SLA_MAP - Mean to Apparent   

ACTION:

Transform star $[\,\alpha,\delta\,]$ from mean place to geocentric apparent. The reference frames and timescales used are post IAU 1976.

CALL:

CALL sla_MAP (RM, DM, PR, PD, PX, RV, EQ, DATE, RA, DA)

GIVEN:

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

$$[\,\alpha,\delta\,]$$ PR,PD D

PX D parallax (arcsec)

RV D radial velocity (km s^-1, +ve if receding)

EQ D epoch and equinox of star data (Julian)

DATE D TDB for apparent place (JD-2400000.5)

RETURNED:

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

NOTES:

1.

EQ is the Julian epoch specifying both the reference frame and the epoch of the position - usually 2000. For positions where the epoch and equinox are different, use the routine sla_PM to apply proper motion corrections before using this routine.

2.

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

3.

The $\alpha$ proper motions are $\dot{\alpha}$ rather than $\dot{\alpha}\cos\delta$, and are per year rather than per century.

4.

This routine may be wasteful for some applications because it recomputes the Earth position/velocity and the precession-nutation matrix each time, and because it allows for parallax and proper motion. Where multiple transformations are to be carried out for one epoch, a faster method is to call the sla_MAPPA routine once and then either the sla_MAPQK routine (which includes parallax and proper motion) or sla_MAPQKZ (which assumes zero parallax and FK5 proper motion).

5.

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_MAPQK (see the previous note) and replacing the precession-nutation matrix into the parameter array directly.

6.

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.