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+.help aop Jun99 "Slalib Package"
+.nf
+
+      SUBROUTINE slAOP (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 optical 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 (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:
+     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
+       slRFRO routine returns a fixed value of the refraction.
+       The complementary routines slAOP (or slAOPQ) and slOAP
+       (or slOAPQ) 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 slMAP 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
+       slFK45 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 slAOPA once followed by one call per star to slAOPQ.
+       Where a range of times within a limited period of a few hours
+       is involved, and the highest precision is not required, call
+       slAOPA once, followed by a call to slAOPT each time the
+       time changes, followed by one call per star to slAOPQ.
+
+   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
+       slOBS 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 slOBS.  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 produced by the present routine are with
+       respect to the celestial pole.  Corrections to the terrestrial
+       pole can be computed using slPLMO.
+
+  Called:  slAOPA, slAOPQ
+
+  P.T.Wallace   Starlink   9 June 1998
+
+  Copyright (C) 1998 Rutherford Appleton Laboratory
+  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+
+.fi
+.endhelp