Sherpa is a modern, general purpose fitting and modeling application available in Python. It contains a set of robust optimization methods that are critical to the forward fitting technique used in parametric data modeling. The Python implementation provides a powerful software package that is flexible and extensible with direct access to all internal data objects. Sherpa affords a highly proficient scientific working environment required by the challenges of modern data analysis. It is implemented as a set of Python modules with computationally-intensive portions written in C++/FORTRAN as extension modules using the Python C-API. It also provides a high level user interface with command-like functions in addition to the classes and functions at the API level. Sherpa is being developed by the Chandra X-ray Center (CXC) and is packaged with the Chandra data analysis software package (CIAO). Sherpa can also be built as a standalone application; it can be extended by the user, or embedded in other applications. It allows for analysis specific to astronomy, but also supports generic modeling and fitting tasks. The 'astro' module includes additional astronomy model functions, FITS image support, instrument models, and utilities. Sherpa's model library includes some commonly used 1D and 2D functions and most of the X-ray spectral models found in the High Energy Astrophysics Science Archive Research Center (HEASARC) XSPEC application. Sherpa also supports user-defined models written in Python, C++, and FORTRAN, allowing users to extend Sherpa with models not included in our model library. Sherpa has a set of optimization methods including LMDIF, implementations of Differential Evolution (Monte Carlo) and Nelder-Mead simplex. These functions minimize differences between data points and model values (as measured by a fit statistic such as the chi-squared, maximum likelihood, or a user-defined statistic). The generic I/O module includes back-end interfaces to read ASCII files using NumPy and astronomy image files (FITS) using PyFITS or CIAO Crates (CXC Data Model library in C++). Sherpa is general enough to fit and model data from a variety of astronomical observatories (e.g., Chandra, ROSAT, Hubble) and over many wavebands (e.g., X-ray, optical, radio). In fact, Sherpa can fit and model any data set that can be represented as collections of 1D or 2D arrays (and can be extended for data of higher dimensionality). Data sets can also be simulated with noise using any model. The visualization module also allows for multiple back-ends. An interface to Matplotlib and CIAO ChIPS (CXC plotting package layered on VTK in C++) are provided for line and histogram plotting. 2D visualization is supported by the Smithsonian Astrophysical Observatory (SAO) imager, DS9. The Sherpa command line uses a configured version of IPython to provide a high level shell with IPython 'magic' and readline support.

B Refsdal, S Doe, D Nguyen, A Siemiginowska, N Bonaventura, D Burke, I Evans, J Evans, A Fruscione, E Galle, J Houck, M Karovska, N Lee, M Nowak, Sherpa: 1D/2D modeling and fitting in Python in Proceedings of the 8th Python in Science conference (SciPy 2009), G Varoquaux, S van der Walt, J Millman (Eds.), pp. 51-57

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