SLA_AMP - Apparent to Mean   

ACTION:

Convert star $[\,\alpha,\delta\,]$ from geocentric apparent to mean place (post IAU 1976).

CALL:

CALL sla_AMP (RA, DA, DATE, EQ, RM, DM)

GIVEN:

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

DATE D TDB for apparent place (JD-2400000.5)

EQ D equinox: Julian epoch of mean place

RETURNED:

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

NOTES:

1.

The distinction between the required TDB and TT is always negligible. Moreover, for all but the most critical applications UTC is adequate.

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.

3.

Where multiple apparent places are to be converted to mean places, for a fixed date and equinox, it is more efficient to use the sla_MAPPA routine to compute the required parameters once, followed by one call to sla_AMPQK per star.

4.

The accuracy is limited by imperfections in the IAU 1976 and 1980 models for precession and nutation. Corrections are tabulated in IERS Bulletin B and at the present epoch are of order 50 milliarcsecond. An improved precession-nutation model can be introduced by using sla_MAPPA and sla_AMPQK (see the previous note) and replacing the precession-nutation matrix into the parameter array directly.

5.

The accuracy is further 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.

REFERENCES:

1.

1984 Astronomical Almanac, pp B39-B41.

2.

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