Indices and tables



The Ancillary Response Function used to describe the effective area curve of an X-ray telescope; that is, the area of the telescope and detector tabulated as a function of energy. The FITS format used to represent ARFs is defined in the OGIP Calibration Memo CAL/GEN/02-002.


A community Python library for Astronomy:


The data reduction and analysis provided by the CXC for users of the Chandra X-ray telescope. Sherpa is provided as part of CIAO, but can also be used separately. The CIAO system is available from


The Input/Output library provided as part of CIAO. It provides read and write access to FITS data files, with speciality support for X-ray data formats that follow OGIP standards (such as ARF and RMF files).


The Chandra X-ray Center.


An external image viewer designed to allow users to interact with gridded data sets (2D and 3D). Is is used by Sherpa to display image data, and is available from It uses the XPA messaging system to communicate with external processes.


The Flexible Image Transport System is a common data format in Astronomy, originally defined to represent imaging data from radio telescopes, but has since been extended to contain a mixture of imaging and tabular data. Information on the various standards related to the format are available from the FITS documentation page at HEASARC.


NASA’s High Energy Astrophysics Science Archive Research Center at Goddard Space Flight Center:


The matplotlib plotting package, which is documented at, is used to provide the plotting support in Sherpa.


The Office for Guest Investigator Programs (OGIP) was a division of the Laboratory for High Energy Astrophysics at Goddard Space Flight Center. The activities of that group have now become the responsibility of the HEASARC FITS Working Group (HFWG), and supports the use of high-energy astrophysics data through multimission standards and archive access. Of particular note for users of Sherpa are the standard documents produced by this group that define the data formats and standards used by high-energy Astrophysics missions.


The standard file format used to store astronomical X-ray spectral data. The format is defined as part of the OGIP set of standards, in particular OGIP memos OGIP/92-007 and OGIP/92-007a. Confusingly, PHA can also refer to the Pulse Height Amplitude (the amount of charge detected) of an event, which is one of the two channel types that can be found in a PHA format file.


The Point Spread Function. This represents the response of an imaging system to a delta function: e.g. what is the shape that a point source would produce when observed by a system. It is dependent on the optical design of the system but can also be influenced by other factors (e.g. for ground-based observatories the atmosphere can add additional blurring).


The Redistribution Matrix Function used to describe the response of an Astronomical X-ray detector. It is a matrix containing the probability of detecting a photon of a given energy at a given detector channel. The FITS format used to represent RMFs is defined in the OGIP Calibration Memo CAL/GEN/02-002.


The phrase World Coordinate System for an Astronomical data set represents the mapping between the measured position on the detector and a “celestial” coordinate. The most common case is in providing a location on the sky (e.g. in Equatorial or Galactic coordinates) for a given image pixel, but it can also be used to map between row on a spectrograph and the corresponding wavelength of light.


The XPA messaging system is used by DS9 to communicate with external programs. Sherpa uses this functionality to control DS9 - by sending it images to display and retriving any regions a used may have created on the image data. The command-line tools used for this commiunication may be available via the package manager for a particular operating system, such as xpa-tools for Ubuntu, or they can be built from source.


This can refer to either the X-ray Spectral fitting package, or the models from this package. XSPEC is distributed by HEASARC and its home page is Sherpa can be built with support for the models from XSPEC.

Sherpa can be built to use XSPEC versions 12.11.1, 12.11.0, 12.10.1 (patch level a or later), 12.10.0, 12.9.1, or 12.9.0.