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- aper

# aper

erfa.aper(theta, astrom)

  In the star-independent astrometry parameters, update only the Earth rotation angle, supplied by the caller explicitly.

  Parameters:
    theta double array
    astrom eraASTROM array
  Returns:
    astrom eraASTROM array

  Notes

  Wraps ERFA function `eraAper`. Note that, unlike the erfa routine, the python wrapper does not change astrom in-place. The ERFA documentation is:

      - - - - - - - -
       e r a A p e r
      - - - - - - - -

      In the star-independent astrometry parameters, update only the
      Earth rotation angle, supplied by the caller explicitly.

      Given:
         theta   double      Earth rotation angle (radians, Note 2)
         astrom  eraASTROM        star-independent astrometry parameters:
          pmt    double       not used
          eb     double[3]    not used
          eh     double[3]    not used
          em     double       not used
          v      double[3]    not used
          bm1    double       not used
          bpn    double[3][3] not used
          along  double       longitude + s' (radians)
          xpl    double       not used
          ypl    double       not used
          sphi   double       not used
          cphi   double       not used
          diurab double       not used
          eral   double       not used
          refa   double       not used
          refb   double       not used

      Returned:
         astrom  eraASTROM        star-independent astrometry parameters:
          pmt    double       unchanged
          eb     double[3]    unchanged
          eh     double[3]    unchanged
          em     double       unchanged
          v      double[3]    unchanged
          bm1    double       unchanged
          bpn    double[3][3] unchanged
          along  double       unchanged
          xpl    double       unchanged
          ypl    double       unchanged
          sphi   double       unchanged
          cphi   double       unchanged
          diurab double       unchanged
          eral   double       "local" Earth rotation angle (radians)
          refa   double       unchanged
          refb   double       unchanged

      Notes:

      1) This function exists to enable sidereal-tracking applications to
         avoid wasteful recomputation of the bulk of the astrometry
         parameters:  only the Earth rotation is updated.

      2) For targets expressed as equinox based positions, such as
         classical geocentric apparent (RA,Dec), the supplied theta can be
         Greenwich apparent sidereal time rather than Earth rotation
         angle.

      3) The function eraAper13 can be used instead of the present
         function, and starts from UT1 rather than ERA itself.

      4) This is one of several functions that inserts into the astrom
         structure star-independent parameters needed for the chain of
         astrometric transformations ICRS <-> GCRS <-> CIRS <-> observed.

         The various functions support different classes of observer and
         portions of the transformation chain:

              functions         observer        transformation

           eraApcg eraApcg13    geocentric      ICRS <-> GCRS
           eraApci eraApci13    terrestrial     ICRS <-> CIRS
           eraApco eraApco13    terrestrial     ICRS <-> observed
           eraApcs eraApcs13    space           ICRS <-> GCRS
           eraAper eraAper13    terrestrial     update Earth rotation
           eraApio eraApio13    terrestrial     CIRS <-> observed

         Those with names ending in "13" use contemporary ERFA models to
         compute the various ephemerides.  The others accept ephemerides
         supplied by the caller.

         The transformation from ICRS to GCRS covers space motion,
         parallax, light deflection, and aberration.  From GCRS to CIRS
         comprises frame bias and precession-nutation.  From CIRS to
         observed takes account of Earth rotation, polar motion, diurnal
         aberration and parallax (unless subsumed into the ICRS <-> GCRS
         transformation), and atmospheric refraction.

      This revision:   2013 September 25

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