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Last modified: 18 October 2011


Known Issues and Limitations for Sherpa 4.3

The Sherpa 4.3 software release still includes some known bugs, and some functionality is subject to caveats. Also, there is some changed or missing functionality with respect to Sherpa 3.4. For complete details on this release, see the release notes, Latest Updates page, and the About Sherpa page.

If you think you have found a bug in Sherpa which is not shown here, please submit a ticket to the CXC Helpdesk describing the problem.


Python Environment

IPython Packaged with CIAO OTS

CIAO includes IPython in the CIAO OTS directory which is used by Sherpa and ChIPS to provide command-line user interfaces. These programs create an IPython profile in the directory $HOME/.ipython-ciao.

If IPython users want any personal customizations to be available when running CIAO, they will have to copy them from $HOME/.ipython to $HOME/.ipython-ciao.

User Python modules must not have the same name as CIAO/Sherpa modules.

When loading a module in Python, Python looks in the current working directory for this module, as well as in the directories listed in the PYTHONPATH environment variable. Therefore, if you have a script named '' in your current working directory, for example, Sherpa will fail to load because the script ./ is loaded, not the code in the module ${ASCDS_INSTALL}/lib/python2.6/site-packages/sherpa. Likewise, the presence of a file named '' in the current working directory will prevent the group module from loading, and will cause Sherpa to open with the following warning message:

unix% sherpa
Welcome to Sherpa: CXC's Modeling and Fitting Package
CIAO 4.3 Sherpa version 1 Thursday, December 2, 2010

WARNING: the group module (from the CIAO tools package) is not installed.
Dynamic grouping functions will not be available.

Please ensure that the names of your Python scripts do not exactly match those of Python modules packaged with CIAO.

User files in current working directory may conflict with Python DS9 interface.

Files with single character names "a" through "y" in the current working directory may conflict with the Python DS9 interface used in Sherpa, preventing 2D data loaded into a Sherpa session from being displayed in DS9 with Sherpa commands such as image_data. Trying to do so will yield an error, "DS9Err: Could not display image." Simply removing the offending file(s) from the current working directory will resolve the issue and restore the ability to plot in DS9 from within the Sherpa session.

Instrument Responses

RMF and ARF energy range must be the same

If an RMF and ARF with different energy ranges are assigned to a data set in Sherpa, Sherpa will print an error message when trying to fit the data: "ValueError: RMF data is invalid or inconsistent". See the CIAO Imaging Spectroscopy threads to learn how to create an ARF and RMF with the same energy range.

RMF energy grid resolution must not exceed that of ARF

Sherpa 4.2 does not currently support the case where the RMF has energy bins of finer resolution than the ARF (an ARF with finer resolution than the RMF energy bins is supported). Sherpa will print an error message when trying to fit data that have response grids unmatched in this way:"need to expand, not shrink effective area." The CIAO why topic on mkacisrmf explains how to remake the ARF such that the grids match.

Background Scaling

Sherpa does not use AREASCAL in background scaling

Unlike XSpec, Sherpa does not incorporate the PHA AREASCAL header keyword value into the background scaling factor used during background subtraction or simultaneous modeling of source and background spectra. Sherpa results match those of XSpec whenever AREASCAL=1.0, which represents the majority of use cases, however there is a problem using Sherpa for cases where the AREASCAL value should be something other than 1.0. The complete background scaling factor currently used by Sherpa is described in the get_bkg_scale ahelp file. This issue will be addressed in a future release of Sherpa.



Last modified: 18 October 2011
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