.help fk524 Jun99 "Slalib Package"
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      SUBROUTINE slFK54 (R2000,D2000,DR2000,DD2000,P2000,V2000,
     :                      R1950,D1950,DR1950,DD1950,P1950,V1950)

     - - - - - -
      F K 5 4
     - - - - - -

  Convert J2000.0 FK5 star data to B1950.0 FK4 (double precision)

  This routine converts stars from the new, IAU 1976, FK5, Fricke
  system, to the old, Bessel-Newcomb, FK4 system.  The precepts
  of Smith et al (Ref 1) are followed, using the implementation
  by Yallop et al (Ref 2) of a matrix method due to Standish.
  Kinoshita's development of Andoyer's post-Newcomb precession is
  used.  The numerical constants from Seidelmann et al (Ref 3) are
  used canonically.

  Given:  (all J2000.0,FK5)
     R2000,D2000     dp    J2000.0 RA,Dec (rad)
     DR2000,DD2000   dp    J2000.0 proper motions (rad/Jul.yr)
     P2000           dp    parallax (arcsec)
     V2000           dp    radial velocity (km/s, +ve = moving away)

  Returned:  (all B1950.0,FK4)
     R1950,D1950     dp    B1950.0 RA,Dec (rad)
     DR1950,DD1950   dp    B1950.0 proper motions (rad/trop.yr)
     P1950           dp    parallax (arcsec)
     V1950           dp    radial velocity (km/s, +ve = moving away)

  Notes:

  1)  The proper motions in RA are dRA/dt rather than
      cos(Dec)*dRA/dt, and are per year rather than per century.

  2)  Note that conversion from Julian epoch 2000.0 to Besselian
      epoch 1950.0 only is provided for.  Conversions involving
      other epochs will require use of the appropriate precession,
      proper motion, and E-terms routines before and/or after
      FK524 is called.

  3)  In the FK4 catalogue the proper motions of stars within
      10 degrees of the poles do not embody the differential
      E-term effect and should, strictly speaking, be handled
      in a different manner from stars outside these regions.
      However, given the general lack of homogeneity of the star
      data available for routine astrometry, the difficulties of
      handling positions that may have been determined from
      astrometric fields spanning the polar and non-polar regions,
      the likelihood that the differential E-terms effect was not
      taken into account when allowing for proper motion in past
      astrometry, and the undesirability of a discontinuity in
      the algorithm, the decision has been made in this routine to
      include the effect of differential E-terms on the proper
      motions for all stars, whether polar or not.  At epoch 2000,
      and measuring on the sky rather than in terms of dRA, the
      errors resulting from this simplification are less than
      1 milliarcsecond in position and 1 milliarcsecond per
      century in proper motion.

  References:

     1  Smith, C.A. et al, 1989.  "The transformation of astrometric
        catalog systems to the equinox J2000.0".  Astron.J. 97, 265.

     2  Yallop, B.D. et al, 1989.  "Transformation of mean star places
        from FK4 B1950.0 to FK5 J2000.0 using matrices in 6-space".
        Astron.J. 97, 274.

     3  Seidelmann, P.K. (ed), 1992.  "Explanatory Supplement to
        the Astronomical Almanac", ISBN 0-935702-68-7.

  P.T.Wallace   Starlink   19 December 1993

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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