Observed to apparent place
type c*(*) type of coordinates - 'R', 'H' or 'A' (see below) ob1 d observed Az, HA or RA (radians; Az is N=0,E=90) ob2 d observed ZD or Dec (radians) date d UTC date/time (modified Julian Date, JD-2400000.5) dut d delta UT: UT1-UTC (UTC seconds) elongm d mean longitude of the observer (radians, East +ve) phim d mean geodetic latitude of the observer (radians) hm d observer's height above sea level (metres) xp d polar motion x-coordinate (radians) yp d polar motion y-coordinate (radians) tdk d local ambient temperature (DegK; std=273.155) pmb d local atmospheric pressure (mB; std=1013.25) rh d local relative humidity (in the range 0.0-1.0) wl d effective wavelength (micron, e.g. 0.55) tlr d tropospheric lapse rate (DegK/metre, e.g. 0.0065)
rap d geocentric apparent right ascension dap d geocentric apparent declination
1) Only the first character of the type argument is significant. 'R' or 'r' indicates that obs1 and obs2 are the observed Right Ascension and Declination; 'H' or 'h' indicates that they are Hour Angle (West +ve) and Declination; anything else ('A' or 'a' is recommended) indicates that obs1 and obs2 are Azimuth (North zero, East is 90 deg) and zenith distance. (Zenith distance is used 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) "Observed" Az,El means the position that would be seen by a perfect theodolite located at the observer. This is related to the observed HA,Dec via the standard rotation, using the geodetic latitude (corrected for polar motion), while the observed HA and RA are related simply through the local apparent ST. "Observed" RA,Dec or HA,Dec thus means 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). By removing from the observed place the effects of atmospheric refraction and diurnal aberration, the geocentric apparent RA,Dec is obtained. 5) Frequently, mean rather than apparent RA,Dec will be required, in which case further transformations will be necessary. The slaAMP etc routines will convert the apparent RA,Dec produced by the present routine into an "FK5" (J2000) mean place, by allowing for the Sun's gravitational lens effect, annual aberration, nutation and precession. Should "FK4" (1950) coordinates be needed, the routines slaFk425 etc will also need to be applied. 6) To convert to apparent RA,Dec the coordinates read from a real telescope, corrections would have to be applied for encoder zero points, gear and encoder errors, tube flexure, the position of the rotator axis and the pointing axis relative to it, non-perpendicularity between the mounting axes, and finally for the tilt of the azimuth or polar axis of the mounting (with appropriate corrections for mount flexures). 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. 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, slaOapqk P.T.Wallace Starlink 30 October 1993