This Guide describes how to reduce ASCA data. By reduce, we mean the preparation of data for analysis, a process which entails first screening your data and then selecting which parts of the screened data to extract as data products (i.e., spectra, light curves and images). Data analysis, i.e., the derivation of scientific results from the data products, is described in detail elsewhere, although the chapters on spectral, temporal and spatial analysis do show how to get started. This Guide assumes that the data have already been loaded, either from the original data tape, or obtained by FTP from the archive. Further details can be found in `The Getting Started Guide' at the URL
http://adfwww.gsfc.nasa.gov/asca/processing_doc/GS/getting_started.html
and other on-line help accessible from the ASCA GOF WWW area at
http://heasarc.gsfc.nasa.gov/docs/asca/ahp_proc_analysis.html.
The principal tool for reducing ASCA data is a program called XSELECT. This Guide is suitable for users who are going to do their data reduction based entirely on XSELECT, as well as users who wish to use their own or preferred software package. Accordingly, the main focus of this Guide is on
For general instructions on how to run XSELECT, users should refer instead to the XSELECT User's Guide available on the WWW at the URL
http://heasarc.gsfc.nasa.gov/docs/software/ftools/xselect/xselect.html.
However, XSELECT commands specific to ASCA are covered here in detail, while the appendix contains a set of XSELECT command sequences for accomplishing the basic reduction tasks.
Between receiving ASCA data from ISAS and sending the data to Guest Observers, the ASCA Guest Observer Facility (GOF) converts the data to FITS format (see §2.2) and performs a set of standard calibrations. Also produced at this stage are mkf files, which contain the time-histories of various parameters to which good data can be referenced, identified and screened. The name mkf comes from the program mkfilter used to produce the files. ASCA data reduction, as performed by GOs, involves the manipulation of these FITS files by a set of tasks, called FTOOLS, which can be called from XSELECT by higher level tasks.
When screening data, XSELECT consults the events data files and the accompanying mkf files to produce a list of selected good time intervals (GTI). These good time intervals are used for extracting a list of screened events (in the same FITS format as before). The final stage, when the GO is happy with the screening, is the filtering (spatial, temporal or spectral) of the events list, which is then binned and projected appropriately for extraction, as data products in their own FITS formats. The products, usually spectra, light curves, or images, can be read into the data analysis programs XSPEC, XRONOS and XIMAGE, respectively, or into any others equipped to handle these formats. Since many of the basic XSELECT commands required to clean data are repetitive, a script called ascascreen has been produced to take care of this task. ascascreen is the usual starting point in the reduction of ASCA data.
The second chapter is devoted to the RAW FITS data files (GIS and SIS) and the mkf files. Familiarity with the basic structure of these files, if not their detailed format, is important when reducing the data they contain.
The third and fourth chapters describe, respectively, those aspects of the GIS and SIS that pertain to data reduction and analysis. The special techniques required by the various instrument modes are described.
The fifth chapter describes what screening criteria have to be applied to the data before it can be analyzed.
The sixth chapter covers how to screen your data with these criteria using XSELECT, ascascreen or tkascascreen.
The seventh chapter covers the next stage of data reduction, how to filter subsets of your screened data before creating data products.
The eighth, ninth and tenth chapters describe, respectively, the extraction and formats of spectra, light curves, and images.
Further details can be found in the appendices. The appendix has a listing of acronyms used in this Guide, relevant WWW URLs, and a set of common XSELECT command sequence examples.
If you are new to ASCA data reduction, then you should start at chapter two, and work your way down through the Table of Contents.
If you are familiar with ASCA data reduction, then the fastest way to find what you need is, again, from the Table of Contents.
On the WWW, each chapter, including this one, is headed by a set of buttons which link you to the other chapters. The sections within each chapter are listed at the beginning, after the introductory section, and are repeated at the end of the chapter. For convenience, the set of chapter buttons is also repeated at the end of each chapter.
This version of the ABC Guide was written when the latest version of FTOOLS available was v3.6, and just before the release of v3.7. FTOOLS (including XSELECT) are typically released biannually, and users should check that they have the latest version. In addition to new releases of FTOOLS, ASCA users need to know about new releases of calibration files, improved screening criteria, bug reports, and other nuances in ASCA data analysis. We list below how to obtain the latest information.
The ASCA GOF home page is located at the URL
http://heasarc.gsfc.nasa.gov/docs/asca/ascagof.html
and the URL for `Processing and Data Analysis' is
http://heasarc.gsfc.nasa.gov/docs/asca/ahp_proc_analysis.html.
The latest information on software and calibration is available from the ASCA GOF home page, particularly items under the heading of `What's New', at the URL
http://heasarc.gsfc.nasa.gov/docs/asca/whatsnew.html.
In addition, both FTOOLS and XANADU have their own Web pages at the URLs
http://heasarc.gsfc.nasa.gov/docs/software/ftools/ftools_menu.html
and
http://heasarc.gsfc.nasa.gov/docs/xanadu/xanadu.html
respectively.
Information about software and calibration updates, bug fixes and enhancements is announced by means of the `ascanews' e-mail exploder. Ascanews is also used to distribute the short-term timelines. If you are not on the mailing list of this exploder, please send an e-mail to listserv@athena.gsfc.nasa.gov which should contain, in the text of the e-mail, the single line:
subscribe ascanews Eileen Collinsfor the person Eileen Collins (please substitute your own name).
The ASCA GOF publishes ASCANews about once a year, a newsletter containing articles about ASCA by members of the ASCA Team. These are available on the WWW at the URL
http://heasarc.gsfc.nasa.gov/docs/asca/newsletters.html
or in paper form. To receive future/back issues of ASCANews contact: ascahelp@athena.gsfc.nasa.gov.
The status of calibration of the ASCA instruments is documented and continuously updated in the `Calibration Uncertainties' Web page, located at the URL:
http://heasarc.gsfc.nasa.gov/docs/asca/cal_probs.html.
Users are urged to become familiar and keep up-to-date with this document in order to fully understand the limitations and systematic errors of any results that they obtain from analysing ASCA data.
If you cannot find the answer to your question in this User's Guide, please send e-mail to ascahelp@athena.gsfc.nasa.gov. The ASCA GOF scientists and support staff will try to respond within one working day.
This is also the e-mail address to which you should send software problem reports. If you do encounter a problem, please:
Please note that the xsel.log file will be overwritten if, after encountering the bug, you restart XSELECT. This means you should rename the file if you want to continue running XSELECT before sending xsel.log to ascahelp.
It is commonplace for ascahelp questions to lead to exchanges of e-mails with a particular ascahelper. However, please send new questions (that are not direct follow-ups to an on-going dialog) to ascahelp rather than to an individual. This both helps to spread the load evenly among ascahelpers, and also is the best way to ensure that a question will be answered in a timely manner.
In general, most questions on FTOOLS or XANADU that arise in the course of ASCA data reduction and analysis should be sent to ascahelp. However, some technical questions, particularly those related to distribution and installation of these software packages, may be more appropriate for ftoolshelp@athena.gsfc.nasa.gov or xanprob@athena.gsfc.nasa.gov respectively. Questions sent to a `wrong' e-mail address will be forwarded to the appropriate experts, but this may involve an additional delay in replying.