slaOap -


void slaOap(type, ob1, ob2, date, dut, elongm, phim, hm, xp, yp, tdk, pmb, rh, wl, tlr, rap, dap)


char *type
double ob1
double ob2
double date
double dut
double elongm
double phim
double hm
double xp
double yp
double tdk
double pmb
double rh
double wl
double tlr
double *rap
double *dap


  Observed to apparent place


     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=273.155)
     pmb    d      local atmospheric pressure (mB; std=1013.25)
     rh     d      local relative humidity (in the range 0.0-1.0)
     wl     d      effective wavelength (micron, e.g. 0.55)
     tlr    d      tropospheric lapse rate (DegK/metre, e.g. 0.0065)


     rap    d      geocentric apparent right ascension
     dap    d      geocentric apparent declination


  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
      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)  "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
      slaAMP 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 slaFk425 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 slaAoppa routine.
      If nothing has changed significantly except the time, the
      slaAoppat 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.

      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
      slaObs routine are west-positive, following astronomical
      usage, and must be reversed in sign before use in the present

  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=0.0.  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 slaObs.  If p,
      the pressure in millibars, is available, an adequate
      estimate of hm can be obtained from the expression

            hm = -8149.9415 * log (p/1013.25);

      (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 = 1013.25 * exp (-hm/8149.9415);

      Note, however, that the refraction is proportional to the
      pressure and that an accurate p value is important for
      precise work.


 slaAoppa, slaOapqk

  P.T.Wallace   Starlink   30 October 1993