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B.3 Focal Plane Chopper Deflection  

The telemetry contains the actual (measured) chopper position. The value represents a voltage which is non-linear to the chopper angle on the sky, and non-synchronous to IR readout data. The conversion is dependent upon the chopper mode and submode.

The initial step is the conversion of the data word to a voltage (V) according to :

               in_volt = C * (RAW_FPC_POSN * Q/P - R)

               C = 1.0         P = 4096
               Q = 20.0V       R = 10.0V

This quantity is converted to mV

                      in_volt = 1000 * in_volt

At this stage the voltage is checked to see if it corresponds to an FCS for FPC_MODE_2 or FPC_MODE_32 else a crude first approximation to the chopper angle is calculated from a simple linear model

               angle_line = - in_volt - b            (arcsec)
                              -----------
                                    a

In the case of FPC_MODE_1, FPC_MODE_31N/P, and FPC_MODE_4, the approximation is then corrected for deviations from the line approximation using the following :

               Angle_approx = angle_line - DU_DC(<angle>)   (arcsec)
                                           -------------
                                                 a

Where DU_DC is a linear interpolation to a correction table. The value of rm <angle> depends on the chopper mode.

Corrections must be made for hysteresis effects in the chopper position the case of FPC_MODE_1 and FPC_MODE_4.

For FPC_MODE_1, the correction is

(1) sky_angle = angle_approx - DU_ACI(+1)    |angle_approx|    (arcsec)
                               ---------  *  -------------
                                   a             150
for angle_approx > 0

(2) sky_angle = angle_approx - DU_ACD(-1)    |angle_approx|    (arcsec)
                               ---------  *  -------------
                                   a              150
for angle_approx < 0


For FPC_MODE_4, the correction is

(1) For saw-tooth profile SUBMODE(0)

sky_angle = angle_approx -  A   [DU_ACI(angle_approx/A)]    (arcsec)
                            ------
                             150*a

(2) for triangular-tooth profile SUBMODE(1)

sky_angle = angle_approx -  A   [DU_ACI(angle_approx/A)]    (arcsec)
                            ------
                             150*a
for increasing angle.

sky_angle = angle_approx -  A   [DU_ACD(angle_approx/A)]    (arcsec)
                            ------
                             150*a
for decreasing angle.

The converted angle is returned. If the FCSs are used then the FPC_STATE variable indicates which one is in use. If an invalid raw position is given, FPC_STATE is set to -1. Any other position gives a value of 0.

Values of the different parameters:

      PARAMETER (C=1.0)            ! initial conversion
      PARAMETER (P=4096)           ! a voltage
      PARAMETER (Q=20.0)
      PARAMETER (R=10.0)


      PARAMETER (A=9.862622)       ! the line approx
      PARAMETER (B=-20.0057)

      PARAMETER (U_ACF1=4238.0)    ! values for the voltage
      PARAMETER (U_DCF1=4238.0)    ! the beam from the FCSs
      PARAMETER (U_OF1=131.0)
      PARAMETER (U_OF2=239.0)
      PARAMETER (TOLERANCE=6.0)

     DATA ANG_DEV  /-180.0,-162.0,-144.0,-126.0,-108.0, -90.0, -72.0,
     #                -54.0, -36.0, -18.0,   0.0,  18.0,  36.0,  54.0,
     #                 72.0,  90.0, 108.0, 126.0, 144.0, 162.0, 180.0/
      DATA DU_DC     /-11.4, -10.4, -11.5, -12.5, -13.6, -15.1, -15.2,
     #                -14.2, -11.5,  -8.1,  -2.9,  -1.0,  -2.9,  -2.4,
     #                 -1.6,  -0.8,   0.3,   1.7,   3.9,   6.1,   9.1/
      DATA NORM_ANG  / -1.0,  -0.9,  -0.8,  -0.7,  -0.6,  -0.5,  -0.4,
     #                 -0.3,  -0.2,  -0.1,   0.0,   0.1,   0.2,   0.3,
     #                  0.4,   0.5,   0.6,   0.7,   0.8,   0.9,   1.0/
      DATA DU_ACD    /  0.0,  -2.8,  -5.7,  -8.7, -11.7, -13.6, -15.5,
     #                -15.2, -14.8, -12.6, -10.3,  -7.8,  -5.4,  -3.1,
     #                 -0.9,   0.6,   2.1,   1.6,   1.0,  -1.3,  -3.6/
      DATA DU_ACI    /  0.0,  -7.4, -14.8, -21.5, -28.2, -32.8, -37.4,
     #                -39.4, -41.4, -41.0, -40.5, -37.3, -34.1, -30.2,
     #                -26.3, -22.6, -18.9, -15.5, -12.0,  -7.8,  -3.6/