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SOFACelestialToIntermediate Method
Transform ICRS star data, epoch J2000.0, to CIRS using the SOFA Atci13 function.

Namespace: ASCOM.Astrometry.SOFA
Assembly: ASCOM.Astrometry (in ASCOM.Astrometry.dll) Version: 6.0.0.0 (6.2.0.2774)
Syntax
public void CelestialToIntermediate(
	double rc,
	double dc,
	double pr,
	double pd,
	double px,
	double rv,
	double date1,
	double date2,
	ref double ri,
	ref double di,
	ref double eo
)

Parameters

rc
Type: SystemDouble
ICRS right ascension at J2000.0 (radians, Note 1)
dc
Type: SystemDouble
ICRS declination at J2000.0 (radians, Note 1)
pr
Type: SystemDouble
RA proper motion (radians/year; Note 2)
pd
Type: SystemDouble
Dec proper motion (radians/year)
px
Type: SystemDouble
parallax (arcsec)
rv
Type: SystemDouble
radial velocity (km/s, +ve if receding)
date1
Type: SystemDouble
TDB as a 2-part Julian Date (Note 3)
date2
Type: SystemDouble
TDB as a 2-part Julian Date (Note 3)
ri
Type: SystemDouble
CIRS geocentric RA (radians)
di
Type: SystemDouble
CIRS geocentric Dec (radians)
eo
Type: SystemDouble
equation of the origins (ERA-GST, Note 5)
Remarks
Notes:
  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 iauPmsafe before use.
  2. The proper motion in RA is dRA/dt rather than cos(Dec)*dRA/dt.
  3. The TDB date date1+date2 is a Julian Date, apportioned in any convenient way between the two arguments. For example, JD(TDB)=2450123.8g could be expressed in any of these ways, among others:
    Date 1Date 2Method
    2450123.8 0.0 JD method
    2451545.0 -1421.3 J2000 method
    2400000.5 50123.2 MJD method
    2450123.5 0.2 Date and time method

    The JD method is the most natural and convenient to use in cases where the loss of several decimal digits of resolution is acceptable. The J2000 method is best matched to the way the argument is handled internally and will deliver the optimum resolution. The MJD method and the date and time methods are both good compromises between resolution and convenience. For most applications of this function the choice will not be at all critical.

    TT can be used instead of TDB without any significant impact on accuracy.

  4. The available accuracy is better than 1 milliarcsecond, limited mainly by the precession-nutation model that is used, namely IAU 2000A/2006. Very close to solar system bodies, additional errors of up to several milliarcseconds can occur because of unmodeled light deflection; however, the Sun's contribution is taken into account, to first order. The accuracy limitations of the SOFA function iauEpv00 (used to compute Earth position and velocity) can contribute aberration errors of up to 5 microarcseconds. Light deflection at the Sun's limb is uncertain at the 0.4 mas level.
  5. Should the transformation to (equinox based) apparent place be required rather than (CIO based) intermediate place, subtract the equation of the origins from the returned right ascension: RA = RI - EO. (The Anp function can then be applied, as required, to keep the result in the conventional 0-2pi range.)
See Also