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7.4 Signal Processing: Staring PHT-S

The signal processing of PHT-S staring mode data starts at the level where one signal in V/s has been derived for each integration ramp. In the following processing steps, the signal records of ramps that are completely saturated are skipped.

7.4.1 Dark signal subtraction

Detailed description: Section 4.2.6

This step is similar to the step described in Section 7.3.2.

Ancillary data required:

The dark signal data are stored in Cal-G file PSDARK (128 pixels of PHT-S), see Sections 14.7 in particular 14.7.1.

7.4.2 Transient correction and calibration of signals

Detailed description: Section 5.2.6

For each pixel $i$ the signals $s(i,t)$ are directly converted to flux densities $F^t_{\nu}(i)$ using a transfer function:

$\displaystyle F^t_{\nu}(i)$ $\textstyle =$ $\displaystyle \varphi(i,s(i,t),t)~~~~[Jy],$ (7.14)
$\displaystyle {\Delta}F^t_{\nu}(i)$ $\textstyle =$ $\displaystyle \frac{{\Delta}s(i,t)}{s(i,t)}F^t_{\nu}(i)~~~~[Jy],$ (7.15)

The function $\varphi(i,s(i,t),t)$ has been implemented as a table with logarithmic signal and time spacings. The value is obtained from linear interpolaton. We give the flux a superscript $t$ to indicate that the flux itself is supposed to be constant and has no time dependence. Time $t$ along a measurement is computed from

t = t_{mid} - t_0~~~~[s],
\end{displaymath} (7.16)

where $t_{mid}$ is the time at the midpoint of a given ramp and $t_0$ the time of the first readout of a given measurement. Signals with $t\,<\,1\,$s are not processed.

This transformation yields transient corrected flux densities $F^t_{\nu}(i)$ along the measurement. Due to the transient behaviour of the PHT-S detectors, the transient correction is larger at the beginning of a measurement where the signal is less stabilised than after a long period of integration. The stabilisation time depends on the absolute brightness of the source.

To include this information, a weight is given to each $F^t_{\nu}(i)$ based on the degree of transient correction:

w'(i,t) = g(i,s(i,t),t).
\end{displaymath} (7.17)

Signals obtained for times $t\,<$ 1 s get $w'(i,t)=0$.

Ancillary data required:

The transfer function $\varphi(i,s,t)$ and corresponding weight table $g(i,s,t)$ are stored in Cal-G files PSDYNAMIC and PSDYNWT, see Section 14.9 for detailed descriptions.

7.4.3 Computation of the weighted mean flux

Detailed description: none

The weighted mean of the array of flux densities $F^t_{\nu}(i)$ is computed using weights obtained from the combination of the weighting factor $w(i,t)$ and the statistical uncertainty ${\Delta}F^t_{\nu}(i)$:

w(i,t) = \frac {w'(i,t)}{{\Delta}^2F^t_{\nu}(i)},
\end{displaymath} (7.18)

and hence,

$\displaystyle \langle F_{\nu}(i) \rangle$ $\textstyle =$ $\displaystyle \frac {\sum_{t>1s} F^t_{\nu}(i)w(i,t)}{\sum_t w(i,t)}~~~[{\rm Jy}],$ (7.19)
$\displaystyle \Delta \langle F_{\nu}(i) \rangle$ $\textstyle =$ $\displaystyle \sqrt{\frac{\sum_{1}^{N}w(i,t) \times
( \langle F_{\nu}(i) \rangle - F^t_{\nu}(i))^2}
{(N-1)\sum_{1}^{N}w(i,t) }}~~~~[{\rm Jy}].$ (7.20)

The calibration for PHT-S is most valid for detector temperatures of $2.7<T_{det}<3.1~$K. Since there were occasions during operations where PHT-S operated outside this temperature interval, a warning will be written to the SPD header in case the temperatures as given by the ERD keywords PSERTEM1 and PSERTEM2 are outside the interval.

If $F_{\nu}(i)$ for pixel $i$ is outside the flux density range then a warning message will be written to the SPD header that the flux for pixel $i$ is out of the calibration range.

Ancillary data required:

The flux limits for which the calibration is reliable are stored in the header of the Cal-G file PSDYNAMIC (Section 14.9) under keywords FLXiL and FLXiU (lower and upper bound, resp.). where $i$ is the pixel number.

next up previous contents index
Next: 7.5 Signal Processing: Chopped Up: 7. Data Processing Level: Previous: 7.3 Signal Processing: Staring
ISO Handbook Volume IV (PHT), Version 2.0.1, SAI/1999-069/Dc