JPRECESS

      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

      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 1.5", 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

       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

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