slaAopqk -

SYNOPSYS

PARAMETERS

double rap

double dap

double *aoprms

double *aob

double *zob

double *hob

double *dob

double *rob

DESCRIPTION

  Quick apparent to observed place.

Given

     rap    double      geocentric apparent right ascension
     dap    double      geocentric apparent declination

       (0)      geodetic latitude (radians)
       (1,2)    sine and cosine of geodetic latitude
       (3)      magnitude of diurnal aberration vector
       (4)      height (hm)
       (5)      ambient temperature (t)
       (6)      pressure (p)
       (7)      relative humidity (rh)
       (8)      wavelength (wl)
       (9)      lapse rate (tlr)
       (10,11)  refraction constants A and B (radians)
       (12)     longitude + eqn of equinoxes + sidereal DUT (radians)
       (13)     local apparent sidereal time (radians)

Returned

     *aob    double      observed azimuth (radians: N=0,E=90)
     *zob    double      observed zenith distance (radians)
     *hob    double      observed hour angle (radians)
     *dob    double      observed declination (radians)
     *rob    double      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
       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 slaMap 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
       slaFk425 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 slaAoppa routine.
       If nothing has changed significantly except the time, the
       slaAoppat routine may be used to perform the requisite
       partial recomputation of aoprms.

Called

 slaDcs2c, slaRefz, slaRefro, slaDcc2s, slaDranrm

  P.T.Wallace   Starlink   31 October 1993

SEE ALSO

• slalib.3