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Astronomical Data Analysis Software and Systems X ASP Conference Series, Vol. 238, 2001 F. R. Harnden Jr., F. A. Primini, and H. E. Payne, eds.

New To ols for the Analysis of ISOPHOT P32 Mapping Data in PIA
C. Gabriel ISO Data Centre, ESA Astrophysics Division, Vil lafranca del Castil lo, P.O. Box 50727, 28080 Madrid, Spain R. Tuffs Max-Planck Institut fur Kernphysik, Heidelberg, Germany Ё 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 µm, 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 (50­200 µm) 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 3в3 array of 46 в46 ) and PHT-C200 (a 2в2 array of 92 в92 ). Mapping in a finite time with a good (Nyquist) sampling therefore requires special observation techniques.

400 c Copyright 2001 Astronomical Society of the Pacific. All rights reserved.


New Tools for ISOPHOT P32 Mapping Data in PIA 3. The Way: ISOPHOT's AOT P32

401

In addition to using ISO s 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 0.15 sec. 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 ±90 arcsec in the Y-spacecraft direction, in 15 and 30 arcsec 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 0.1­100 sec, depending on the absolute flux level as well as the flux changes involved (Acosta, Gabriel, & Castaneda ~ 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 fur Kernphysik by Ё one of us, is a collection of IDL routines, which: · solves the non-linear optimisation problem for the set of sky brightnesses illuminating the detector on the grid of sky sampling (arbitrary source morphology), · optionally solves for detector starting state, · optionally solves for detector model parameters (through self-calibration), which by default are predetermined. 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. 5.1. Detector Model
2

The signal to every time is given by the sum of two components: S = S1 + S with a slow S1 and a fast S2 part: S1 = (1 - 1 )S (1 - e-t/1 )+ S01 e-t/
1

(1)


402

Gabriel and Tuffs S2 = 2 S (1 - e-t/2 )+ S02 e-t/
2

(2)

The prediction of a change in photocurrent after a flux change is known as the "jump condition": S
01

= 1 (S



-S

p

)+ S1p S

02

=S

2p

(3)

with p as the index for the previous flux level. 5.2. Determination of Parameters

The "default" parameters determination was performed using · starting exposures of internal calibrators for slow components · standard celestial calibration sources for fast components, exploiting the redundancy · illumination dependency in parametrisation of s and s. 6. The Results

P32 Tools is already giving very good results, both for point sources and extended ob jects, 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. 7. 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. 8. 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 µmand 100 µmusing 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]5007 emission map of the Crab Nebula taken with the Goddard FabryPerot 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 µm ([O III]), 63 µm ([O I]) and 88 µm ([O III]), as observed by the spectra


New Tools for ISOPHOT P32 Mapping Data in PIA

403

Figure 1.

Crab Nebula maps obtained by PIA plain processing

Figure 2.

Crab Nebula maps obtained by P32 Tools processing

taken with the ISO-LWS spectrometer in both wavelength regions covered by the ISOPHOT C100 60 µm and 100 µm broadband filters. References Acosta-Pulido, J. A., Gabriel, C., & Castaneda, H. 2000, Experimental Astron~ omy 10, Kluwer Academic Publishers, 333­346 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