4.3 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 CTYPEn values are an RA-xxxx/DEC-xxxx pair. Table 4.3 shows the four values defined.

 

'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.