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7.7 Signal Processing: PHT32 Raster Maps

Detailed description: see also Sections 3.5, 3.10.2

PHT32 applied the fast chopper scanning mode in combination with a spacecraft raster grid to obtain oversampled linear scans or maps. In general, the chopper dwell time is short with respect to the reset interval. The number of ramps collected per chopper plateau is in most cases 4. In addition, the number of readouts per ramp can also be as low as 4. If the first readouts per ramp and destructive readouts were to be removed as described in Sections 7.2.1 and 7.2.2, only a few usable readouts would be left on a chopper plateau. This would make the signal processing very sensitive to glitches.

An alternative method for determining the average signal per chopper plateau is applied in OLP. This method uses the same readout processing methods as used for the chopped photometric observations. Since the PHT32 signal processing already include special processing on readout level, the ramp processing steps have been modified.

Ancillary data required:

none

7.7.1 Obtaining the average signal per chopper plateau

Detailed description: none

The average signal per chopper plateau is obtained by first applying ramp processing steps 7.2.3 until 7.2.8. The discarding of destructive readouts (7.2.1) and discarding of disturbed non-destructive readouts (7.2.2) are not carried out. For each detector pixel a difference signal is computed from readout and ( $k{\geq}2$ ) of a given ramp:

$\begin{displaymath} s_i(k) = \frac {V_i(k) - V_i(k-1)} {t(k) - t(k-1)}~~~~~~~~~~~~~[{\rm V/s}], \end{displaymath}$

(7.68)

where is a valid readout, and is the corresponding time. After all difference signals on a chopper plateau have been determined, an outlier resistent mean, a median, lower and upper quartile values are obtained.

The subsequent processing steps are common to the signal processing steps for staring PHT-P and PHT-C: reset interval correction (7.3.1), dark current subtraction (7.3.2) and correcttion for non-linear detector responsivity (7.3.3).

Ancillary data required:

none

7.7.2 Correction for chopper vignetting

Detailed description: section 4.5.3

The throughput of the ISO and ISOPHOT optics depends on the chopper deflection. The level of variation is of order of a few percent for the maximum chopper deflection in case of the C100 and C200 subsystems. Especially for weak sources on a strong background this phenomenon can cause spurious detections or negative signals for some pixels on the arrays.

For each average and median signal $\langle s \rangle$ the correction $c_{chop}$ is applied:

$\begin{displaymath} {\langle s_c \rangle} = c_{chop}(f, i, {\theta}_{chop}) {\times} {\langle s \rangle}~~~~~~~~~~~~~[{\rm V/s}], \end{displaymath}$

(7.69)

where is the detector pixel, is the filter, and $\theta_{chop}$ is the chopper deflection. In case of an absence of chopper vignetting $c_{chop}=~1$ .

Ancillary data required:

The correction for C100 and C200 are stored in the Cal-G files PC1VIGN and PC2VIGN, see section 14.11.

Next: 7.8 Signal Processing: PHT-S Up: 7. Data Processing Level: Previous: 7.6 Signal Processing: Chopped

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