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Last modified: 24 October 2023


Contributed CIAO Sherpa (Python) Extension Packages

On this page we provide links to software packages developed by the CXC and external users which extend the functionality of Sherpa.

Please note that the CIAO HelpDesk does not provide support for this software. If you need help using one of these packages, contact the package developer at the email address listed below.

Sherpa for Python Users

Sherpa for Python

Sherpa is a modeling and fitting application for Python users which can be built and used independently of CIAO (it is the same code, although due to releases during the year outside of CIAO the functionality may be slightly different).

Sherpa is an Open Source project with the source code repository placed on GitHub The complete tar files are available for download as well as the full project repository which can be 'cloned'. The Sherpa Project welcomes contributions from the users via GitHub. There are several Sherpa Python releases in each calendar year. Information on the releases can be found on the Zenodo Sherpa search page.

Requirements: Python 3.5 or later, a recent version of NumPy, FFTW 3.3 or later

Primary developers are the Sherpa Team at SAO-CXC, although input - such as new features, bug or documentation fixes, and bug reports - by the community are welcomed. Post questions and issues on the Sherpa GitHub page.

Specialized Science Tools


Developed by Tom Aldcroft at SAO-CXC (email: aldcroft at

Calculate useful values for a given cosmology. This is a Python version of the Cosmology Calculator (Ned Wright) and uses code adapted from (James Schombert).

Requirements: CIAO, Sherpa


Users may prefer to use the AstroPy Cosmology module as it is under active development and contains more functionality than cosmocalc.


Created by Tom Aldcroft at SAO-CXC, developed by Doug Burke at SAO-CXC (email: dburke at

CIAO Sherpa extension package to facilitate deprojection of two-dimensional annular X-ray spectra to recover the three-dimensional source properties.

Requirements: AstroPy, SciPy, Sherpa


Developed by Hans Moritz Guenther (email: hgunther at

A convolution model to use with HST/COS data. It will convolve the model with the local line spread functions appropriate for this data.

Requirements: CIAO, Sherpa

BXA - Bayesian X-ray Analysis

Developed by Johannes Buchner (email: jbuchner - at -

BXA connects the nested sampling algorithm MultiNest to the X-ray spectral analysis environment Sherpa for Bayesian Parameter Estimation and Model comparison.

Requirements: CIAO, Sherpa, PyMultiNest, and MultiNest.


Packages that have been moved into Sherpa

The following packages have been moved into Sherpa and no-longer need to be installed separately.


This was moved into Sherpa at the CIAO 4.5 release: pyblocxs.

Developed by Sherpa Team at SAO-CXC (email: asiemiginowska at

pyBLoCXS is a sophisticated Markov chain Monte Carlo (MCMC) based algorithm designed to carry out Bayesian Low-Count X-ray Spectral (BLoCXS) analysis in the Sherpa environment.

Requirements: Sherpa 4.2.1 or later


This was moved into Sherpa at the CIAO 4.7 release: datastack.

Developed by Tom Aldcroft at SAO-CXC (email: aldcroft at

Datastack is a CIAO Sherpa extension package for manipulating and fitting a stack of related data sets. It provides stack-enabled (i.e. vectorized) versions of the key Sherpa commands used to load data, set source models, get and set parameters, fit, and plot.

Requirements: CIAO, Sherpa