# Sherpa and CIAO¶

The Sherpa package was developed by the Chandra X-ray Center (CXC) as a general purpose fitting and modeling tool, with specializations for handling X-ray Astronomy data. It is provided as part of the CIAO analysis package, where the code is the same as that available from the Sherpa GitHub page, with the following modifications:

• the I/O backend uses the CIAO library Crates rather than astropy;
• a set of customized IPython routines are provided as part of CIAO that automatically loads Sherpa and adjusts the appearance of IPython (mainly changes to the prompt);
• and the CIAO version of Sherpa includes the optional XSPEC model library (sherpa.astro.xspec).

The online documentation provided for Sherpa as part of CIAO, namely http://cxc.harvard.edu/sherpa/, can be used with the standalone version of Sherpa, but note that the focus of this documentation is the session-based API provided by the sherpa.astro.ui and sherpa.ui modules. These are wrappers around the Object-Oriented interface described in this document, and data management and utility routines.