In the original AIPS convention, the native system was required to be locally parallel to the standard celestial coordinate system at the tangent point. GC relax this requirement. The axes of the native sky coordinate system derived by deprojection need not be in the same direction as those of the standard celestial coordinate system. GC provide equations for the transformation. The following keywords are reserved:
LONGPOLE is understood to have a default value of 0° or 180° where possible. The choice depends on the latitude of the reference point in the standard system and in the native system; if the latitude in the standard system is greater, LONGPOLE is 0°; if the latitude in the native system is greater, it is 180°. With this convention, celestial latitude and native latitude increase in the same direction at the tangent point. A detailed discussion of the transformations and the application of the reserved keywords can be found in GC and is beyond the scope of this Guide.
GC adopt the AIPS convention of using the first four characters of the value field of the CTYPEn keywords to identify the standard system, as shown in Table 4.2.
RA-, | DEC- | equatorial coordinates (, ) |
GLON, | GLAT | galactic coordinates (, b) |
ELON, | ELAT | ecliptic coordinates (, ) |
SLON, | SLAT | supergalactic coordinates |
HLON, | HLAT | helioecliptic coordinates |
Galactic coordinates as defined above are in the ``new'' system adopted by the IAU in 1950, where the origin, 0° latitude and 0° longitude, is in the direction of the galactic center. Identifications for older systems or future systems should be in the form (xLAT, xLON) but not one of the designations used in Table 4.2. Ecliptic coordinates refer to the mean ecliptic and equinox of the date of observation, in the post-IAU 1976 system.
The values of the CTYPEn provide both the world coordinate system and the projection used. For example, if the data matrix contained a sky map in right ascension and declination projected on the plane using the tangent projection, the CTYPEn values would be
CTYPE1 = 'RA--TAN'
CTYPE2 = 'DEC-TAN'
The coordinates must be logically consistent; for example, 'RA--TAN' should not be paired with 'GLON-ARC'.