Quick apparent to observed place.
rap double geocentric apparent right ascension dap double geocentric apparent declination
(0) geodetic latitude (radians) (1,2) sine and cosine of geodetic latitude (3) magnitude of diurnal aberration vector (4) height (hm) (5) ambient temperature (t) (6) pressure (p) (7) relative humidity (rh) (8) wavelength (wl) (9) lapse rate (tlr) (10,11) refraction constants A and B (radians) (12) longitude + eqn of equinoxes + sidereal DUT (radians) (13) local apparent sidereal time (radians)
*aob double observed azimuth (radians: N=0,E=90) *zob double observed zenith distance (radians) *hob double observed hour angle (radians) *dob double observed declination (radians) *rob double observed right ascension (radians)
1) This routine returns zenith distance rather than elevation in order to reflect the fact that no allowance is made for depression of the horizon. 2) The accuracy of the result is limited by the corrections for refraction. Providing the meteorological parameters are known accurately and there are no gross local effects, the predicted apparent RA,Dec should be within about 0.1 arcsec for a zenith distance of less than 70 degrees. Even at a topocentric zenith distance of 90 degrees, the accuracy in elevation should be better than 1 arcmin; useful results are available for a further 3 degrees, beyond which the slaRefro routine returns a fixed value of the refraction. The complementary routines slaAop (or slaAopqk) and slaOap (or slaOapqk) are self-consistent to better than 1 micro- arcsecond all over the celestial sphere. 3) It is advisable to take great care with units, as even unlikely values of the input parameters are accepted and processed in accordance with the models used. 4) "Apparent" place means the geocentric apparent right ascension and declination, which is obtained from a catalogue mean place by allowing for space motion, parallax, precession, nutation, annual aberration, and the Sun's gravitational lens effect. For star positions in the FK5 system (i.e. J2000), these effects can be applied by means of the slaMap etc routines. Starting from other mean place systems, additional transformations will be needed; for example, FK4 (i.e. B1950) mean places would first have to be converted to FK5, which can be done with the slaFk425 etc routines. 5) "Observed" Az,El means the position that would be seen by a perfect theodolite located at the observer. This is obtained from the geocentric apparent RA,Dec by allowing for Earth orientation and diurnal aberration, rotating from equator to horizon coordinates, and then adjusting for refraction. The HA,Dec is obtained by rotating back into equatorial coordinates, using the geodetic latitude corrected for polar motion, and is the position that would be seen by a perfect equatorial located at the observer and with its polar axis aligned to the Earth's axis of rotation (n.b. not to the refracted pole). Finally, the RA is obtained by subtracting the HA from the local apparent ST. 6) To predict the required setting of a real telescope, the observed place produced by this routine would have to be adjusted for the tilt of the azimuth or polar axis of the mounting (with appropriate corrections for mount flexures), for non-perpendicularity between the mounting axes, for the position of the rotator axis and the pointing axis relative to it, for tube flexure, for gear and encoder errors, and finally for encoder zero points. Some telescopes would, of course, exhibit other properties which would need to be accounted for at the appropriate point in the sequence. 7) The star-independent apparent-to-observed-place parameters in aoprms may be computed by means of the slaAoppa routine. If nothing has changed significantly except the time, the slaAoppat routine may be used to perform the requisite partial recomputation of aoprms.
slaDcs2c, slaRefz, slaRefro, slaDcc2s, slaDranrm P.T.Wallace Starlink 31 October 1993