-OBS.
(2) In the GO FITS file version 1.1 definition, we have not agreed FITS keyword mechanisms to stored information about user defined co-ordinates, not for requested to track solar system object simply by specifying name (with co-ordinates derived from the ephemerides). Accordingly, we use the COORDSYS value of OTHER to indicate this information is not available.