BPRECESS

       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.

       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 1.5", 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"

       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
       Converted to IDL V5.0   W. Landsman   September 1997
       Fixed bug where A term not initialized for vector input
            W. Landsman        February 2000

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