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Last modified: 8 July 2011

Frequently Asked Questions

This page provides answers to common questions about Sherpa functionality; feel free to submit a new one via the Helpdesk.


Command Comparison

  1. What is the difference between the 'load', 'read', and 'unpack' Sherpa commands?
  2. What do the Sherpa 'get' commands do?
  3. How do Sherpa 'load' commands handle ASCII files?
  4. How can I convert my Sherpa 3.4 scripts to Sherpa 4.3?


  1. How can I modify a single plot in a display containing multiple plots?
  2. How can I plot a model-fitted spectrum in photon flux units (photons s-1 cm-2 keV-1)?
  3. How can I plot an unfolded source spectrum in flux units, independent of a model spectrum?
  4. How can I set the axis scaling of plots to linear or logarithmic?
  5. How can I change the size of axis labels in a plot?
  6. How can I plot the ungrouped fitted source model on my grouped data?
  7. How can I plot my data in units of Counts versus Energy(keV)?


  1. How can I define my own fit statistic in Sherpa?
  2. How can I specify which measurement errors to use in fitting a PHA data set?

Parameter Estimation

  1. What do the WARNING messages returned by the confidence methods mean?

Computer Environment

  1. How can I preserve the output of a show command in my terminal window after quitting the command?

PHA Data

  1. How does Sherpa apply the BACKSCAL value recorded in the header of a spectrum file?
  2. How does Sherpa calculate errors on PHA data values given as a rate rather than counts?
  3. How does Sherpa handle Level=3 PHA spectra downloaded from the Chandra Source Catalog?
  4. How can I simulate a PHA data set in Sherpa, with/without added Poisson noise?[New]

Image Data

  1. How can I set the coordinate system to be used in the analysis of my image data?

Model evaluation

  1. How do I change an X-Spec xset variable?
  2. How do I change the AtomDB version used by X-Spec Apec models?
  3. What is the function of a Sherpa model's "cache" attribute?[New]


  1. How can I specify the number of system cores to use for computationally intensive functions?



  1. How can I log the command output from my Sherpa fitting session?
  2. How can I suppress the screen output from Sherpa commands?

Last modified: 8 July 2011
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