slaFk425 -

SYNOPSYS

PARAMETERS

double r1950

double d1950

double dr1950

double dd1950

double p1950

double v1950

double *r2000

double *d2000

double *dr2000

double *dd2000

double *p2000

double *v2000

DESCRIPTION

  Convert B1950.0 FK4 star data to J2000.0 FK5.

  (double precision)

  This routine converts stars from the old, Bessel-Newcomb, FK4
  system to the new, IAU 1976, FK5, Fricke 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 B1950.0,FK4)

     r1950,d1950     double    B1950.0 RA,dec (rad)
     dr1950,dd1950   double    B1950.0 proper motions (rad/trop.yr)
     p1950           double    parallax (arcsec)
     v1950           double    radial velocity (km/s, +ve = moving away)

  Returned:  (all J2000.0,FK5)

     *r2000,*d2000   double    J2000.0 RA,dec (rad)
     *dr2000,*dd2000 double    J2000.0 proper motions (rad/jul.yr)
     *p2000          double    parallax (arcsec)
     *v2000          double    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)  Conversion from Besselian epoch 1950.0 to Julian epoch
      2000.0 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 FK425 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.

  Defined in slamac.h:  D2PI

  P.T.Wallace   Starlink   20 December 1993

SEE ALSO

• slalib.3