slaOapqk -

SYNOPSYS

PARAMETERS

char *type

double ob1

double ob2

double *aoprms

double *rap

double *dap

DESCRIPTION

  Quick observed to apparent place.

Given

     type   char        type of coordinates - 'r', 'h' or 'a' (see below)
     ob1    double      observed az, HA or RA (radians; az is n=0,e=90)
     ob2    double      observed ZD or Dec (radians)

       (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

     *rap    double      geocentric apparent right ascension
     *dap    double      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.
       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.

   5)  "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 slaFk524 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.

  called:  slaDcs2c, slaDcc2s, slaRefro, slaDranrm

  P.T.Wallace   Starlink   30 October 1993

SEE ALSO

• slalib.3