The Chandra X-ray Observatory

CIAO's modeling and fitting package

Sherpa is the CIAO modeling and fitting application. It enables the user to construct complex models from simple definitions and fit those models to data, using a variety of statistics and optimization methods (see Gallery of Examples). For the most up-to-date changes and additions to Sherpa functionality, see the Latest Updates page.

Sherpa lets you: fit 1-D data sets (simultaneously or individually), including: spectra, surface brightness profiles, light curves, general ASCII arrays. fit 2-D images/surfaces in the Poisson/Gaussian regime. access the internal data arrays. build complex model expressions. import and use your own models. choose appropriate statistics for modeling Poisson or Gaussian data. import new statistics, with priors if required by analysis. visualize a parameter space with simulations or using 1-D/2-D cuts of the parameter space. calculate confidence levels on the best-fit model parameters. choose a robust optimization method for the fit: Levenberg-Marquardt, Nelder-Mead Simplex or Monte Carlo/Differential Evolution. use Python to create complex analysis and modeling functions, build the batch mode analysis or extend the provided functionality to meet the required needs.

The new Sherpa infrastructure greatly enhances the default Sherpa functions, and provides users with an environment for developing complex and sophisticated analysis.

Sherpa is designed for use in a variety of modes: as a user-interactive application and in batch mode. Sherpa is an importable module for scripting languages (Python or S-Lang) and is available as a C/C++ library for software developers. In addition, users may write their own Python and S-Lang scripts for use in Sherpa; refer to the page Should I Use Sherpa in Python or S-Lang? page for help in deciding which language to use.

The About Sherpa page outlines new features and provides a summary of missing items with respect to Sherpa 3.4. See also the SciPy2009 conference proceedings for detailed information about Sherpa's capabilities. Please send feedback and questions on Sherpa to the Helpdesk.

The CXC is committed to helping Sherpa users transition to new syntax as smoothly as possible. If you have existing Sherpa scripts or save files, submit them to us via the CXC Helpdesk and we will provide the CIAO/Sherpa 4.2 syntax to you.

Citing Sherpa in a Publication

If you are writing a paper and would like to cite Sherpa, we recommend the following papers:

\bibitem[Freeman et al.(2001)]{2001SPIE.4477...76F} Freeman, P., Doe, S., 
\& Siemiginowska, A.\ 2001, \procspie, 4477, 76

The specific version of CIAO and CALDB (if applicable) used for the analysis should be mentioned as well.

A reference for the Python interface to Sherpa is also available:

\bibitem[Doe et al.(2007)]{2007ASPC..376..543D} Doe, S., et al.\ 2007, 
Astronomical Data Analysis Software and Systems XVI, 376, 543

Further guidelines are available from the Acknowledgment of Use of Chandra Resources.