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Introduction
This guide describes ASCA Revision 2 data distributions.
Its purpose is to give you the information you need to
start working with your data.
Specifically, this guide will tell you:
Following these chapters there is a glossary.
This guide does not tell you how to
reduce or analyze your data. For that we refer you to the
ASCA ABC Guide to Data Reduction and other documentation.
If you have old data processed under Revision 1, this guide may still be
useful, but you may want to also consult the
Getting Started Guide for Revision 1 Data.
This guide describes data processed under Revision 2 Data Processing
(hereafter referred to as rev2). Below we highlight the major differences
between rev2 and rev1, which preceded it.
You can tell which revision was used to process your data by looking at
the processing header page included with your hardcopy.
Processing script versions 7.0.0 and higher are rev2.
Versions 5.9.9 and earlier are rev1. Versions 6.0.0 -- 6.9.9 refer to rev1+,
which is a hybrid of rev1 and rev2. Rev1+ employed the same software
as rev2 and offered some of its advantages, but mostly mimicked the
data products of rev1.
Rev1+ data processing began August 30, 1996, and rev2 processing began
June 18, 1997.
No More Raw Files
Rev1 included a large number of "raw" files, which were FITS event files
created by the ASCA telemetry reader frfread. In rev1
these were the basic starting point for data reduction.
The problem was that frfread produced a new file every time
ASCA made a slight change in mode. Since the spacecraft often switches
between modes, it was not uncommon to have a thousand of these raw files
for a single observation.
This was cumbersome for both the users and the processing team.
Rev2 does away with the raw files in favor of the unfiltered files,
which are basically concatenations of the raw files.
In FTOOLS 4.0 and later, ascascreen will read the ".unf" files
by default. With earlier versions of the FTOOLS you must specify
ascascreen -e unf.
Exposure Maps and Exposure-Corrected Images
Rev1 produced FITS format images for each screened event file in detector and
sky coordinates.
These have been replaced by exposure maps and exposure-corrected images
which combine data for all event files from like detectors (i.e. one
image for GIS2 and GIS3 and one image for SIS0 and SIS1).
For more information on these see the descriptions of the contents of the
images directory in the file descriptions
section.
Source Detection
Rev2 runs a source detection algorithm which locates the bright points in
ASCA's field of view. It then produces region filter files for each
detected source which are used to extract spectra and light curves for each source. The details of the source detection algorithm can be found in
the
processing script documentation.
Spectra
Rev2 extracts spectra for each detected source. It then generates response
matrices (RMFs and ARFs) for these spectra.
Light Curves
Rev2 extracts three kinds of light curves:
- Binned light curves, which combine data from all filtered event files
and are useful for spotting interesting features in your first look at
the data.
- Housekeeping GIS L1 count rate monitor light curves produced by the
ghkcurve FTOOL. These are useful for bright sources.
- Unbinned light curves,
which are essentially event files extracted for each
source, which can be used for fine timing analysis. Note that these
files do not carry extraction region information, so spectra should
not be extracted from them.
New Directory Structure
Rev1 distributed all files in six directories:
"telem", "raw", "unscreened, "screened", "product" and "aux".
The new data products in rev2 make it desirable to have a new
directory structure. First, the "raw" directory has been eliminated, since there
are no more raw files. Second, the "product" directory has been split into
"images", "spectra", and "lcurves" directories to better accommodate the new
data products. Finally, a separate "calib" directory is used to hold the
calibration files which were formerly in "aux".
HTML Documentation
Rev2 produces a set of HTML web pages which document the observation, the
way it was processed, and the files containing its data.
These are much easier to read than the plain ASCII files formerly
produced in rev1.
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