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      Precess positions from J2000.0 (FK5) to B1950.0 (FK4)


      Calculates the mean place of a star at B1950.0 on the FK4 system from
      the mean place at J2000.0 on the FK5 system.

Calling Sequence

      bprecess, ra, dec, ra_1950, dec_1950, [ MU_RADEC = , PARALLAX =
                                      RAD_VEL =, EPOCH = ]


      RA,DEC - Input J2000 right ascension and declination in *degrees*.
              Scalar or N element vector


      RA_1950, DEC_1950 - The corresponding B1950 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
      B1950 system. The parallax and radial velocity will have a very
      minor influence on the B1950 position.
      EPOCH - scalar giving epoch of original observations, default 2000.0d
          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 186.
      Also see Aoki et al (1983), A&A, 128,263
      BPRECESS 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 reverse of
              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 J1950 can be up to 12", mainly in right ascension. If
      better accuracy than this is needed then BPRECESS should be used.
      An unsystematic comparison of BPRECESS with the IPAC precession
      routine (http://nedwww.ipac.caltech.edu/forms/calculator.html) always
      gives differences less than 0.15".


      The SAO2000 catalogue gives the J2000 position and proper motion for
      the star HD 119288. Find the B1950 position.
      RA(2000) = 13h 42m 12.740s Dec(2000) = 8d 23' 17.69''
      Mu(RA) = -.0257 s/yr Mu(Dec) = -.090 ''/yr
      IDL> mu_radec = 100D* [ -15D*.0257, -0.090 ]
      IDL> ra = ten(13, 42, 12.740)*15.D
      IDL> dec = ten(8, 23, 17.69)
      IDL> bprecess, ra, dec, ra1950, dec1950, mu_radec = mu_radec
      IDL> print, adstring(ra1950, dec1950,2)
              ===> 13h 39m 44.526s +08d 38' 28.63"

Revision History

      Written, W. Landsman October, 1992
      Vectorized, W. Landsman February, 1994
      Treat case where proper motion not known or exactly zero November 1994
      Handling of arrays larger than 32767 Lars L. Christensen, march, 1995
      Fixed bug where A term not initialized for vector input
            W. Landsman February 2000
      Use V6.0 notation W. Landsman Mar 2011

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