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SUBROUTINE sla_PRECES (SYSTEM, EP0, EP1, RA, DC)
*+
* - - - - - - -
* P R E C E S
* - - - - - - -
*
* Precession - either FK4 (Bessel-Newcomb, pre IAU 1976) or
* FK5 (Fricke, post IAU 1976) as required.
*
* Given:
* SYSTEM char precession to be applied: 'FK4' or 'FK5'
* EP0,EP1 dp starting and ending epoch
* RA,DC dp RA,Dec, mean equator & equinox of epoch EP0
*
* Returned:
* RA,DC dp RA,Dec, mean equator & equinox of epoch EP1
*
* Called: sla_DRANRM, sla_PREBN, sla_PREC, sla_DCS2C,
* sla_DMXV, sla_DCC2S
*
* Notes:
*
* 1) Lowercase characters in SYSTEM are acceptable.
*
* 2) The epochs are Besselian if SYSTEM='FK4' and Julian if 'FK5'.
* For example, to precess coordinates in the old system from
* equinox 1900.0 to 1950.0 the call would be:
* CALL sla_PRECES ('FK4', 1900D0, 1950D0, RA, DC)
*
* 3) This routine will NOT correctly convert between the old and
* the new systems - for example conversion from B1950 to J2000.
* For these purposes see sla_FK425, sla_FK524, sla_FK45Z and
* sla_FK54Z.
*
* 4) If an invalid SYSTEM is supplied, values of -99D0,-99D0 will
* be returned for both RA and DC.
*
* P.T.Wallace Starlink 20 April 1990
*
* Copyright (C) 1995 Rutherford Appleton Laboratory
*-
IMPLICIT NONE
CHARACTER SYSTEM*(*)
DOUBLE PRECISION EP0,EP1,RA,DC
DOUBLE PRECISION PM(3,3),V1(3),V2(3)
CHARACTER SYSUC*3
DOUBLE PRECISION sla_DRANRM
* Convert to uppercase and validate SYSTEM
SYSUC=SYSTEM
IF (SYSUC(1:1).EQ.'f') SYSUC(1:1)='F'
IF (SYSUC(2:2).EQ.'k') SYSUC(2:2)='K'
IF (SYSUC.NE.'FK4'.AND.SYSUC.NE.'FK5') THEN
RA=-99D0
DC=-99D0
ELSE
* Generate appropriate precession matrix
IF (SYSUC.EQ.'FK4') THEN
CALL sla_PREBN(EP0,EP1,PM)
ELSE
CALL sla_PREC(EP0,EP1,PM)
END IF
* Convert RA,Dec to x,y,z
CALL sla_DCS2C(RA,DC,V1)
* Precess
CALL sla_DMXV(PM,V1,V2)
* Back to RA,Dec
CALL sla_DCC2S(V2,RA,DC)
RA=sla_DRANRM(RA)
END IF
END
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