SUBROUTINE sla_AOP ( RAP, DAP, DATE, DUT, ELONGM, PHIM, HM, : XP, YP, TDK, PMB, RH, WL, TLR, : AOB, ZOB, HOB, DOB, ROB ) *+ * - - - - * A O P * - - - - * * Apparent to observed place, for sources distant from the solar * system. * * Given: * RAP d geocentric apparent right ascension * DAP d geocentric apparent declination * 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 (K; std=273.15D0) * 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 (K/metre, e.g. 0.0065D0) * * Returned: * AOB d observed azimuth (radians: N=0,E=90) * ZOB d observed zenith distance (radians) * HOB d observed Hour Angle (radians) * DOB d observed Declination (radians) * ROB d observed Right Ascension (radians) * * Notes: * * 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 * 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) "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 sla_MAP 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 * sla_FK425 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 sla_AOPPA once followed by one call per star to sla_AOPQK. * Where a range of times within a limited period of a few hours * is involved, and the highest precision is not required, call * sla_AOPPA once, followed by a call to sla_AOPPAT each time the * time changes, followed by one call per star to sla_AOPQK. * * 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 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 nearly proportional to the * pressure and that an accurate P value is important for precise * work. * * 13) The azimuths etc produced by the present routine are with * respect to the celestial pole. Corrections to the terrestrial * pole can be computed using sla_POLMO. * * Called: sla_AOPPA, sla_AOPQK * * Last revision: 2 December 2005 * * Copyright P.T.Wallace. All rights reserved. * * License: * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program (see SLA_CONDITIONS); if not, write to the * Free Software Foundation, Inc., 59 Temple Place, Suite 330, * Boston, MA 02111-1307 USA * *- IMPLICIT NONE DOUBLE PRECISION RAP,DAP,DATE,DUT,ELONGM,PHIM,HM, : XP,YP,TDK,PMB,RH,WL,TLR,AOB,ZOB,HOB,DOB,ROB DOUBLE PRECISION AOPRMS(14) CALL sla_AOPPA(DATE,DUT,ELONGM,PHIM,HM,XP,YP,TDK,PMB,RH,WL,TLR, : AOPRMS) CALL sla_AOPQK(RAP,DAP,AOPRMS,AOB,ZOB,HOB,DOB,ROB) END