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Gabriel, C. & Tuffs, R. J. 2001, in ASP Conf. Ser., Vol. 238, Astronomical Data Analysis Software and Systems X, eds. F. R. Harnden, Jr., F. A. Primini, & H. E. Payne (San Francisco: ASP), 400

New Tools for the Analysis of ISOPHOT P32 Mapping Data in PIA

C. Gabriel
ISO Data Centre, ESA Astrophysics Division, Villafranca del Castillo, P.O. Box 50727, 28080 Madrid, Spain

R. Tuffs
Max-Planck Institut für Kernphysik, Heidelberg, Germany

Abstract:

The Infrared Space Observatory AOT P32 is a dedicated mapping mode combining the raster capability of ISO with the chopping capability of the ISOPHOT instrument for obtaining images in the far infrared, maximising the spatial resolution. We present diffraction limited maps of the Crab Nebula at 60 and 100$\mu$m, as an illustration of a new tool for the P32 data analysis, integrated within the ISOPHOT Interactive Analysis (PIA).

1. Introduction

The Astronomical Observation Template (AOT) P32 was one of the observational modes (Heinrichsen, Gabriel, Richards, & Klaas 1997) defined for the instrument ISOPHOT (Lemke et al. 1996) on board the Infrared Space Observatory ( ISO; Kessler et al. 1996). It tried to solve the question of observing extended structures with large detector pixels in the far infrared by a high sky registration and redundancy. The non-linear response of the Ge:Ga detectors affects the calibration of such a mode to a large extent, making the correction of those transient effects one of the most difficult tasks in the ISOPHOT data analysis.

2. The Challenge: Mapping in the Far Infrared

Obtaining photometric maps of extended sources in the far infrared (50-200$\mu$m) is challenging because of the typical large sizes of the detector pixels involved in addition to the diffraction effects in this wavelength regime. The ISOPHOT detector arrays used in the far infrared were PHT-C100 (a 3$\times$3 array of 46$^$$\times$46$^$) and PHT-C200 (a 2$\times$2 array of 92$^$$\times$92$^$). Mapping in a finite time with a good (Nyquist) sampling therefore requires special observation techniques.

3. The Way: ISOPHOT's AOT P32

In addition to using ISOs capability of performing raster observations and the array structure of ISOPHOT's long wavelength detectors, the Astronomical Observation Template P32 used the focal plane chopper of ISOPHOT to rapidly modulate the satellite effective pointing, in steps of a third of the detector pixel pitch, on timescales ranging down to $\sim$0.15sec. The result is very good sky registration, with the oversampling and redundancy necessary for the best achievable spatial resolution.

The chopper deflection is up to $\pm90$arcsec in the Y-spacecraft direction, in 15 and 30arcsec steps respectively for the C100 and C200 detectors. The arrays are on the Y-Z spacecraft plane and the total number of positions seen by a chopper sweep is therefore 13 for C100 and 7 for C200. Every sky position is therefore registered three times in the Y-spacecraft direction within a chopper cycle by a detector pixel and its neighbour pixels. The raster steps in Y and Z directions (with different user defined oversampling factors) also ensure a uniform coverage and high redundancy.

4. The Main Problem: Detector Transients

The ISOPHOT C200 (Ge:Ga stressed) and especially the C100 (Ge:Ga) photoconductor detectors have a complex non-linear response as a function of illumination history on timescales of $\sim$0.1-100sec, depending on the absolute flux level as well as the flux changes involved (Acosta, Gabriel, & Castañeda 2000). The P32 observation mode, with its high frequency flux modulation, as described above, is in principle always in a non-stabilized state. Under- and overshooting signal effects, caused by a short term ``hook'' response, complicate the calibration of observations performed in this mode.

5. The Solution: P32_Tools Package

This package, originally developed at Max-Planck Institut für Kernphysik by one of us, is a collection of IDL routines, which:

A complete set of diagnostic plots, images, and text information is produced, which enables the user to assess the quality and reliability of the data treatment.

6. Detector Model

The signal to every time is given by the sum of two components: $S = S_1 + S_2$ with a slow $S_1$ and a fast $S_2$ part:

\begin{displaymath}
S_1 = (1 - \beta_1) S_\infty (1 - e^{-t/\tau_1}) + S_{01} e^{-t/\tau_1}
\end{displaymath} (1)


\begin{displaymath}
S_2 = \beta_2 S_\infty (1 - e^{-t/\tau_2}) + S_{02} e^{-t/\tau_2}
\end{displaymath} (2)

The prediction of a change in photocurrent after a flux change is known as the ``jump condition'':
\begin{displaymath}
S_{01} = \beta_1 (S_\infty - S_{\infty p}) + S_{1p}
S_{02} = S_{2p}
\end{displaymath} (3)

with $p$ as the index for the previous flux level.

7. Determination of Parameters

The ``default'' parameters determination was performed using

8. The Results

P32_Tools is already giving very good results, both for point sources and extended objects, although it is still in a testing and enhancement phase.

The nature of the problem, together with the fact that disturbances from cosmic ray hits are difficult to handle in an automatic way in this observation mode, require highly interactive work on the data.

9. The Integration within PIA

P32_Tools has been fully integrated within PIA (Gabriel, Acosta-Pulido, Heinrichsen, Morris, & Tai 1997; Gabriel & Acosta-Pulido 1999) for a better and easier access to all the capabilities already given. This also allowed us to make use of PIA's graphical data handling. Graphical menus for driving processing and parameter handling, display of results and information were specially developed, but wide use is made of already existent PIA tools.

10. An Example, FIR images of the Crab Nebula

P32 observations of the Crab Nebula have been preliminarily reduced with the P32_Tools package. These observations were performed with the aim of trying to understand the origin of the ``InfraRed Bump'' discovered by IRAS. A comparison of the obtained images at 60 $\mu$m and 100 $\mu$m using the default PIA processing (Figure 1), and the ones obtained using the P32_Tools package (Figure 2) show a remarkable improvement in angular resolution, indicating that the ``Infrared Bump'' probably arises from compact line emitting structure superposed on the smooth synchrotron emission.

There is a good correlation between the drift corrected P32 maps and an [O III]$\lambda$5007 emission map of the Crab Nebula taken with the Goddard Fabry-Perot Imager (Lawrence et al. 1995). The P32 maps are largely tracing oxygen. Prominent oxygen fine structure emission lines are present in the nebula at 52$\mu$m ([O III]), 63$\mu$m ([O I]) and 88$\mu$m ([O III]), as observed by the spectra taken with the ISO-LWS spectrometer in both wavelength regions covered by the ISOPHOT C100 60$\mu$m and 100$\mu$m broadband filters.

Figure 1: Crab Nebula maps obtained by PIA plain processing
\begin{figure}
\epsfclipon
\plottwo{P1-11a.ps}{P1-11b.ps}
\epsfclipoff
\end{figure}

Figure 2: Crab Nebula maps obtained by P32_Tools processing
\begin{figure}
\epsfclipon
\plottwo{P1-11c.ps}{P1-11d.ps}
\epsfclipoff
\end{figure}

References

Acosta-Pulido, J. A., Gabriel, C., & Castañeda, H. 2000, Experimental Astronomy 10, Kluwer Academic Publishers, 333-346

Gabriel, C., Acosta-Pulido, J., Heinrichsen, I., Morris, H., & Tai, W.-M. 1997, in ASP Conf. Ser., Vol. 125, Astronomical Data Analysis Software and Systems VI, ed. G. Hunt & H. E. Payne (San Francisco: ASP), 108

Gabriel, C. & Acosta-Pulido, J., 1999, ESA SP-427, 73

Heinrichsen, I., Gabriel, C., Richards, P., & Klaas, U. 1997, ESA SP-401

Kessler, M. F., et al. 1996, A&A, 315, L27

Lawrence, S. S., et al. 1995, AJ, 109, 2635

Lemke, D., et al. 1996, A&A, 315, L64


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