Convert B1950.0 FK4 star data to J2000.0 FK5 assuming zero proper motion in an inertial frame (double precision)

This routine converts stars from the old, Bessel-Newcomb, FK4 system to the new, IAU 1976, FK5, Fricke system, in such a way that the FK5 proper motion is zero. Because such a star has, in general, a non-zero proper motion in the FK4 system, the routine requires the epoch at which the position in the FK4 system was determined.

r1950,d1950

B1950.0 FK4 RA,Dec at epoch (rad)

bepoch

Besselian epoch (e.g. 1979.3)

RETURNS

[ r2000, d2000 ]

J2000.0 FK5 RA,Dec (rad)

Notes

1)

The epoch BEPOCH is strictly speaking Besselian, but if a Julian epoch is supplied the result will be affected only to a negligible extent.

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.

Example

	[ r2000, d2000 ] = fk45(r1950, d1950, 1950)

fk54(r2000, d2000, bepoch)

convert from FK5 to FK4.

Convert a J2000.0 FK5 star position to B1950.0 FK4 assuming zero proper motion and parallax.

This routine converts star positions from the new, IAU 1976, FK5, Fricke system to the old, Bessel-Newcomb, FK4 system.

r2000,d2000

J2000.0 FK5 RA,Dec (rad)

bepoch

Besselian epoch (e.g. 1950)

RETURNS

[ r1950, d1950 ]

B1950.0 FK4 RA,Dec (rad) at epoch BEPOCH

Notes

1)

The proper motion in RA is dRA/dt rather than cos(Dec)*dRA/dt.

2)

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 routines before and after this routine is called.

3)

Unlike in the slaFK524 routine, the FK5 proper motions, the parallax and the radial velocity are presumed zero.

4)

It is the intention that FK5 should be a close approximation to an inertial frame, so that distant objects have zero proper motion; such objects have (in general) non-zero proper motion in FK4, and this routine returns those fictitious proper motions.

5)

The position returned by this routine is in the B1950 reference frame but at Besselian epoch bepoch. For comparison with catalogues the bepoch argument will frequently be 1950.0.

P.T.Wallace

Starlink 30 October 1993

Example

	[ r1950, d1950 ] = fk54(r2000, d2000, 1950)

SEE ALSO

slalink.1

programs available with the slalink extensions to tawk.

slalib.3

The Star Link Astrometry library.

tabtable.1

starbase data tables introduction (this page).

tablefun.3

starbase data tables introduction (this page).

tawk

awk language extensions implimented at SAO.

mawk

an implimentation of the awk programming language by Mike Brennan (brennanboeing.com).