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## SOFAObservedToCelestial Method |

Observed place at a groundbased site to to ICRS astrometric RA,Dec using the SOFA Atoc13 function.

Syntax

public int ObservedToCelestial( string type, double ob1, double ob2, double utc1, double utc2, double dut1, double elong, double phi, double hm, double xp, double yp, double phpa, double tc, double rh, double wl, ref double rc, ref double dc )

- type
- Type: SystemString

type of coordinates - "R", "H" or "A" (Notes 1,2) - ob1
- Type: SystemDouble

observed Az, HA or RA (radians; Az is N=0,E=90) - ob2
- Type: SystemDouble

observed ZD or Dec (radians) - utc1
- Type: SystemDouble

UTC as a 2-part quasi Julian Date (Notes 3,4) - utc2
- Type: SystemDouble

UTC as a 2-part quasi Julian Date (Notes 3,4) - dut1
- Type: SystemDouble

UT1-UTC (seconds, Note 5) - elong
- Type: SystemDouble

longitude (radians, east +ve, Note 6) - phi
- Type: SystemDouble

geodetic latitude (radians, Note 6) - hm
- Type: SystemDouble

height above ellipsoid (m, geodetic Notes 6,8) - xp
- Type: SystemDouble

polar motion coordinates (radians, Note 7) - yp
- Type: SystemDouble

polar motion coordinates (radians, Note 7) - phpa
- Type: SystemDouble

pressure at the observer (hPa = mB, Note 8) - tc
- Type: SystemDouble

ambient temperature at the observer (deg C) - rh
- Type: SystemDouble

relative humidity at the observer (range 0-1) - wl
- Type: SystemDouble

wavelength (micrometers, Note 9) - rc
- Type: SystemDouble

ICRS astrometric RA (radians) - dc
- Type: SystemDouble

ICRS astrometric Dec (radians)

Status: +1 = dubious year (Note 4), 0 = OK, -1 = unacceptable date

Remarks

Notes:

- "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.
- Only the first character of the type argument is significant. "R" or "r" indicates that ob1 and ob2 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 ob1 and ob2 are azimuth (north zero, east 90 deg) and zenith distance.
- 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 iauDtf2d to convert from calendar date and time of day into 2-part quasi Julian Date, as it implements the leap-second-ambiguity convention just described.

- 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 iauDat for further details.
- 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.
- The geographical coordinates are with respect to the 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.
- 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.
- 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 follows:

**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.

- 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).
- 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 astrometric 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 iauAtco13 and iauAtoc13 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.

- 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.

See Also