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+.help oap Jun99 "Slalib Package"
+.nf
+
+      SUBROUTINE slOAP (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
+      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)  "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
+      slAMP 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 slFK54 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 slAOPA routine.
+      If nothing has changed significantly except the time, the
+      slAOPT 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
+      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 used by the present routine are with respect
+      to the celestial pole.  Corrections from the terrestrial pole
+      can be computed using slPLMO.
+
+  Called:  slAOPA, slOAPQ
+
+  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