SUBROUTINE sla_OAP (TYPE, OB1, OB2, DATE, DUT, ELONGM, PHIM, : HM, XP, YP, TDK, PMB, RH, WL, TLR, : RAP, DAP) *+ * - - - - * O A P * - - - - * * Observed to apparent place * * Given: * 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=273.155D0) * PMB d local atmospheric pressure (mB; std=1013.25D0) * RH d local relative humidity (in the range 0D0-1D0) * WL d effective wavelength (micron, e.g. 0.55D0) * TLR d tropospheric lapse rate (DegK/metre, e.g. 0.0065D0) * * Returned: * RAP d geocentric apparent right ascension * DAP d geocentric apparent declination * * Notes: * * 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 * sla_REFRO routine returns a fixed value of the refraction. * The complementary routines sla_AOP (or sla_AOPQK) and sla_OAP * (or sla_OAPQK) 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 * sla_AMP 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 sla_FK524 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 sla_AOPPA routine. * If nothing has changed significantly except the time, the * sla_AOPPAT 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 * sla_OBS 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=0D0. 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 sla_OBS. If P, * the pressure in millibars, is available, an adequate * estimate of HM can be obtained from the expression * * HM ~ -29.3D0*TSL*LOG(P/1013.25D0). * * where TSL is the approximate sea-level air temperature in * deg K (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 * station, HM as follows: * * P ~ 1013.25D0*EXP(-HM/(29.3D0*TSL)). * * Note, however, that the refraction is proportional to the * pressure and that an accurate P value is important for * precise work. * * 13) The azimuths etc used by the present routine are with respect * to the celestial pole. Corrections from the terrestrial pole * can be computed using sla_POLMO. * * Called: sla_AOPPA, sla_OAPQK * * P.T.Wallace Starlink 6 September 1999 * * Copyright (C) 1999 P.T.Wallace and CCLRC *- IMPLICIT NONE CHARACTER*(*) TYPE DOUBLE PRECISION OB1,OB2,DATE,DUT,ELONGM,PHIM,HM, : XP,YP,TDK,PMB,RH,WL,TLR,RAP,DAP DOUBLE PRECISION AOPRMS(14) CALL sla_AOPPA(DATE,DUT,ELONGM,PHIM,HM,XP,YP,TDK,PMB,RH,WL,TLR, : AOPRMS) CALL sla_OAPQK(TYPE,OB1,OB2,AOPRMS,RAP,DAP) END