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.help fk425 Jun99 "Slalib Package"
.nf
SUBROUTINE slFK45 (R1950,D1950,DR1950,DD1950,P1950,V1950,
: R2000,D2000,DR2000,DD2000,P2000,V2000)
- - - - - -
F K 4 5
- - - - - -
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 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)
Returned: (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)
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.
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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.endhelp
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