[aob, zob, hob, dob, rob] = ERFA.atioq(ri, di, astrom)Quick CIRS to observed place transformation.
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 or eraApco.
ri double CIRS right ascension
di double CIRS declination
astrom ASTROM* 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)
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)
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This function returns zenith distance rather than altitude in order to reflect the fact that no allowance is made for depression of the horizon.
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The accuracy of the result is limited by the corrections for refraction, which use a simple Atan(z) + Btan^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 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.
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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.
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The CIRS RA,Dec is obtained from a star catalog mean place by allowing for space motion, parallax, the Sun's gravitational lens effect, annual aberration and precession-nutation. For star positions in the ICRS, these effects can be applied by means of the eraAtci13 (etc.) functions. Starting from classical "mean place" systems, additional transformations will be needed first.
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"Observed" Az,El means the position that would be seen by a perfect geodetically aligned theodolite. This is obtained from the CIRS 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, and is the position that would be seen by a perfect equatorial with its polar axis aligned to the Earth's axis of rotation. Finally, the RA is obtained by subtracting the HA from the local ERA.
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The star-independent CIRS-to-observed-place parameters in ASTROM may be computed with eraApio or eraApco. If nothing has changed significantly except the time, eraAper may be used to perform the requisite adjustment to the astrom structure.
eraS2c spherical coordinates to unit vector
eraC2s p-vector to spherical
eraAnp normalize angle into range 0 to 2pi
This revision: 2020 December 7
Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.