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Subsections


7.9 Signal Processing: PHT04 Staring Multi-Aperture
Photometry

In this section processing steps are described which apply to a specific subset of multi-aperture photometry (PHT04) observations. We refer to Müller 2000a, [41] and Müller 2000b, [42] for details. Different from all other ISOPHOT observations, the outcome of these processing steps is not a dedicated photometry product but an indication whether the source is extended or not. Since the analysis depends only on the signal variation between different apertures and not on the absolute photometric calibration, it was decided to include the processing steps in Derive_SPD.

The processing steps start at average signal per measurement level and applies only to PHT04 observations with suitable settings. Subsequently, the background subtracted relative source signals are derived whereby the background signal is determined from the largest aperture measurements. Finally, the resulting curve of growth is compared to the expected curve based on calibration observations on point sources.

The outcome of the analysis is given in SPD header of the photometry product.


7.9.1 Sorting out eligible PHT04 observations

Detailed description: None

The analysis of background subtracted relative source signals can only take place for PHT04 observations which satisfy the following criteria:

If one of these two criteria are satisfied the SPD header keyword BACKSRSS, the background subtracted relative source signal processing flag, is set to `TRUE' otherwise `FALSE'. In case of the latter the analysis is not applied. Instead a clear warning is given in the SPD header.

Ancillary data required:

none


7.9.2 Analysis of the background subtracted source signal

Detailed description: Section 4.5.1

The analysis assumes that the processing steps for staring measurements with PHT-P have already been performed up to the average signal per measurement, i.e. all processing steps of Section 7.3.

To subtract the background signal level, the background signal per aperture area $S_{back}$ and its uncertainty ${\Delta}S_{back}$ are determined from the 180$''$ and 120$''$ apertures:


\begin{displaymath} S_{back} = \frac{s(180)f_{PSF}(120)-s(120)f_{PSF}(180)} {A(180)f_{PSF}(120)-A(120)f_{PSF}(180)}~~~[{\rm V/s}], \end{displaymath} (7.72)

and uncertainty


\begin{displaymath} {\Delta}S_{back} = S_{back}\sqrt{(\frac{{\Delta}s(180)}{s(180)})^2 + (\frac{{\Delta}s(120)}{s(120)}})^2~~~[{\rm V/s}], \end{displaymath} (7.73)

where

Once the background level $S_{back}$ has been determined, the background contribution to the total signal of a measurement with a given aperture can be derived:


\begin{displaymath} s_{back}(i) = S_{back}A(i)A^*_{corr}(i)~~~[{\rm V/s}], \end{displaymath} (7.74)

and
\begin{displaymath} {\Delta}s_{back}(i) = {\Delta}S_{back}A(i)A^*_{corr}(i)~~~[{\rm V/s}], \end{displaymath} (7.75)

where $s_{back}(i)$ and ${\Delta}s_{back}(i)$ are the background signal for aperture $i$ and its corresponding uncertainty, and $A^*_{corr}(i)$ is a correction factor such that $A(i)A^*_{corr}(i)$ gives the empirical aperture area for aperture $i$ assuming that the theoretical area of the 180$''$ aperture is correct. In Table 7.2 we have listed the values of $A_{corr}$ for a given aperture diameter.


Table 7.2: Aperture correction factors $A^*_{corr}$, the value 1.0 indicates that the correction factor could not be determined empirically.
$filter$ 5$''$ 7.6$''$ 10$''$ 13.8$''$ 18$''$ 23$''$ 52$''$ 79$''$ 99$''$ 120$''$ 180$''$
P1_3.29 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
P1_3.60 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
P1_4.85 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
P1_7.30 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.65 0.80 1.00
P1_7.70 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
P1_10.0 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.50 0.65 1.00
P1_11.3 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
P1_11.5 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
P1_12.8 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.57 0.63 0.72 1.00
P1_16.0 0.90 0.90 0.90 0.75 0.75 0.80 0.62 0.68 0.70 0.78 1.00
P2_20 0.40 0.40 0.40 0.40 0.40 0.65 1.00 1.08 1.01 1.00 1.00
P2_25 0.45 0.45 0.45 0.45 0.45 0.45 0.48 0.82 0.91 0.97 1.00
P3_60 0.30 0.30 0.30 0.30 0.40 0.40 2.30 1.75 1.17 1.08 1.00
P3_100 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Several entries in Table 7.2 could not be determined from in-orbit observations, these have been set to `1.0'. If one of these entries have to be used, a warning will be issued in the SPD product header and the header keyword `FILTCORF' is set to FALSE.

For each measurement a background subtracted source signal $s_{src}(i)$ plus uncertainty is computed:


$\displaystyle s_{src}(i)$ $\textstyle =$ $\displaystyle {s(i)-s_{back}(i)}~~~[{\rm V/s}]$ (7.76)
$\displaystyle {\Delta}s_{src}(i)$ $\textstyle =$ $\displaystyle \sqrt{{\Delta}s(i)^2 + {\Delta}s_{back}(i)^2}$ (7.77)

If the source signal is is non-zero, then the background subtracted source signal normalised to the 180$''$ aperture $R(i)$ is computed:


$\displaystyle R(i)$ $\textstyle =$ $\displaystyle s_{src}(i)/s_{src}(180'')$ (7.78)
$\displaystyle {\Delta}R(i)$ $\textstyle =$ $\displaystyle R(i)\sqrt{(\frac{{\Delta}s(i)}{s(i)})^2 + (\frac{{\Delta}s_{src}(180'')}{s_{src}(180'')})^2},$ (7.79)

otherwise $R(i)$ and ${\Delta}R(i)$ are set to zero, the header keyword NORMCALC is set to FALSE, and a warning message is issued in the SPD product header.

The calculations are repeated for each aperture $i$ and the results are stored in the SPD header.

Ancillary data required:

none


7.9.3 Summary of SPD warning messages for PHT04

Detailed description: Chapter 5, Section 4.5.4

The results of the analysis of the PHT04 aperture photometry are completely stored in the SPD product header. We refer to Section 13.3.2 for a full description of the header keywords. The processing of PHT04 measurements is more constrained than other SPD processing steps. Consequently, several warning messages are issued in case the measurements fall outside the given processing constraints.

These messages are transferred to the Auto-Analysis Result product header if they occur in the SPD header.

Ancillary data required:

none

next up previous contents index
Next: 7.10 In-band Power Calibration Up: 7. Data Processing Level: Previous: 7.8 Signal Processing: PHT-S
ISO Handbook Volume IV (PHT), Version 2.0.1, SAI/1999-069/Dc