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SUBROUTINE slPRCS (SYSTEM, EP0, EP1, RA, DC)
*+
* - - - - - - -
* P R C E
* - - - - - - -
*
* Precession - either FK4 (Bessel-Newcomb, pre IAU 1976) or
* FK5 (Fricke, post IAU 1976) as required.
*
* Given:
* SYSTEM int precession to be applied: 1 = FK4 or 2 = 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: slDA2P, slPRBN, slPREC, slDS2C,
* slDMXV, slDC2S
*
* 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 slPRCS (1, 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 slFK45, slFK54, slF45Z and
* slF54Z.
*
* 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
*-
IMPLICIT NONE
INTEGER SYSTEM
DOUBLE PRECISION EP0,EP1,RA,DC
DOUBLE PRECISION PM(3,3),V1(3),V2(3)
DOUBLE PRECISION slDA2P
* Convert to uppercase and validate SYSTEM
IF (SYSTEM.NE.1.AND.SYSTEM.NE.2) THEN
RA=-99D0
DC=-99D0
ELSE
* Generate appropriate precession matrix
IF (SYSTEM.EQ.1) THEN
CALL slPRBN(EP0,EP1,PM)
ELSE
CALL slPREC(EP0,EP1,PM)
END IF
* Convert RA,Dec to x,y,z
CALL slDS2C(RA,DC,V1)
* Precess
CALL slDMXV(PM,V1,V2)
* Back to RA,Dec
CALL slDC2S(V2,RA,DC)
RA=slDA2P(RA)
END IF
END
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