Introduction to SAS
By the way: what is all that about?
Why do I need it?
XMM-Newton data are available in two formats:
- Observation Data Files (ODF), i.e. reformatted telemetry in FITS format. They contain un-calibrated quantities on a chip-by-chip or science window basis for the X-ray cameras and Optical Monitor, respectively
- Pipeline Processing System (PPS) products, a collection of validated, top-level scientific products including event and source lists, multiwavelength images and cross-correlation products, generated at the Survey Science Center (SSC).
Even if one starts the analysis of an XMM-Newton datasets with the PPS products, the SAS is necessary to extract standard (spectra, light curves) and/or customized science products. Moreover, SAS allows the users to reproduce the reduction pipelines run to get the PPS products (or, at least, a substantial part of them) from the ODFs files. This step is advisable, whenever substantial changes in the software and/or instrument calibrations occurred from the time when the ODF were processed by the SSC.
... so all my FTOOLS/LHEASOFT or SPEX knowledge is not useful?
Don't jump too early to this pessimistic conclusion! Whenever relevant, XMM-Newton data files are FITS (or compressed FITS). When appropriate, data files produced by the SAS tasks (e.g.: images, spectra, time series) have been designed to be OGIP-compliant. They provide therefore full compatibility with the most commonly used analysis packages, such as: FTOOLS (FITS file manipulation), XANADU (timing and spectral analysis), SPEX (spectral analysis) SAOIMAGE, SAOTNG, DS9 (image display and analysis). However, you do not need to know the FTOOLS/LHEASOFT package to work with XMM-Newton data. SAS includes a powerful and extensive suite of FITS file manipulation packages, based on the Data Access Layer library.
On the other hand, SAS does not include tools for spectral, timing or image analysis (although being able to generate all files - spectra, light curve, response matrices, exposure maps - which are required for the scientific analysis).
"Bird's eye" view of the SAS capabilities
In extreme summary, you absolutely need the SAS if you want to:
- reduce XMM-Newton data. A set of "reduction metatasks" is available in the SAS to transform the ODF into different levels of scientific products, directly usable for science analysis:
- EPIC: epproc and emproc produce calibrated and concatenated (i.e.: 1 single file including events from all the detector chips) event lists for the EPIC cameras. Similarly, emchain and epchain mirror the PPS SSC pipeline (a comparison between the chain and the proc tasks is discussed here).
- RGS: the SAS reduction pipeline, rgsproc generates, alongside with calibrated and concatenated event lists, background-subtracted spectra and response matrices corresponding to a given sky coordinate or source in an EPIC source list
- OM: the SAS reduction pipelines omichain and omfchain (applicable to Image and Fast Mode windows, respectively) generate flat-fielded, and geometrically distortion corrected sky images, source lists and time series light curves
- create, maintain or update your set of XMM-Newton calibration data, or build a Calibration Index File (CIF), via the SAS task cifbuild
- create scientific products according to your own taste and (data screening) criteria. The scientific product extractor in the SAS realm is evselect, whith its GUI interface xmmselect. evselect is able to extract spectra, images and pseudo-images, light curves, histograms and filtered event lists. Data screening criteria can be applied "on-the-fly", using the powerful capabilities of the selectlib library
- generate the instrument spectral transfer functions ("response matrices" in the X-ray astronomy jargon) to perform quantitative spectral analysis of X-ray cameras data, via the arfgen plus rmfgen, and rgsrmfgen tasks for EPIC and RGS, respectively
- generate EPIC exposure maps, through the eexpmap task
- estimate the amount of pile-up affecting your favorite source in an EPIC field-of-view, through the epatplot task
SAS is very useful for you if you want to:
- browse the content of your ODF, through the GUI-based odfbrowser
- manipulate XMM-Newton data files, through the capabilities offered by the Data Access Layer library
- visualize the XMM-Newton calibration files, through the GUI-driven calview
- generate Good Time Interval (GTI) files, applicable to later data screening for the optimization of scientific product extraction, based on housekeeping (hkgtigen) or scientific (tabgtigen) light curves
- merge EPIC event lists
- perform source detection on EPIC fields, via the source detection task edetect_chain
- mosaic images from different exposures and/or instruments, through the emosaic tasks
- perform barycentric corrections (earthbarycen)
You do not need SAS if you want to:
- perform timing or spectral science analysis of calibrated scientific products
Again, please refer to the on-line documentation for a full view of the whole SAS capabilities. The SAS GUI interface (launched by typing sas on the command line after a successful installation) includes the complete list of all the available SAS tasks, with a short description of their functionality.
A schematic view of the data analysis process for XMM-Newton data, and of the role the SAS plays in it is given here.
You are probably now curious enough, and want to start playing with the SAS. The SAS start-up may be interesting for you. Other recipes to accomplish specific data reduction tasks with the SAS are available in the threads page.
If you have installed the config-doc tar archive (cf. the SAS installation page), you have a copy of the documentation on your disk. You can type the command sashelp to reach the documentation entry page. Note if you already have a running Netscape session, the disk where the SAS is installed must be visible from the machine where Netscape was started.
You may work with any other html browser like for example "mozilla" by defining the SAS_BROWSER environment variable as pointing to the exact place where this binary is placed.
Should sashelp not work, or shouldn't you have Netscape installed, start any browser on $SAS_DIR/README.html.
For each SAS task both a PostScript and an HTML version of the documentation are available. For those interested, the ChangeLogs of each SAS package, with a low level description of all the changes to the code (since the project's inception, roughly October 1997), are also available.
A release note describes the difference between the current and the former version.