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Data Analysis and Processing Software

--by Keith Arnaud, GSFC

Introduction

The basis of the ASCA processing and analysis software system is the use of FITS format files to store all data and calibration information. The first step in the processing is to convert the telemetry data (the so-called FRF) into FITS format. The following three classes of files are created:

science data
For most observing modes, this consists of a list of events with attributes (detector position, time, etc).

housekeeping data
All the instrument and spacecraft housekeeping stored in a compressed format.

auxiliary data
Parameters based on orbit and attitude information, e.g. the angle between the satellite pointing direction and the bright limb of the Earth. This file also includes selected housekeeping parameters in an uncompressed format for ease of use.

No information is lost in the conversion from raw telemetry data to FITS. These three classes of files will be distributed to GOs along with FITS format files that give the satellite orbit and attitude during the observation. A calibration database is available through anonymous ftp.

Further processing and analysis of these files can then be performed using a suite of programs (FTOOLS) written to manipulate FITS data structures. These FTOOLS can run either within IRAF or as stand-alone programs from the operating system. The initial processing of data at GSFC uses FTOOLS run from Unix shell scripts to perform a number of tasks that GOs probably will not want to duplicate. However, individual GOs will want to do some processing and reduction of their data. Since using the FTOOLS requires more knowledge of the detailed structure of the data files than most users will wish to acquire, there is a higher level, interactive, command-driven program (XSELECT) that performs the data reduction functions commonly required. The output from XSELECT then can be used in analysis programs such as those provided by XANADU, IRAF/PROS, or IDL.

File Descriptions

Science data
The science file contains the science data and the time intervals over which the data were accumulated. For most observation modes, the science data consists of a list of events ordered by time. Each event also is labelled by detector and sky positions, the spectrometer channel, and instrument-specific \parameters. More adventurous observers may have data in SIS FAST mode, where the Y position is a time coordinate, or GIS MPC mode, where the science data consist of a series of spectra. The standard science data file follows the design of the ROSAT Rationalized FITS file.

Housekeeping data
The housekeeping file contains the housekeeping parameters stored as a time-ordered list with each parameter only appearing when its value changes. An FTOOL can be used to extract from this file the more conventional table showing the parameter value against time. In practice, very little use has been found for the housekeeping parameters apart from the small number that are also included in the auxiliary file. The housekeeping file is distributed for completeness but most GOs will not want to keep it on disk.

Auxiliary data
The auxiliary file contains tabular data giving various parameters against time. Useful quantities included are the SAA flag, the angle between the pointing direction and the limb of the bright Earth, the angle between the pointing direction and the Sun, the pointing stability, the cut-off rigidity, and the important instrument housekeeping parameters. This file is used to select the good data times.

Initial Processing

Some initial data processing is performed at GSFC on all GO datasets. All software used is available for GOs to run at their own institutions if they wish, indeed, most of the programs used are simply FTOOLS. The processing performed is as follows:

  • The FRF telemetry file is converted to FITS format science and housekeeping files. A new science file is started each time there is a significant change of observation mode.

  • The auxiliary file is created.

  • The detector and sky positions for each event are calculated based on calibration data and the attitude history for the observation. The same FTOOL (ASCALIN) also calculates the PI channel for GIS events by correcting for the temporal and spatial gain variations.

  • A short automatic analysis sequence is run to generate "quick-look" plots.
The processing described above will be performed within a week of the arrival of the FRF data at GSFC. All intermediate products of the processing will be distributed to GOs.

Data Cleaning

GOs will have to clean their science data before going on to create the higher level analysis products such as images, light curves, and spectra. This data selection may be done using the program XSELECT and is described in detail in the GOF document An abc of ASCA Data Reduction. We summarize the important features below.

GIS data selection
The most important GIS data to reject are those falling outside the central 20 arcminute radius region. This outer ring is poorly calibrated and has a larger (and variable) background. The section of the image containing the inflight calibration source also should be excluded. Both these rejections can be performed in XSELECT using a spatial region filter to produce a new science data file containing good events. It is also a good idea to reject data close to the Earth's limb.

SIS data selection
Bright Earth contamination is the worst problem with SIS data, so a selection criterion should be applied based on the angle between the pointing direction and the bright limb of the Earth. Once the bright Earth contamination is removed then the CCD "hot" and "flickering" pixels can be identified and extracted. A command in XSELECT runs an FTOOL that performs this task by identifying pixels whose count rate is not consistent with those around them. CCD Hysteresis effects also make it advisable to reject the first few frames of data immediately after SAA and any transitions between day and night. This rejection also can be made in a simple way using XSELECT.

In summary, the program XSELECT can be used to select data based on the values of housekeeping and auxiliary data (and simple functions thereof). A number of heuristics have been determined by the ASCA team, but this work is certainly not complete and GOs are encouraged have a go themselves.

Creating Data Products

Once a clean set of science data has been created the next step is to create products for use by analysis software. The basic product is an event list. This product can be read into IRAF/PROS or XIMAGE, just like the ROSAT event files. The only caveat on this statement is that SIS data in FAINT mode do not have a single PHA but have instead nine PHA values, one for each of the pixels in the 3x3 event box. An FTOOL called FAINT converts this FAINT mode data into a form with only one PHA per event and this file then can be used. The FAINT task can be run from within XSELECT in a transparent manner. XSELECT also includes a command to accumulate images, light curves or spectra from the clean event files and then to save this accumulation to standard FITS format files. A large range of selection criteria can be used when accumulating these products. For instance, an image can be accumulated by XSELECT then displayed using SAOimage. Regions can be selected and used as filters in XSELECT to create spectra and light curves from the selected part of the image. Analogously, a light curve can be created and displayed. Time ranges can be selected using the cursor and these time ranges used in the creation of other images, light curves or spectra.

Calibration-derived Products

Scientific analysis of the data products requires a number of files that depend on instrument calibrations. These are supplied through a combination of FTOOLS and a calibration database accessible by anonymous ftp. Spectral response files are available from the calibration database for the standard situations and FTOOLS are under developement to produce the response in more complicated circumstances. An FTOOL to create exposure maps is also under developement. We expect that these FTOOLS will continue evolving for the next few months, as the instruments become better understood.

Data Analysis Software

No high-level data analysis software has been written specifically for ASCA, since several multi-mission systems already exist. The following systems are those which have been tested with ASCA data:

XANADU
The programs XSPEC, XIMAGE, and XRONOS have been used extensively by the ASCA team, and can read all the products written by XSELECT.

IRAF/PROS
The science event file can be read into PROS and imaging and timing analysis performed. At present spectral analysis of ASCA data is not available.

IDL
A number of ASCA team members have been using IDL for data analysis using the available astronomy libraries and their own tasks.

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