erfa.atco13(rc, dc, pr, pd, px, rv, utc1, utc2, dut1, elong, phi, hm, xp, yp, phpa, tc, rh, wl)[source]

ICRS RA,Dec to observed place. The caller supplies UTC, site coordinates, ambient air conditions and observing wavelength.

rcdouble array
dcdouble array
prdouble array
pddouble array
pxdouble array
rvdouble array
utc1double array
utc2double array
dut1double array
elongdouble array
phidouble array
hmdouble array
xpdouble array
ypdouble array
phpadouble array
tcdouble array
rhdouble array
wldouble array
aobdouble array
zobdouble array
hobdouble array
dobdouble array
robdouble array
eodouble array


Wraps ERFA function eraAtco13. The ERFA documentation is:

- - - - - - - - - -
 e r a A t c o 1 3
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ICRS RA,Dec to observed place.  The caller supplies UTC, site
coordinates, ambient air conditions and observing wavelength.

ERFA models are used for the Earth ephemeris, bias-precession-
nutation, Earth orientation and refraction.

   rc,dc  double   ICRS right ascension at J2000.0 (radians, Note 1)
   pr     double   RA proper motion (radians/year, Note 2)
   pd     double   Dec proper motion (radians/year)
   px     double   parallax (arcsec)
   rv     double   radial velocity (km/s, +ve if receding)
   utc1   double   UTC as a 2-part...
   utc2   double   ...quasi Julian Date (Notes 3-4)
   dut1   double   UT1-UTC (seconds, Note 5)
   elong  double   longitude (radians, east +ve, Note 6)
   phi    double   latitude (geodetic, radians, Note 6)
   hm     double   height above ellipsoid (m, geodetic, Notes 6,8)
   xp,yp  double   polar motion coordinates (radians, Note 7)
   phpa   double   pressure at the observer (hPa = mB, Note 8)
   tc     double   ambient temperature at the observer (deg C)
   rh     double   relative humidity at the observer (range 0-1)
   wl     double   wavelength (micrometers, Note 9)

   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 (CIO-based, radians)
   eo     double        equation of the origins (ERA-GST, radians)

Returned (function value):
          int      status: +1 = dubious year (Note 4)
                            0 = OK
                           -1 = unacceptable date


1)  Star data for an epoch other than J2000.0 (for example from the
    Hipparcos catalog, which has an epoch of J1991.25) will require
    a preliminary call to eraPmsafe before use.

2)  The proper motion in RA is dRA/dt rather than cos(Dec)*dRA/dt.

3)  utc1+utc2 is quasi Julian Date (see Note 2), apportioned in any
    convenient way between the two arguments, for example where utc1
    is the Julian Day Number and utc2 is the fraction of a day.

    However, JD cannot unambiguously represent UTC during a leap
    second unless special measures are taken.  The convention in the
    present function is that the JD day represents UTC days whether
    the length is 86399, 86400 or 86401 SI seconds.

    Applications should use the function eraDtf2d to convert from
    calendar date and time of day into 2-part quasi Julian Date, as
    it implements the leap-second-ambiguity convention just

4)  The warning status "dubious year" flags UTCs that predate the
    introduction of the time scale or that are too far in the
    future to be trusted.  See eraDat for further details.

5)  UT1-UTC is tabulated in IERS bulletins.  It increases by exactly
    one second at the end of each positive UTC leap second,
    introduced in order to keep UT1-UTC within +/- 0.9s.  n.b. This
    practice is under review, and in the future UT1-UTC may grow
    essentially without limit.

6)  The geographical coordinates are with respect to the ERFA_WGS84
    reference ellipsoid.  TAKE CARE WITH THE LONGITUDE SIGN:  the
    longitude required by the present function is east-positive
    (i.e. right-handed), in accordance with geographical convention.

7)  The polar motion xp,yp can be obtained from IERS bulletins.  The
    values are the coordinates (in radians) of the Celestial
    Intermediate Pole with respect to the International Terrestrial
    Reference System (see IERS Conventions 2003), measured along the
    meridians 0 and 90 deg west respectively.  For many
    applications, xp and yp can be set to zero.

8)  If hm, the height above the ellipsoid of the observing station
    in meters, is not known but phpa, the pressure in hPa (=mB),
    is available, an adequate estimate of hm can be obtained from
    the expression

          hm = -29.3 * tsl * log ( phpa / 1013.25 );

    where tsl is the approximate sea-level air temperature in K
    (See Astrophysical Quantities, C.W.Allen, 3rd edition, section
    52).  Similarly, if the pressure phpa is not known, it can be
    estimated from the height of the observing station, hm, as

          phpa = 1013.25 * exp ( -hm / ( 29.3 * tsl ) );

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

9)  The argument wl specifies the observing wavelength in
    micrometers.  The transition from optical to radio is assumed to
    occur at 100 micrometers (about 3000 GHz).

10) 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 observed
    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 30 arcmin (radio) at the horizon.

    Without refraction, the complementary functions eraAtco13 and
    eraAtoc13 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.

11) "Observed" Az,ZD means the position that would be seen by a
    perfect geodetically aligned theodolite.  (Zenith distance is
    used rather than altitude in order to reflect the fact that no
    allowance is made for depression of the horizon.)  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 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.

12) 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.

   eraApco13    astrometry parameters, ICRS-observed, 2013
   eraAtciq     quick ICRS to CIRS
   eraAtioq     quick CIRS to observed

This revision:   2022 May 3

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