3.3 XAS files

We give here an user's overview of XAS data files, while programming details will be given elsewhere.

Overview

There are the following basic kind of files used by XAS :

the FOT telemetry files, i.e. the semi-raw (mostly binary) data in mission-dependent format
the reduced data files, which are described in detail below, and are produced (in mission-independent format) by the user using XAS programs
ancillary user files, which are usually (tiny) ASCII files like time windows or log files produced by some programs
the calibration files used by the software and distributed with it.

XAS data files

XAS data files are binary files using native internal representation of the specific operating system, and using a mission-independent format. All families of XAS files, listed below, share a common organization consisting of :

a mini-header in front of the file
a data area with either image (n rows of m pixels) or tabular (n rows with p logical columns of different widths) organization
an extensible header at the end of the file, which is organized as a sequence of keywords

The following are the types of XAS files :

images (of type .image) containing counts or other parameters as a function of spatial position
pseudo-images (equally of type .image) containing counts or other parameters as a function of any two (non-spatial) coordinates
response matrices (of type .matrix) containing instrumental response as a function of energy and channel. They are also in image format. An associated histogram (of type .histo) carries the reference energy grid, in form of an unidimensional image.
all the above files are in image format, there are as many records as image rows, and each row contains the right number of REAL*4 pixels (the software can support also INTEGER*2 images but their use is discouraged).
all files which follows are instead in tabular format.
spectra (of type .spectrum) containing a count/s or count histogram as a function of energy or of another "channelized" parameter. These files have (usually) four (depth one) columns :
1. the lower channel boundary (in channels or in keV)
2. the upper channel boundary (in channels or in keV)
3. the data value (in cts/s or cts)
4. its error (in cts/s or cts)
time profiles (of type .time) containing any quantity as a function of time. These files may have up to 7 columns, although usually not all columns are present :
1. the start time of the bin (in double precision, elapsed seconds since midnight of the first day of the observation). This is present virtually in all cases.
2. the bin size (present only if not constant)
3. the deadtime (present in a few cases, currently not yet used)
4. the data value (in cts/s, cts, temperature, voltage or other units), which may also have a depth greater than one
5. its error
6. an optional second data column
7. and its optional error
photon lists (of type .photon) containing a list of events. For each events one may have a number of columns containing information like X, Y, energy, time etc. according to the particular case.
generic tabular files are also possible

Where do XAS data files reside ?

XAS uses its own system-independent file naming convention (VOS names), which is translated into system dependent names by appropriate routines. This is in practice not a concern for the general user, and in particular for the Unix user, since VOS names virtually (sic!) coincide with Unix names. A name may be of the form :

/dir/dir/.../name.type
dir/dir/.../name.type
name.type
name

The type (usually one of the standard ones given above) is often omitted and assigned automatically by the programs, or by context.

Also the path (either absolute or relative) is usually omitted, and is instead built by programs.

It is important to remember that, irrespective of what is the current working directory, XAS programs will look for files (without an absolute path) or create them in specific directories previously decided by the user, and namely

in a "FOT directory" for FOT telemetry files
in a "data directory" for all reduced data products
in a "print directory" for ancillary output (log files etc.)
in a system (or user-specified) directory for calibration files

Relative paths are relative to this particular directory !

The full path of each of these directories is constructed on the basis of several environment variables, in order to allow each user the maximum flexibility. We give here examples for the "data directory", while the cases of the "FOT" and "print" directory are similar, replacing fotdir or printdir to datadir, and optionally replacing fotorder or printorder to order (if this is not done one obtains the same hierarchy).

The full path for the "data directory" is constructed of up to 5 parts (each part being in turn a path), e.g.

/part1/part2/part3/part4/part5

part1 is always a logical root, the value of rootdir
The other four parts (of which three can be omitted) are controlled by the four variables datadir, target, date, instrument, and occur in the order specified by variable order

Compare the following examples :

Flat arrangement

xasset rootdir /home/me
xasset datadir mydata

Default order assumed, data files are in /home/me/mydata

Complex arrangement

xasset rootdir /home/me
xasset datadir mydata
xasset target Crab
xasset date Sep06
xasset instrument mecs
xasset order ctdi

Data files are in /home/me/mydata/Crab/Sep06/mecs

Changing order

If one instead does e.g. xasset order cidt data files go in /home/me/mydata/mecs/Sep06/Crab
All permutations are possible.

One can also omit a part, e.g. if one does xasset order cd data files go in /home/me/mydata/Sep06
If one xasunset order the default path /home/me/mydata is restored.

Using different orders

Let us assume that one wants to do a standard reduction for a particular object, or observation, or dataset called pinco, then the same reduction for another dataset called panco and finally combine the results :

xasset rootdir /home/me
xasset datadir mydata
xasset target pinco
xasset order ct

Files for first data set are created as /home/me/mydata/pinco/name.type

xasset target panco

Files for other data set are created as /home/me/mydata/panco/name.type

xasunset order

The system will look for data files in /home/me/mydata, therefore one may refer to the files of either data sets with relative paths like pinco/name.type or panco/name.type and call merged files just as name.type. The same result will occur doing xasunset target

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