erfa.atoiq(type, ob1, ob2, astrom)[source]

Quick observed place to CIRS, given the star-independent astrometry parameters.

typeconst char array
ob1double array
ob2double array
astromeraASTROM array
ridouble array
didouble array


Wraps ERFA function eraAtoiq. The ERFA documentation is:

- - - - - - - - -
 e r a A t o i q
- - - - - - - - -

Quick observed place to CIRS, given the star-independent astrometry

Use of this function is appropriate when efficiency is important and
where many star positions are all to be transformed for one date.
The star-independent astrometry parameters can be obtained by
calling eraApio[13] or eraApco[13].

   type   char[]     type of coordinates: "R", "H" or "A" (Note 1)
   ob1    double     observed Az, HA or RA (radians; Az is N=0,E=90)
   ob2    double     observed ZD or Dec (radians)
   astrom eraASTROM* star-independent astrometry parameters:
    pmt    double       PM time interval (SSB, Julian years)
    eb     double[3]    SSB to observer (vector, au)
    eh     double[3]    Sun to observer (unit vector)
    em     double       distance from Sun to observer (au)
    v      double[3]    barycentric observer velocity (vector, c)
    bm1    double       sqrt(1-|v|^2): reciprocal of Lorenz factor
    bpn    double[3][3] bias-precession-nutation matrix
    along  double       longitude + s' (radians)
    xpl    double       polar motion xp wrt local meridian (radians)
    ypl    double       polar motion yp wrt local meridian (radians)
    sphi   double       sine of geodetic latitude
    cphi   double       cosine of geodetic latitude
    diurab double       magnitude of diurnal aberration vector
    eral   double       "local" Earth rotation angle (radians)
    refa   double       refraction constant A (radians)
    refb   double       refraction constant B (radians)

   ri     double          CIRS right ascension (CIO-based, radians)
   di     double          CIRS declination (radians)


1) "Observed" Az,ZD means the position that would be seen by a
   perfect geodetically aligned theodolite.  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
   (CIO-based) RA are related simply through the Earth rotation
   angle and the site longitude.  "Observed" RA,Dec or HA,Dec thus
   means the position that would be seen by a perfect equatorial
   with its polar axis aligned to the Earth's axis of rotation.

2) Only the first character of the type argument is significant.
   "R" or "r" indicates that ob1 and ob2 are the observed right
   ascension (CIO-based) and declination;  "H" or "h" indicates that
   they are hour angle (west +ve) and declination;  anything else
   ("A" or "a" is recommended) indicates that ob1 and ob2 are
   azimuth (north zero, east 90 deg) and zenith distance.  (Zenith
   distance is used rather than altitude in order to reflect the
   fact that no allowance is made for depression of the horizon.)

3) The accuracy of the result is limited by the corrections for
   refraction, which use a simple A*tan(z) + B*tan^3(z) model.
   Providing the meteorological parameters are known accurately and
   there are no gross local effects, the predicted intermediate
   coordinates should be within 0.05 arcsec (optical) or 1 arcsec
   (radio) for a zenith distance of less than 70 degrees, better
   than 30 arcsec (optical or radio) at 85 degrees and better than
   20 arcmin (optical) or 25 arcmin (radio) at the horizon.

   Without refraction, the complementary functions eraAtioq and
   eraAtoiq are self-consistent to better than 1 microarcsecond all
   over the celestial sphere.  With refraction included, consistency
   falls off at high zenith distances, but is still better than
   0.05 arcsec at 85 degrees.

4) 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.

   eraS2c       spherical coordinates to unit vector
   eraC2s       p-vector to spherical
   eraAnp       normalize angle into range 0 to 2pi

This revision:   2022 August 30

Copyright (C) 2013-2023, NumFOCUS Foundation.
Derived, with permission, from the SOFA library.  See notes at end of file.