8.3. CELESTIAL COORDINATE SYSTEM REPRESENTATIONS
celestial coordinate system (α, δ). The algorithm to be used to define the spherical
projection must be encoded in the CTYPEi keyword as the three-letter algorithm code,
the allowed values for which are specified in Table 8.3 and defined in references  and
. The target celestial coordinate system is also encoded into the left-most portion
of the CTYPEi keyword as the coordinate type.
For the final step, the parameter LONPOLEa must be specified, which is the native lon-
gitude of the celestial pole, φp. For certain projections (such as cylindricals and conics,
which are less commonly used in astronomy), the additional keyword LATPOLEa must be
used to specify the native latitude of the celestial pole. See  for the transformation
equations and other details.
The accepted celestial coordinate systems are: the standard equatorial (RA-- and
DEC-), and others of the form xLON and xLAT for longitude-latitude pairs, where x is G
for Galactic, E for ecliptic, H for helioecliptic and S for supergalactic coordinates. Since
the representation of planetary, lunar, and solar coordinate systems could exceed the
26 possibilities afforded by the single character x, pairs of the form yzLN and yzLT may
be used as well.
RADESYSa -- [character; default: FK4, FK5, or ICRS: see below]
Name of the reference frame of equatorial or ecliptic coordinates, whose value
must be one of those specified in Table 8.4. The default value is FK4 if the value of
EQUINOXa < 1984.0, FK5 if EQUINOXa ≥ 1984.0, or ICRS if EQUINOXa is not given.
EQUINOXa -- [floating-point; default: see below]
Epoch of the mean equator and equinox in years, whose value must be non-
negative. The interpretation of epoch depends upon the value of RADESYSa if
present: Besselian if the value is FK4 or FK4-NO-E, Julian if the value is FK5; not
applicable if the value is ICRS or GAPPT.
EPOCH -- [floating-point]
This keyword is deprecated and should not be used in new FITS files. It is reserved
primarily to prevent its use with other meanings. The EQUINOX keyword shall be
used instead. The value field of this keyword was previously defined to contain
a floating-point number giving the equinox in years for the celestial coordinate
system in which positions are expressed.
DATE-OBS -- [floating-point]
This reserved keyword is defined in §22.214.171.124.
MJD-OBS -- [floating-point; default: DATE-OBS if given, otherwise no default]
Modified Julian Date (JD 2,400,000.5) of the observation, whose value corre-
sponds (by default) to the start of the observation, unless another interpretation
is explained in the comment field. No specific time system (e.g. UTC, TAI, etc.)
is defined for this or any of the other time-related keywords. It is recommended