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      Precess astronomical coordinates from B1950 to J2000


      Calculate the mean place of a star at J2000.0 on the FK5 system from the
      mean place at B1950.0 on the FK4 system.
      Use BPRECESS for the reverse direction J2000 ==> B1950

Calling Sequence

      jprecess, ra, dec, ra_2000, dec_2000, [ MU_RADEC = , PARALLAX =
              RAD_VEL =, EPOCH = ]


      RA,DEC - input B1950 right ascension and declination in *degrees*.
              Scalar or vector


      RA_2000, DEC_2000 - the corresponding J2000 right ascension and
              declination in *degrees*. Same number of elements as RA,DEC
              but always double precision.
      MU_RADEC - 2xN element double precision vector containing the proper
                  motion in seconds of arc per tropical *century* in right
                  ascension and declination.
      PARALLAX - N_element vector giving stellar parallax (seconds of arc)
      RAD_VEL - N_element vector giving radial velocity in km/s
      The values of MU_RADEC, PARALLAX, and RADVEL will all be modified
      upon output to contain the values of these quantities in the
      J2000 system. Values will also be converted to double precision.
      The parallax and radial velocity will have a very minor influence on
      the J2000 position.
      EPOCH - scalar giving epoch of original observations, default 1950.0d
          This keyword value is only used if the MU_RADEC keyword is not set.


      The algorithm is taken from the Explanatory Supplement to the
      Astronomical Almanac 1992, page 184.
      Also see Aoki et al (1983), A&A, 128,263
      JPRECESS distinguishes between the following two cases:
      (1) The proper motion is known and non-zero
      (2) the proper motion is unknown or known to be exactly zero (i.e.
              extragalactic radio sources). In this case, the algorithm
              in Appendix 2 of Aoki et al. (1983) is used to ensure that
              the output proper motion is exactly zero. Better precision
              can be achieved in this case by inputting the EPOCH of the
              original observations.
      The error in using the IDL procedure PRECESS for converting between
      B1950 and J2000 can be up to 12", mainly in right ascension. If
      better accuracy than this is needed then JPRECESS should be used.


      The SAO catalogue gives the B1950 position and proper motion for the
      star HD 119288. Find the J2000 position.
          RA(1950) = 13h 39m 44.526s Dec(1950) = 8d 38' 28.63''
          Mu(RA) = -.0259 s/yr Mu(Dec) = -.093 ''/yr
      IDL> mu_radec = 100D* [ -15D*.0259, -0.093 ]
      IDL> ra = ten(13,39,44.526)*15.D
      IDL> dec = ten(8,38,28.63)
      IDL> jprecess, ra, dec, ra2000, dec2000, mu_radec = mu_radec
      IDL> print, adstring(ra2000, dec2000,2)
              ===> 13h 42m 12.740s +08d 23' 17.69"


      "When transferring individual observations, as opposed to catalog mean
      place, the safest method is to tranform the observations back to the
      epoch of the observation, on the FK4 system (or in the system that was
      used to to produce the observed mean place), convert to the FK5 system,
      and transform to the the epoch and equinox of J2000.0" -- from the
      Explanatory Supplement (1992), p. 180

Revision History

      Written, W. Landsman September, 1992
      Corrected a couple of typos in M matrix October, 1992
      Vectorized, W. Landsman February, 1994
      Implement Appendix 2 of Aoki et al. (1983) for case where proper
      motion unknown or exactly zero W. Landsman November, 1994
      Converted to IDL V5.0 W. Landsman September 1997
      Fixed typo in updating proper motion W. Landsman April 1999
      Make sure proper motion is floating point W. Landsman December 2000
      Use V6.0 notation W. Landsman Mar 2011

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