SLA_AMPQK - Quick Apparent to Mean   

ACTION:

Convert star $[\,\alpha,\delta\,]$ from geocentric apparent to mean place (post IAU 1976). Use of this routine is appropriate when efficiency is important and where many star positions are all to be transformed for one epoch and equinox. The star-independent parameters can be obtained by calling the sla_MAPPA routine.

CALL:

CALL sla_AMPQK (RA, DA, AMPRMS, RM, DM)

GIVEN:

$$[\,\alpha,\delta\,]$$ RA,DA

AMPRMS D(21) star-independent mean-to-apparent parameters:

(1) time interval for proper motion (Julian years)

(2-4) barycentric position of the Earth (AU)

(5-7) heliocentric direction of the Earth (unit vector)

$$\times 2 /$$ (8)

(9-11) v: barycentric Earth velocity in units of c

$$\sqrt{1-\left\vert\mbox{\bf v}\right\vert^2}$$ (12)

$$3\times3$$ (13-21)

RETURNED:

$$[\,\alpha,\delta\,]$$ RM,DM

NOTES:

1.

The accuracy is limited by the routine sla_EVP, called by sla_MAPPA, which computes the Earth position and velocity using the methods of Stumpff. The maximum error is about 0.3 milliarcsecond.

2.

Iterative techniques are used for the aberration and light deflection corrections so that the routines sla_AMP (or sla_AMPQK) and sla_MAP (or sla_MAPQK) are accurate inverses; even at the edge of the Sun's disc the discrepancy is only about 1 nanoarcsecond.

REFERENCES:

1.

1984 Astronomical Almanac, pp B39-B41.

2.

Lederle & Schwan, 1984. Astr.Astrophys. 134, 1-6.