SAS startup thread

Let us assume we are working in /home/user/my_work and have our XMM observation data files in /home/user/sasdata/<my_obs>, where <my_obs> identifies a specific XMM observation. We do not need to have our XMM data in the working directory.


To start working with SAS, we need to answer three questions:

1) Where is the SAS software in my system?

2) Where are stored all the Calibration files?

3) Where are my XMM observation data?

To answer the first question, we need to define two environment variables, SAS_DIR and SAS_PATH. To do it you need to know where in your system you (or somebody) have installed SAS. Let us assume that your favorite SAS release is in

/some_dir/xmmsas_20040318_1831

so we could define SAS_DIR as follows

setenv SAS_DIR /some_dir/xmmsas_20040318_1831

and the SAS_PATH variable as follows

setenv SAS_PATH $SAS_DIR

that is, the same than SAS_DIR. In these definitions we have used the proper syntax for the C-Shell (tcsh as well). Finally:

source $SAS_DIR/sas-setup.csh

For Bourne or Bash shells, the proper syntax is

export SAS_DIR=/some_dir/xmmsas_20040318_1831

export SAS_PATH=$SAS_DIR

. $SAS_DIR/sas-setup.sh

For the remaining of this text we will continue using the C-Shell/tcsh syntax.

To answer the second question we need to know where in the system are stored all the calibration data or Current Calibration Files (CCF). Usually this is a common place for everybody since they will be of utility for all users of XMM data. Let us assume this is /ccf/pub. Then we need to define SAS_CCFPATH as follows

setenv SAS_CCFPATH /ccf/pub

Later, we will need to define which calibration files must be used during the data analysis and reduction. However, there is something more we have to do, before we reach this point.

The third question seems at first sight the easiest to answer and in fact we have already answered it when we have set our XMM data directory /home/user/sasdata/<my_obs>. But the complete answer needs a little of work.

XMM-Newton data are provided in the form of a bundle of files called Observation Data Files (ODF). We call such a bundle of files, an ODF. All the files of an ODF contain information for a single XMM-Newton observation, concerning the different science exposures, which instrument were used, the exposure modes, the exposure filters, etc.

In order to work with our ODF we need first to identify which calibration files (CCF) we need to use. That is done by creating a Calibration Index File (CIF) which will tell our reduction processes which CCFs we need to use out of all available in SAS_CCFPATH, for the specific date of our observation data.

Let us start setting the SAS_ODF variable as

setenv SAS_ODF /home/user/sasdata/<my_obs>

pointing to the place where we have our data. To create the CIF, we need to run the first pure SAS command,

cifbuild

without options. The output will be the file ccf.cif, in our working directory /home/user/my_work. The ccf.cif is a FITS file which can be viewed with fv (FITSIO), containing the references to all the required CCFs, without specifying their directory paths (withccfpath=no, default option).

Once the CIF is created we can set SAS_CCF as follows

setenv SAS_CCF ccf.cif

The last step is to update the summary file coming with the ODF bundle (*SUM.ASC) with all the observational information required to process the data. This is done with another SAS command, odfingest, which will take such information from all the ODF components and produce a 7.0.0 detailed summary file *SUM.SAS. So then,

odfingest

and now we re-define the SAS_ODF variable to point to the 7.0.0 summary file, as follows

setenv SAS_ODF `ls -1 *SUM.SAS`

The summary file has ASCII format. It is interesting to view this file with any text editor and to notice the value assigned for the PATH tag at the beginning of the file. If everything has been done as indicated in this document, such value should be

PATH /home/user/sasdata/<my_obs>