Apparent to observed place, for optical sources distant from the solar system.
rap double geocentric apparent right ascension dap double geocentric apparent declination date double UTC date/time (Modified Julian Date, JD-2400000.5) dut double delta UT: UT1-UTC (UTC seconds) elongm double mean longitude of the observer (radians, east +ve) phim double mean geodetic latitude of the observer (radians) hm double observer's height above sea level (metres) xp double polar motion x-coordinate (radians) yp double polar motion y-coordinate (radians) tdk double local ambient temperature (DegK; std=273.155) pmb double local atmospheric pressure (mB; std=1013.25) rh double local relative humidity (in the range 0.0-1.0) wl double effective wavelength (micron, e.g. 0.55) tlr double tropospheric lapse rate (DegK/metre, e.g. 0.0065)
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) This routine takes time to execute, due mainly to the rigorous integration used to evaluate the refraction. For processing multiple stars for one location and time, call slaAoppa once followed by one call per star to slaAopqk. Where a range of times within a limited period of a few hours is involved, and the highest precision is not required, call slaAoppa once, followed by a call to slaAoppat each time the time changes, followed by one call per star to slaAopqk. 8) The date argument is UTC expressed as an MJD. This is, strictly speaking, wrong, because of leap seconds. However, as long as the delta UT and the UTC are consistent there are no difficulties, except during a leap second. In this case, the start of the 61st second of the final minute should begin a new MJD day and the old pre-leap delta UT should continue to be used. As the 61st second completes, the MJD should revert to the start of the day as, simultaneously, the delta UTC changes by one second to its post-leap new value. 9) The delta UT (UT1-UTC) is tabulated in IERS circulars and elsewhere. It increases by exactly one second at the end of each UTC leap second, introduced in order to keep delta UT within +/- 0.9 seconds. 10) IMPORTANT -- TAKE CARE WITH THE LONGITUDE SIGN CONVENTION. The longitude required by the present routine is east-positive, in accordance with geographical convention (and right-handed). In particular, note that the longitudes returned by the slaObs routine are west-positive, following astronomical usage, and must be reversed in sign before use in the present routine. 11) The polar coordinates xp,yp can be obtained from IERS circulars and equivalent publications. The maximum amplitude is about 0.3 arcseconds. If xp,yp values are unavailable, use xp=yp=0.0. See page B60 of the 1988 Astronomical Almanac for a definition of the two angles. 12) The height above sea level of the observing station, hm, can be obtained from the Astronomical Almanac (Section J in the 1988 edition), or via the routine slaObs. If p, the pressure in millibars, is available, an adequate estimate of hm can be obtained from the expression hm = -8149.9415 * log(p/1013.25); (See Astrophysical Quantities, C.W.Allen, 3rd edition, section 52.) Similarly, if the pressure p is not known, it can be estimated from the height of the observing
p = 1013.25 * exp(-hm/8149.9415); Note, however, that the refraction is proportional to the pressure and that an accurate p value is important for precise work.
slaAoppa, slaAopqk P.T.Wallace Starlink 31 October 1993