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Coordinate Keywords
There are a
number of standard equatorial coordinate systems. HW proposed the
following keywords to supplement the reserved keywords discussed
in section 3.1.1.1.
- RADECSYS (character) standing for
RA/Dec system specifies the reference frame for the
equatorial coordinate system. It should be used when the CTYPE
values are an RA-xxxx/DEC-xxxx pair. Table 4.3
shows the four values defined.
Table 4.3:
Reference Frames for Equatorial Coordinate Systems
'FK4 ' |
mean place old (before the 1976 IAU) system |
'FK4-NO-E' |
mean place old system but without e-terms |
'FK5 ' |
mean place new (after the 1976 IAU) system |
'GAPPT ' |
geocentric, apparent place, after the 1976 IAU
system |
The default value is 'FK4 '. Under the FK4, the date of
equinox (value of the EQUINOX keyword) is interpreted as a
Besselian epoch and under FK5 as a Julian epoch. If there is no
EQUINOX keyword (or EPOCH for old FITS files), the equinox
is assumed to be 1950.0 for FK4 and 2000.0 for FK5.
- MJD-OBS (floating) specifies the time of observation in the
form of a Modified Julian Date (JD - 2400000.5), in the International
Atomic Time time scale. Whether this point refers to the starting,
midpoint, or ending time is not defined. Use comments to say. Use of
this keyword permits greater precision than DATE-OBS. Because the
FK4 reference frame is not inertial--there is a small but significant
rotation relative to distant objects--the epoch or time of observation
is required to specify when the mean place was correct.
Next: Current Status
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William Pence
2004-01-07