Name

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      PREMAT

Purpose

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      Return the precession matrix needed to go from EQUINOX1 to EQUINOX2.

Explantion

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      This matrix is used by the procedures PRECESS and BARYVEL to precess
      astronomical coordinates

Calling Sequence

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      matrix = PREMAT( equinox1, equinox2, [ /FK4 ] )

Inputs

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      EQUINOX1 - Original equinox of coordinates, numeric scalar.
      EQUINOX2 - Equinox of precessed coordinates.

Output

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      matrix - double precision 3 x 3 precession matrix, used to precess
              equatorial rectangular coordinates

Optional Input Keywords

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      /FK4 - If this keyword is set, the FK4 (B1950.0) system precession
              angles are used to compute the precession matrix. The
              default is to use FK5 (J2000.0) precession angles

Examples

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      Return the precession matrix from 1950.0 to 1975.0 in the FK4 system
      IDL> matrix = PREMAT( 1950.0, 1975.0, /FK4)

Procedure

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      FK4 constants from "Computational Spherical Astronomy" by Taff (1983),
      p. 24. (FK4). FK5 constants from "Astronomical Almanac Explanatory
      Supplement 1992, page 104 Table 3.211.1.

Revision History

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      Written, Wayne Landsman, HSTX Corporation, June 1994
      Converted to IDL V5.0 W. Landsman September 1997