6.1 Processing overview

The ISO data processing is split into three parts, allowing the observer to choose himself from which data product he wants to start his own analysis of the data. A global overview of the processing is given in Figure 6.1.

  
Figure 6.1: Overview of the ISO data processing

The ISO processing is an automated process where the end products (Edited Raw Data, Standard Processed Data and Auto Analysis Results) are quality checked (see section 6.2. The following is a brief overview of the three steps in the processing. The algorithms used in the last two steps (Derive-SPD and Auto Analysis) are described in more detail in sections 6.3 and 6.4.

TDF first scan/Derive ERD

The telemetry data from the satellite is transmitted to the Ground Station in blocks called 'formats', each format containing the data for a 2 second period. These data are stored in the Ground Segment as so-called Telemetry Distribution Files (TDFs). The TDFs contain the raw scientific data from the Prime Instrument and satellite and instrument housekeeping information for a complete revolution. They are, thus, the prime input for the ISO data processing system. The first step in the ISO processing for an observation is to extract from a TDF the raw data relevent to that observation. This is carried out in two stages:

• 'TDF first scan' reads the TDF (and other ancilliary files) to create a set of pointers to relevent events in the observation, such as; the start and end of the observation, the start and end of LWS illuminator flashes, the start and end of mechanism (Grating or FP) scans. This is stored in the 'Compact Status History' (LSTA) file.
• Using the Compact Status History, 'Derive ERD' extracts the information for the observation into the set of Edited Raw Data (ERD) files for the observation. The data extracted includes the instrument science data, instrument housekeeping data and relevent satellite information (e.g housekkeping, pointing and orbital information). The ERD produced is a complete set of the data required to process an observation, but, as no conversion from engineering values has been made, it requires a detailed knowlwdge of the instrument to process and is therefore not recommended as a starting point.

Derive SPD (SPL)

The second stage in the ISO data processing most of the instrument specific peculiarities are removed and some basic calibrations are performed. In Figure 6.2 a schematic overview of the Derive SPD process is given.

  
Figure 6.2: Schematic overview of Derive-SPD. The names in the dashed boxes indicate auxiliary or calibration files.

The output of this process (Standard Processed Data or SPD) contains only scientific data, still in engineering units (i.e. not wavelength or flux calibrated), and in chronological order. Derive SPD processes a Target Dedicated Time (TDT) which can consist of more than one AOT. The individual data types (i.e. different AOTs or subsystems in the instrument) can be identified using LWS Compact Status History. This files is produced by TDF first scan. Derive SPD processes the raw detector read outs into photocurrent by fitting the raw data ramps. It also removes saturated points and glitches due to particle impacts. Derive SPD also processes the measurements of the internal illuminators into a calibration file that is used by Auto Analysis.

Auto Analysis (AAL)

This final stage in the ISO processing performs all the astronomical calibrations of the data. The main output of this stage is the LSAN file, containing the flux and wavelength calibrated spectrum for a single AOT. The AAL process performs the wavelength calibration, flux calibration, corrects for the spectral responsivity of the instrument, responsivity drift correction, and absolute responsivity correction. A secondary output of this process is the LSNR file, which contains the same data as the LSAN file, but without the absolute responsivity correction and responsivity drift corrections applied. Three other informational files are also produced by Auto-Analysis.

A schematic overview of the Auto Analysis process is given in Figure 6.3.

  
Figure 6.3: Overview of Auto Analysis. The files in dashed boxes are the calibration files used by AA.

The calibrations performed by Auto Analysis also include a correction for the spectral bandwidth of the instrument and a correction for the effective aperture for a point source (internal to the responsivity correction, that was measured on a point source). Auto Analysis does not include:

• averaging of repeated spectral scans
• joining together of spectra on adjacent detectors
• generation of maps for raster scans
• subtraction of the local astronomical background
• correction for the effective aperture for an extended source