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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 depends on the chopper mode.

The initial step is the conversion of the data word ( $X^{raw}_{FPC}$ = RAW_FPC_POSN ) to a voltage ( $V_{ini}$ ) according to:

$\begin{displaymath} V_{ini} = C (\frac{X^{raw}_{FPC}{\times}Q}{P}-R)~~~~~~~~[{\rm mV}], \end{displaymath}$

(B.6)

where


  	=	1000, the conversion to mV
  	=	20V
  	=	4096
  	=	10.0V

At this stage a crude first approximation to the chopper angle ( $\alpha$ ) is calculated from a simple linear model:

$\begin{displaymath} \alpha_{linear} = -\frac{V_{ini}+b}{a}~~~~[{\rm arcsec}] \end{displaymath}$

(B.7)

Since there are several chopping modes (see Table 13.2), the software has to sort out the conversion based on the chopper mode to be processed. In case of rectangular chopper mode

$\begin{displaymath} \alpha_{approx} = \alpha_{linear} - \frac{{\Delta}U_{DC}(\alpha_{linear})}{a}~~~~[{\rm arcsec}] \end{displaymath}$

(B.8)

where ${\Delta}U_{DC}(\alpha)$ is a correction obtained from linear interpolation in a hardcoded table given in Table B.3. An additional correction for hysteresis is computed according to:

$\begin{displaymath} \alpha_{sky} = \alpha_{approx} - (\frac{{\Delta}U_{ACI}(-3.... ...}\vert}{150})~~~[{\rm arcsec}]~~{\rm if}~\alpha_{approx}>\,0, \end{displaymath}$

(B.9)

$\begin{displaymath} \alpha_{sky} = \alpha_{approx} - (\frac{{\Delta}U_{ACD}(0)}... ...}\vert}{150})~~~[{\rm arcsec}]~~{\rm if}~\alpha_{approx}<\,0, \end{displaymath}$

(B.10)

For scanning modes (such as used in AOT PHT32) the chopper amplitude ( ${\alpha}_{amp}$ ) is required. In case of sawtooth and triangular chopper mode, ${\alpha}_{amp}$ is determined from:

$\begin{displaymath} {\alpha}_{amp} = N_{step}{\times}{\alpha}_{inc}, \end{displaymath}$

(B.11)

where $N_{step}$ is the number of chopper steps and ${\alpha}_{inc}$ the increment in arcsec as given in the compact status (product PSTA, keywords PSTACSTE and PSTACINC, see Section 13.2.7). For the other modes ${\alpha}_{amp}$ is equal to the value of PSTACAMP, provided in the compact status.

The approximation is then corrected for deviations from the line approximation:

$\begin{displaymath} \alpha_{approx} = \alpha_{linear} - \frac{{\Delta}U_{DC}({\alpha}_{amp})}{a}~~~~[{\rm arcsec}] \end{displaymath}$

(B.12)

Next, corrections must be made for hysteresis effects:

For saw-tooth profile SUBMODE(0)

$\begin{displaymath} \alpha_{sky} = \alpha_{approx} - {\alpha}_{amp} \frac{{\De... ...ha_{approx}/{\alpha}_{amp})} {150{\times}a}~~~[{\rm arcsec}] \end{displaymath}$ (B.13)
for triangular-tooth profile SUBMODE(1)

$\begin{displaymath} \alpha_{sky} = \alpha_{approx} - {\alpha}_{amp} \frac{{\De... ... {150{\times}a}~~~[{\rm arcsec}]~~{\rm for~increasing~angle}, \end{displaymath}$ (B.14)

$\begin{displaymath} \alpha_{sky} = \alpha_{approx} - {\alpha}_{amp} \frac{{\De... ... {150{\times}a}~~~[{\rm arcsec}]~~{\rm for~decreasing~angle}. \end{displaymath}$ (B.15)

In the correction the ratio $\alpha_{approx}/{\alpha}_{amp}$ is equivalent to ${\alpha}_{NORM}$ in interpolation Table B.3.

For chopper modes 12 and 13 (central field of view, see Table13.2) the chopper deflection is still computed according to Equation B.12.

Values of the different parameters:


      =	   9.862622 
      =	   20.0057
   =	4238.0 
   =	4238.0 
    =	 131.0
    =	 239.0
    =	 6.0

**Table B.3:** *Parameters for deriving the chopper deflection.*
${\alpha}_{NORM}$	${\alpha}$	${\Delta}U_{DC}(\alpha)$	${\Delta}U_{ACD}(\alpha)$	${\Delta}U_{ACI}(\alpha)$
1.0	180.0	11.4	0.0	0.0
0.9	162.0	10.4	2.8	7.4
0.8	144.0	11.5	5.7	14.8
0.7	126.0	12.5	8.7	21.5
0.6	108.0	13.6	11.7	28.2
0.5	90.0	15.1	13.6	32.8
0.4	72.0	15.2	15.5	37.4
0.3	54.0	14.2	15.2	39.4
0.2	36.0	11.5	14.8	41.4
0.1	18.0	8.1	12.6	41.0
0.0	0.0	2.9	10.3	40.5
0.1	18.0	1.0	7.8	37.3
0.2	36.0	2.9	5.4	34.1
0.3	54.0	2.4	3.1	30.2
0.4	72.0	1.6	0.9	26.3
0.5	90.0	0.8	0.6	22.6
0.6	108.0	0.3	2.1	18.9
0.7	126.0	1.7	1.6	15.5
0.8	144.0	3.9	1.0	12.0
0.9	162.0	6.1	1.3	7.8
1.0	180.0	9.1	3.6	3.6

Next: B.4 Description of the Up: B. Conversion Algorithms for Previous: B.2 Selected FCS Power

ISO Handbook Volume IV (PHT), Version 2.0.1, SAI/1999-069/Dc