.help aopqk Jun99 "Slalib Package"
.nf

      SUBROUTINE slAOPQ (RAP, DAP, AOPRMS, AOB, ZOB, HOB, DOB, ROB)

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      A O P Q
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  Quick apparent to observed place (but see note 8, below, for
  remarks about speed).

  Given:
     RAP    d      geocentric apparent right ascension
     DAP    d      geocentric apparent declination
     AOPRMS d(14)  star-independent apparent-to-observed parameters:

       (1)      geodetic latitude (radians)
       (2,3)    sine and cosine of geodetic latitude
       (4)      magnitude of diurnal aberration vector
       (5)      height (HM)
       (6)      ambient temperature (T)
       (7)      pressure (P)
       (8)      relative humidity (RH)
       (9)      wavelength (WL)
       (10)     lapse rate (TLR)
       (11,12)  refraction constants A and B (radians)
       (13)     longitude + eqn of equinoxes + sidereal DUT (radians)
       (14)     local apparent sidereal time (radians)

  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
       observed RA,Dec predicted by this routine 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)  "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)  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)  At zenith distances beyond about 76 degrees, the need for
       special care with the corrections for refraction causes a
       marked increase in execution time.  Moreover, the effect
       gets worse with increasing zenith distance.  Adroit
       programming in the calling application may allow the
       problem to be reduced.  Prepare an alternative AOPRMS array,
       computed for zero air-pressure;  this will disable the
       refraction corrections and cause rapid execution.  Using
       this AOPRMS array, a preliminary call to the present routine
       will, depending on the application, produce a rough position
       which may be enough to establish whether the full, slow
       calculation (using the real AOPRMS array) is worthwhile.
       For example, there would be no need for the full calculation
       if the preliminary call had already established that the
       source was well below the elevation limits for a particular
       telescope.

  9)   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:  slDS2C, slREFZ, slRFRO, slDC2S, slDA2P

  P.T.Wallace   Starlink   22 February 1996

  Copyright (C) 1996 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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