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7.2 Ramp Processing

7.2.1 Discarding destructive read-outs

Detailed description: section 3.2.2

The destructive read-out is the last read-out of a ramp prior to the reset of the read-out electronics and is in general disturbed by the pulse pattern of the electronics. Destructive read-outs usually do not follow the highly linear relationship between time and read-out voltage as is the case for the non-destructive read-outs.

In Derive_SPD the destructive read-out is assumed unreliable and is discarded.

Ancillary data required:

None

7.2.2 Discarding disturbed non-destructive read-outs

Detailed description: section 4.3.1

At the beginning of an integration ramp immediately after a reset, the first non-destructive read-out is unreliable. In case of a large number of non-destructive read-outs per ramp, more than one read-out can be disturbed.

In Derive_SPD the first fraction x of an integration ramp is discarded, where x depends on the number of non-destructive read-outs.

Ancillary data required:

The value of x as a function of number of non-destructive read-outs. The table is stored in Cal G file PSELNDR, see section 13.4.

7.2.3 Discarding data with OTF or OPF ``off''

Detailed description: none

During a sky measurement the source can drift out of its nominal pointing due to telescope pointing instabilities. When this happens the on-target flag (OTF) is automatically set to ``off''.

Similarly, if the chopper is not correctly positioned, a chopper on-position flag (OPF) is also set to ``off''.

Initially, until revolution 524, the OTF is set to off when the actual pointing drifts out of a cone with radius of 10'' around the intended pointing. Due to the good pointing performance of ISO the OTF cone has been reduced to 2'' for ISOPHOT as of revolution 524.

The OTF and OPF flags are contained in the read-out records of the ERD products. If either of these flags is set to off then the current integration ramp is abandoned and excluded from further processing. A record is kept per pixel per measurement of the number of integration ramps that are rejected for this reason.

For raster scans, it is assumed that the spacecraft moves off target to the next raster point when one of the two raster point identifiers changes. To avoid the inclusion of off-target data while the spacecraft is slewing and repositioning all data are rejected for a fixed (1 sec) period after the change in the raster point identifier. After this period the OTF is checked: if the OTF is off then data are rejected until the OTF is on again.

Ancillary data required:

None

7.2.4 Sorting out the different data-types

Detailed description: none

The different measurements that were performed as part of an AOT (like sky, FCS, and dark measurements) need to be identified and separated before they can be processed. For a given TDT all output data records of the same detector and measurement type will be stored inside the same SPD product.

Different measurements are sorted out by performing a cross-correlation between the ERD record and the corresponding CSH record with the same time key. The following flags are then inserted at the beginning of each data record in Derive_SPD to identify the type of measurement:

Raster point ID
Aperture ID
Chopper position
Filter ID
Polarizer ID

Note that the filter ID also determines the detector.

Ancillary data required:

None

7.2.5 Conversion of digitized numbers to CRE output voltages

Detailed description: section 3.2.3

The detector interface electronics (DIE) in the PHT external electronics unit subtracts a commandable offset from the CRE voltage and amplifies the difference with a commandable gain factor before the analogue to digital conversion. The CRE output voltage must be reconstructed from the digitized numbers (DN).

There are two DIE chains. Each detector subsystem has a default connection to one of them (see 3.2.3). A change in default connection is indicated by the ``cross status'' flag in the CSH (actually never used during the mission). The CSH also contains the value of the selected gain of the differential amplifier in the DIE electronics. The CRE output voltage and DNs are related by the following formula for a given DIE electronics chain and selected gain:

$\begin{displaymath} {\rm CRE [V] = (D_{o}-DN-GAIN_{2}\times (DI-2048))\times \frac{20.0 [V]}{4096\times GAIN_{1}} + U_{off} [V]}\end{displaymath}$ (7.1)

where,

: D ${\rm _{o}}$ = fixed offset
: GAIN₂ = offset gain
: DI = selected OFFSET data word (0...4095)
: GAIN₁ = signal gain
: U $_{{\rm off}}$ = GAIN₁ dependent offset

Ancillary data required:

1.: The selected OFFSET from the measurement record in the CSH.
2.: The DIE transfer function table which is a Cal G files PDIE1TRANS and PDIE2TRANS, see 13.3.

7.2.6 Discarding saturated read-outs

Detailed description: section 4.3.5

If the source is significantly brighter than anticipated, integration ramps can saturate, i.e. the CRE output voltage has reached its maximum value prior to reset. The read-outs taken during times of detector saturation are useless and must be discarded. Saturation also occurs in case the responsivity of the detector exceeds the nominal value by a large factor. In practice this frequently happened towards the end of the science window where both FCS and sky measurement got saturated due to high responsivity. For detectors P3, C100 and C200 many FCS measurements taken during revolutions 94-191 were saturated due to over-illumination by changed FCS behaviour.

Threshold voltages for each detector are listed in section 4.3.5. As soon as the threshold voltage has been crossed, all following read-outs up to the end of the ramp are discarded.

Ancillary data required:

In the current OLP version it is assumed for all detectors that saturation is reached for CRE output voltages greater than 1.0 V.

7.2.7 Correction for non-linearities of the integration ramps

Detailed description: sections 4.3.2 and 4.3.4

Integration ramps are not perfectly straight but show deviations from linearity. The non-linearity is caused by two independent effects:

non-linearities due to the CRE (section 4.3.2)
downward curving due to de-biasing. Only low-bias germanium detectors P3, C100 and in particular C200 show this effect (section 4.3.4).

The integration ramps are corrected for non-linearities before signals are derived.

In Derive_SPD it is assumed that the corrections are only a function of the absolute value of the CRE output voltage. In addition it is assumed that non-linearities due to both CRE and debiasing (as is the case for the germanium detectors P3, C100 and C200) can be corrected using one function which only depends on the CRE output voltage.

The ramps are linearized using tables which contain for a given CRE output voltage the correction voltage to be subtracted. In the tables the sampling of the CRE voltages is sufficiently fine to allow searching for the table value closest to the measured CRE voltage and using its corresponding correction.

Ancillary data required:

The CRE Transfer Function Table per detector pixel and clock frequency are stored in Cal G files PC1CRELIN, PC2CRELIN, and PPCRELIN (section 13.5). There is no ramp linearisation for PHT-S measurements.

7.2.8 Ramp deglitching

Detailed description: section 4.4

A radiation hit or glitch shows up as a voltage increase between read-outs which is larger than the increase expected from the source photo-current. In case the hit is very energetic, the voltage increase can be so high that it saturates a ramp or even causes a responsivity transient. In SPD two different deglitching algorithms are applied: the first algorithm is described in this section and is based on an analysis of the read-outs per ramp; the second one checks for any outliers in the signals of a given chopper plateau and is described in section 7.3.4.

For ramp deglitching, an iterative algorithm has been implemented which identifies and removes excessive increases in CRE output voltage. The algorithm has the following settings:

N_it = 4 number of iterations

$N_{\sigma}$ = 4.5 minimum number of standard deviations for glitch detection

N_min = 25 minimum number of read-outs/ramp for application of algorithm

For integration ramps with less than N_min read-outs no ramp deglitching is applied and the deglitching can only take place at signal level (section 7.3.4). For an integration ramp consisting of N > N_min read-outs with voltages V(1), V(2),...V(N), taken at times t(1), t(2),...t(N), the slope between each consecutive read-out is calculated:

$\begin{displaymath} S(i) = \frac {V(i+1) - V(i)} {t(i+1) - t(i)}~~~~~~~~~~~~~V/s.\end{displaymath}$ (7.2)

In addition, the mean, S, and standard deviation, $\sigma$ (S), of these slopes are determined. In case,

$\begin{displaymath} S(j) \gt S + N_\sigma*\sigma(S)~~~~~~~~~~~~~~~~~V/s,\end{displaymath}$ (7.3)

then the voltages j+1 to N are corrected conform to the mean slope,

$\begin{displaymath} V_{corrected}(i+1) = (V(i+1)-V(i)) - (V(j+1)-V(j)) + {\Delta}V~~~~V, ~~~i=j,...,N-1\end{displaymath}$ (7.4)

where ${\Delta}V$ is the mean voltage difference for all read-outs. This procedure is repeated N_it times to remove successively smaller glitches.

Ancillary data required:

None

7.2.9 Extraction of the signals and their uncertainties

Detailed description: section 3.2.2

The slope of each ramp s_ph (or signal in V/s) is proportional to the photo-current which is a measure for the number of photons falling on the detector per unit time. In the SPD processing all valid read-outs between two reset intervals are used to fit a first order polynomial. The uncertainty ${\Delta}s_{ph}$ is the rms of the fit residuals.

Ancillary data required:

None

Next: 7.3 Signal Processing Up: 7 Data Processing Level: Previous: 7.1 Overview

ISOPHOT Data Users Manual, Version 4.1, SAI/95-220/Dc

N_it	=	4	number of iterations
$N_{\sigma}$	=	4.5	minimum number of standard deviations for glitch detection
N_min	=	25	minimum number of read-outs/ramp for application of algorithm