SUBROUTINE ZWCOMP(SAMP,DC,M,WAVE,STATUS)
C
C Module Number: 13.3.2.1
C
C Module Name: zwcomp
C
C Keyphrase:
C ----------
C Compute HRS wavelengths
C
C Description:
C ------------
C  The Wavelength at the specified photocathode sample position
C  and spectral order is computed using dispersion coefficients
C  giving sample position as a function of order and wavelength.
C  The dispersion equation is solved using Newtons iterative method
C  with the input wavelength as an initial guess.
C
C FORTRAN Name: zwcomp.for
C
C
C Keywords of Accessed Files :
C --------------------------
C       none
C
C Modules Called:
C ---------------
C SDAS:
C       umsput
C History:
C --------
C Version       Date      Author        Description
C       1       Oct 86  D. Lindler      Designed and coded
C       2       dec 87  D. Lindler      New sdas i/o and standards
C------------------------------------------------------------------------
C     INCLUDE FILE FOR THE IRAF77 FORTRAN INTERFACE TO THE IRAF VOS
C
C
C     FILE I/O ACCESS MODES
C
      INTEGER   RDONLY
      PARAMETER (RDONLY = 1)
      INTEGER   RDWRIT
      PARAMETER (RDWRIT = 2)
      INTEGER   WRONLY
      PARAMETER (WRONLY = 3)
      INTEGER   APPEND
      PARAMETER (APPEND = 4)
C
C     CODES FOR DATA TYPES
C
      INTEGER   TYBOOL
      PARAMETER (TYBOOL = 1)
      INTEGER   TYCHAR
      PARAMETER (TYCHAR = 2)
      INTEGER   TYINT
      PARAMETER (TYINT = 4)
      INTEGER   TYREAL
      PARAMETER (TYREAL = 6)
      INTEGER   TYDOUB
      PARAMETER (TYDOUB = 7)
C
C     UMSPUT DESTINATIONS -- CB, DAO, 4-SEP-87
C
      INTEGER STDOUT
      PARAMETER (STDOUT = 1)
      INTEGER STDERR
      PARAMETER (STDERR = 2)
C
C     UHDAS HEADER PARM TYPES -- CB, DAO, 5-SEP-87
C
      INTEGER GENHDR
      PARAMETER (GENHDR = 0)
      INTEGER IMSPEC
      PARAMETER (IMSPEC = 1)
C
C     THIS SECTION IS FOR PARAMETERS RELEVANT TO TABLE I/O.
C
C     THESE MAY BE SET BY UTPPTI AND/OR READ BY UTPGTI:
C
C                                       LENGTH OF ROW (UNIT = SIZE OF REAL)
      INTEGER   TBRLEN
      PARAMETER (TBRLEN = 1)
C                                       INCREASE ROW LENGTH
      INTEGER   TBIRLN
      PARAMETER (TBIRLN = 2)
C                                       NUMBER OF ROWS TO ALLOCATE
      INTEGER   TBALLR
      PARAMETER (TBALLR = 3)
C                                       INCREASE ALLOC NUM OF ROWS
      INTEGER   TBIALR
      PARAMETER (TBIALR = 4)
C                                       WHICH TYPE OF TABLE? (ROW OR COLUMN)
      INTEGER   TBWTYP
      PARAMETER (TBWTYP = 5)
C                                       MAXIMUM NUMBER OF USER PARAMETERS
      INTEGER   TBMXPR
      PARAMETER (TBMXPR = 6)
C                                       MAXIMUM NUMBER OF COLUMNS
      INTEGER   TBMXCL
      PARAMETER (TBMXCL = 7)
C                                       TYPE = ROW-ORDERED TABLE
      INTEGER   TBTYPR
      PARAMETER (TBTYPR = 11)
C                                       TYPE = COLUMN-ORDERED TABLE
      INTEGER   TBTYPC
      PARAMETER (TBTYPC = 12)
C
C     THESE MAY BE READ BY UTPGTI BUT MAY NOT BE SET:
C
C                                       NUMBER OF ROWS WRITTEN TO
      INTEGER   TBNROW
      PARAMETER (TBNROW = 21)
C
C     END IRAF77.INC
C
C INPUT PARAMETERS
C
        DOUBLE PRECISION SAMP
C                                    --->PHOTOCATHODE SAMPLE NUMBER
        DOUBLE PRECISION DC(7)
C                                    --->DISPERSION COEFFICIENTS
        DOUBLE PRECISION M
C                                    --->SPECTRAL ORDER NUMBER
C
C INPUT/OUTPUT PARAMTER
C
        DOUBLE PRECISION WAVE
C                                    --->WAVELENGTH
C
C OUTPUT PARAMETER
C
        INTEGER         STATUS
C                                    --->ERROR STATUS
C
C LOCAL VARIABLES
C
        DOUBLE PRECISION AA,BB,CC
C                                    --->POLYNOMIAL COEF.
        INTEGER         ITER
C                                    --->ITERATION COUNTER
        CHARACTER*130   CONTXT
C                                    --->WHAT HAPPENED
        DOUBLE PRECISION SCOMP
C                                    --->COMPUTE SAMPLE POSITION
        DOUBLE PRECISION SLOPE
C                                    --->SLOPE OF POLYNOMIAL
        REAL            EPS
C                                    --->CONVERGENCE TEST 0.01 DIODES
        INTEGER ISTAT
C
C  DATA DECLARATIONS
C
        DATA EPS/0.01/
C------------------------------------------------------------------------
C
C
C COMPUTE POLYNOMIAL COEFFICIENTS
C
        AA=DC(1)+DC(4)*M
        BB=DC(2)*M + DC(5) + DC(6)*M*M
        CC=DC(3)*M*M + DC(7)*M
C
C PERFORM NEWTON ITERATION
C
        ITER=0
C                                    --->ITERATION COUNT
        SCOMP=AA+BB*WAVE+CC*WAVE*WAVE
C                                    --->INITIAL GUESS
50      IF (ABS(SAMP-SCOMP).LE.EPS)GO TO 60
                ITER=ITER+1
                SLOPE=BB+2*CC*WAVE
C                                    --->SLOPE OF LINE
                IF( (ABS(SLOPE).LT.1.0E-10) .OR. (ITER.GT.20) .OR.
     &                  (ABS(SLOPE).GT.1.0E10))THEN
                        CONTXT='WAVELENGTH COMP. FAILED TO CONVERGE'
                        STATUS=1
                        GO TO 999
                ENDIF
C
C COMPUTE NEW WAVELENGTH
C
                WAVE=WAVE+(SAMP-SCOMP)/SLOPE
                SCOMP=AA+BB*WAVE+CC*WAVE*WAVE
        GO TO 50
60      STATUS=0
        GO TO 1000
999     CALL UMSPUT(CONTXT,STDOUT+STDERR,0,ISTAT)
1000    RETURN
        END