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Last modified: 18 December 2023

Frequently Asked Questions

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


Warnings and General Behavior

  1. WARNING: chips is not supported in CIAO 4.12+, falling back to matplotlib.

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?

Data access

  1. Writing out data to a file [New]
  2. Accessing the data used to create a plot [New]


  1. How can I set the axis scaling of plots to linear or logarithmic before creating them? [Updated]
  2. How can I create multiple plots in the same window? [Updated]
  3. How can I include multiple datasets in the same plot?
  4. How can I plot the ungrouped fitted source model on my grouped data?
  5. How can I plot my data in units of Counts versus Energy(keV)?

Plotting with Matplotlib

  1. Changing the limits of a plot [New]
  2. How can I set the axis scaling of existing plots to linear or logarithmic? [Updated]
  3. How can I modify a single plot in a display containing multiple plots?
  4. How can I plot a model-fitted spectrum in photon flux units (photons s-1 cm-2 keV-1)?
  5. How can I plot an unfolded source spectrum in flux units, independent of a model spectrum?
  6. Adding a label to a plot [New]
  7. Adding a line to a plot [New]
  8. How can I change the size of axis labels in a plot?
  9. How can I adjust multiple plots with matplotlib? [Updated]
  10. Including extra data in plots with matplotlib

Plotting with bokeh

  1. Starting the bokeh backend [New]


  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? [Updated]
  5. How can I group net counts?
  6. Why is there data being plotted or fit outside the energy range I noticed? [Updated]
  7. Can I use an XSPEC save file (.xcm) with Sherpa? [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 create and use XSPEC convolution models?
  2. How do I use an XSPEC table model? [New]
  3. How do I create an XSPEC table model? [New]
  4. How do I use an XSPEC user model in Sherpa? [New]
  5. How do I change an XSPEC xset variable?
  6. How do I change the AtomDB version used by XSPEC Apec models?
  7. What is the function of a Sherpa model's "cache" attribute?


  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? [Updated]


  1. How can I assign XSPEC-style numbering to Sherpa model parameters for use in my script?