CIAO's Modeling & Fitting Application

Sherpa Highlights in CIAO 3.2

The Chandra X-Ray Center introduced version 3.2 of the CIAO software package on 15 December 2004.

• Improvements to the file-based PSF models (fpsf1d, fpsf2d) and the point-source models (ptsrc1d, ptsrc2d) have been made.

• The Levenberg-Marquardt fitting method has improved handling of problem datasets.

• Two changes have been made to improve the projection error estimation methods:

  • the projection method now catches overflows and underflows, and calculates the errors for the remaining parameters rather than immediately exiting, and
  • the allowable range on many model parameters has been restricted to reduce the chances of overflow occurring.

See the Sherpa release notes for a complete list of CIAO 3.2 changes.

About Sherpa

Sherpa, CIAO's generalized modeling and fitting engine, allows users to construct complex models and to fit models to data in N dimensions. It has a library of optimization methods and fit statistics. Sherpa is "domain independent", i.e. it does not require particular axes to be fit. It is also mission independent, with no particular tie to Chandra data. For example, it has been used to analyze HST spectra.

Sherpa supports S-Lang, an interpreted programming language that can be used for scripting and data manipulation. Existing S-Lang scripts and utilities are available for download on the CIAO scripts page.

The GUIDE package within Sherpa links Sherpa results (stored in a MDL file) to the ATOMDB, enabling the identification of spectral lines and the use of their properties in further fitting.

In order to run Sherpa, you must download and install CIAO.

Citing Sherpa in a Publication

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

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