ISO Cross-Calibration Final Report Chapter 1. Introduction

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1.1 Scope

This is the last of a series of Reports generated every six months by the ISO Data Centre (IDC, hereafter) during the ISO Post-Operations Phase (POPS, hereafter) which tries to summarize the final status of ISO (cross-)calibration at the end of POPS. It includes information on the calibration progress achieved in the calibration of all four ISO instruments with special emphasis on cross-calibration aspects within the framework of the POPS Cross-Calibration Plan.

The previous four Status Reports can be found here:

First POPS Cross-Calibration Status Report (July 1999)

Second POPS Cross-Calibration Status Report (February 2000)

Third POPS Cross-Calibration Status Report (July 2000)

Fourth POPS Cross-Calibration Status Report (January 2001)

This final issue contains the Final Reports provided by each of the ISO Calibration Working Groups (see below) and the final status of internal and external Cross-Calibration of ISO with other ground-based and space facilities.

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1.2 Structure of cross-calibration activities

The global IDC strategy for approaching cross-calibration during the Post-Operations phase, as described in the ISO POPS Cross-Calibration Plan, was based on topic-oriented Working Groups, each one of them addressing one specific calibration topic.

Working Groups (WGs, hereafter) were formed by members from the IDC and from the different NDCs. They were also open to contributions from external individuals interested in actively participating in ISO calibration activities.

Eight Working Groups (WGs, hereafter) were created during Post-Operations working on:

- Beam Profiles/Pointing

- Glitches

- Line profiles

- Photometric calibration

- Polarisation

- Spectral Matching

- Transients

- Interactive Analysis

Of course, the topics addressed by each of these WGs overlapped in many cases since many cuts through the calibration issue were possible. For example, studies of Transients were fundamental to a full understanding of the instruments' photometric calibration, which was the main goal of the Photometric Calibration WG. The WG scheme provided them with a platform to exchange their knowledge and to progress with respect to their own subject.

It was the responsibility of cross-calibration manpower to maintain an overview of the work of the several Working Groups, cooperate with instrument-specific calibration manpower in efforts to understand inconsistencies of calibration relevant to more than one instrument and establish links between the different Working Groups when necessary favouring a fluid exchange of useful information.

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1.3 Final Goals: General

The objectives of cross-calibration can be summarized as follows:

• Verify that, from a more global point of view, the calibrations of the individual ISO instruments are correct, and ensure a certain level of homogeneity and consistency of the data and how they were calibrated (e.g. compatibility of models, bandpass zero-points, and of calibration philosophy and procedures).
• Compare the results of different ISO instruments and verify the errors quoted for each instrument-specific calibration. Where these results appear to be inconsistent seek to understand and to eliminate those differences.
• Compare ISO results with external data (e.g. ground based observations, KAO data, IRAS, MSX, COBE,..). Where results appear to be inconsistent seek to understand and to eliminate those differences.

The intention was to have a flexible plan/strategy with these goals as a reference materialized in a detailed schedule of cross-calibration activities for each of the Working Groups listed above.

Responsibility for cross-calibration planning was with the IDC in close interaction with the NDCs, which were responsible for instrument-specific calibration.

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1.4 Final Goals: per Working Group

1.4.1 Beam profiles/Pointing WG

• Characterization of the instruments' beam profiles.
• Address common problems (asymmetries, vignetting, ghosts,..) and their possible relation to the ISO telescope point spread function (PSF).
• Allow cross-fertilisation of models available.
• Evaluation of common deconvolution tools, to apply to extended source data, once beam profile calibration files become available.
• Support improvement of ISO pointing reconstruction in OLP.

1.4.2 Glitches WG

• Statistical analysis and characterization of glitches in the four ISO instruments. Study of the relation with the space radiation environment.
• Compare glitch statistics and glitch height distributions with model predictions and simulations.
• Enhance the quality of the reduced ISO data, improving the de-glitching and de-tailing algorithms in the pipeline and/or IA.
• Keep the knowledge on the radiation environment and its effects on the detectors, so that it can be applied to future missions.

1.4.3 Line Profiles WG

• Characterize the SWS and LWS observed instrumental profiles.
• Study the stability of these instrumental profiles as a function of time, forward and backward scans, detector, line strength and point-like versus extended sources.
• Analysis and interpretation of deviations found from line profile definition; recommendations and caveats.
• Comparison with theoretical instrumental profiles and resolution.

1.4.4 Photometric Calibration WG

• Definition of the photometric system and description of calibration sources/ models used in ISO data reduction.
• Characterization of the photometric accuracies achieved by each ISO AOT (and the corresponding photometric 'error budget').
• Make periodic 4-instrument photometric comparisons on selected sample targets and extend this comparison to calibration models and to photometric results obtained with other facilities (IRAS,MSX,COBE, ground-based,..) using OLP data products.
• ake similar comparisons on a subsample of targets using data reduced with the latest available versions of the instrument IAs and compare this with OLP photometric accuracies.
• Testing of photometric improvements as new algorithms or calibration files become available stimulating their inclusion into the corresponding IAs first and eventually into the pipeline.
• Identification and analysis of photometric calibration problems affecting OLP data products providing recommendations and caveats to the users of the ISO Legacy Archive.

1.4.5 Polarisation WG

• Characterization of the polarimetric capabilities of ISO. Angles, detection limits, wavelength dependency, internal consistency (PHT,CAM) and comparison with external data.

1.4.6 Spectral Matching WG

• Analysis of spectral matching between LWS and SWS in the overlapping region. Same with CAM-CVF and PHT-S spectra. Comparison with models.
• Identification of possible systematic errors in the measured spectral response to extended sources by comparing the observations from a variety of instruments. Effects on the determination of spectral line fluxes.
• Expansion of cross-calibration between the instruments beyond the wavelength regions which they have in common.

1.4.7 Transients WG

• Characterize transient effects in the 4 ISO instruments and determine the best correction method as a function of detector type, observing mode and characteristics of the source, and estimation of effects on photometric accuracy.
• Stimulate the development and implementation of S/W tools to detect and correct transient effects in IAs first and eventually in the pipeline.

1.4.8 Interactive Analysis WG

• Coordinate the information flow between developers of the different instruments' IAs, making sure that each team is aware of what other teams are doing and that S/W tools developed by one of the teams are made available to other teams if they consider it useful.
• Stimulate the implementation of new S/W tools in the four ISO instruments' IAs.
• Raise the profile of the various IA packages as means to enhance data reduction.
• Investigate communication channels between the IA users and IA developers in both directions.

Pedro Garcia-Lario, ISO Data Centre

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