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+      SUBROUTINE slDE2H (HA, DEC, PHI, AZ, EL)
+*+
+*     - - - - -
+*      D E 2 H
+*     - - - - -
+*
+*  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
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 of the License, or
+*    (at your option) any later version.
+*
+*    This program is distributed in the hope that it will be useful,
+*    but WITHOUT ANY WARRANTY; without even the implied warranty of
+*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*    GNU General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the
+*    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+*    Boston, MA  02110-1301  USA
+*
+*  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION HA,DEC,PHI,AZ,EL
+
+      DOUBLE PRECISION D2PI
+      PARAMETER (D2PI=6.283185307179586476925286766559D0)
+
+      DOUBLE PRECISION SH,CH,SD,CD,SP,CP,X,Y,Z,R,A
+
+
+*  Useful trig functions
+      SH=SIN(HA)
+      CH=COS(HA)
+      SD=SIN(DEC)
+      CD=COS(DEC)
+      SP=SIN(PHI)
+      CP=COS(PHI)
+
+*  Az,El as x,y,z
+      X=-CH*CD*SP+SD*CP
+      Y=-SH*CD
+      Z=CH*CD*CP+SD*SP
+
+*  To spherical
+      R=SQRT(X*X+Y*Y)
+      IF (R.EQ.0D0) THEN
+         A=0D0
+      ELSE
+         A=ATAN2(Y,X)
+      END IF
+      IF (A.LT.0D0) A=A+D2PI
+      AZ=A
+      EL=ATAN2(Z,R)
+
+      END