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      SUBROUTINE slDE2H (HA, DEC, PHI, AZ, EL)

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      D E 2 H
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  Equatorial to horizon coordinates:  HA,Dec to Az,El

  (double precision)

  Given:
     HA      d     hour angle
     DEC     d     declination
     PHI     d     observatory latitude

  Returned:
     AZ      d     azimuth
     EL      d     elevation

  Notes:

  1)  All the arguments are angles in radians.

  2)  Azimuth is returned in the range 0-2pi;  north is zero,
      and east is +pi/2.  Elevation is returned in the range
      +/-pi/2.

  3)  The latitude must be geodetic.  In critical applications,
      corrections for polar motion should be applied.

  4)  In some applications it will be important to specify the
      correct type of hour angle and declination in order to
      produce the required type of azimuth and elevation.  In
      particular, it may be important to distinguish between
      elevation as affected by refraction, which would
      require the "observed" HA,Dec, and the elevation
      in vacuo, which would require the "topocentric" HA,Dec.
      If the effects of diurnal aberration can be neglected, the
      "apparent" HA,Dec may be used instead of the topocentric
      HA,Dec.

  5)  No range checking of arguments is carried out.

  6)  In applications which involve many such calculations, rather
      than calling the present routine it will be more efficient to
      use inline code, having previously computed fixed terms such
      as sine and cosine of latitude, and (for tracking a star)
      sine and cosine of declination.

  P.T.Wallace   Starlink   9 July 1994

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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