Starting Sherpa

Starting Sherpa for the First Time

When Sherpa is started from within the CIAO environment, the user is informed of the CIAO and Python version numbers:

unix% sherpa
-----------------------------------------------------
Welcome to Sherpa: CXC's Modeling and Fitting Package
-----------------------------------------------------
Sherpa 4.17.0

Python 3.11.6 | packaged by conda-forge | (main, Oct  3 2023, 10:40:35) [GCC 12.3.0]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.30.0 -- An enhanced Interactive Python. Type '?' for help.

IPython profile: sherpa
Using matplotlib backend: tkagg

CIAO installation: ciao-install versus conda

The information presented when Sherpa is started depends on

1. how CIAO was installed (ciao-install or conda)
2. what version of Python was used (only relevant for the conda installation since ciao-install only supports Python 3.11)

The Sherpa script loads the Sherpa (Python) module, as well as the Crates module, which handles file input and output, and the Matplotlib plotting package.

Default Sherpa settings are defined in the $HOME/.sherpa.rc file, which is processed when Sherpa is started; this file can be modified as described in the section Customizing the Sherpa Environment.

The Sherpa application uses the $HOME/.ipython-ciao/ directory to store configuration information. The first time Sherpa is started you will see a message asking whether you want to update this directory. For example:

unix% sherpa

 ATTENTION: The IPython profile found in /home/ciaoenthusiast/.ipython-ciao/profile_sherpa is not compatible with this version of sherpa.

            Update to a compatible version? [Y/N]:

Unless you have manually changed your $HOME/.ipython-ciao/profile_sherpa file, it is safe to answer "Y" here. The outdated file is renamed with a timestamp to preserve it. The new profile is installed read-only. If you would like to modify how Sherpa uses IPython functions, or call exec/execfile to load Python scripts when Sherpa starts, a customization file should be added to ~/.ipython-ciao/profile_sherpa/startup/. If you would like IPython profiles to be somewhere other than ~/.ipython-ciao, you can set IPYTHONDIR to point to the desired directory.

ChIPS warning

If you have transitioned to Sherpa with a version earlier than CIAO 4.12, then it is likely that you will see this warning when Sherpa starts:

WARNING: chips is not supported in CIAO 4.12+, falling back to matplotlib.
WARNING: Please consider updating your $HOME/.sherpa.rc file to suppress this warning.

Sherpa will work correctly, and create plots with Matplotlib. To stop the warning from appearing please update your Sherpa RC file as described in the FAQ entry.

Command-line options

The following are the remaining command-line options which may be specified when Sherpa is started:

unix% sherpa -h
-- sherpa
  usage: sherpa [-x] [-n] [-b] [-rcfile file] [norcfile] <file>
     -x       launch sherpa shell in separate display terminal
     -n       do not print banner
     -b       batch mode
     -rcfile  load a user specifiable preference file
     -norcfile do not load any user specifiable preference file (overrides -rcfile)
     <file> Python command file to execute

See the section Command-line Options for Sherpa for more details on these Sherpa startup options.

Getting Help

There are several ways to access the Sherpa help files.

Please use the Python help when possible

The in-built Python help command provides more up-to-date information on a command than the ahelp command. The "downside" is that the documentation is written to match the standards used by other Scientific packages, such as NumPy and AstroPy, rather than to follow the ahelp approach and layout.

sherpa> help plot_fit_ratio

sherpa> help set_stat

-------------> help(group_counts)

sherpa> !ahelp -b QEXAMPLELIST reg_proj

Finding a Command

Here are some tips on how to find the name of a command while working in Sherpa.

sherpa> *_arf*?
create_arf
get_arf
get_arf_plot
get_bkg_arf
load_arf
load_bkg_arf
load_multi_arfs
plot_arf
save_arf
set_arf
unpack_arf

sherpa> plot_fit__
plot_fit_delchi() plot_fit_ratio() plot_fit_resid()

Running CIAO and Unix commands from the Sherpa Prompt

Any line beginning with "!" is passed from Sherpa to the shell. This allows you to run CIAO and shell commands from the Sherpa prompt.

For example:

sherpa> !pwd
/data/ciao_demo/threads/acis-s/1838/primary

sherpa> !ls
acisf01838_000N001_bpix1.fits  acisf01838N001_src2.fits
acisf01838N001_evt2.fits       pcadf084244404N001_asol1.fits

sherpa> !dmkeypar acisf01838N001_evt2.fits OBJECT echo+
G21.5-0.9

sherpa> !dmcopy "acisf01838N001_evt2.fits[energy=300:10000]" 1838_energy.fits

Exiting Sherpa

Type "quit" or use <CTRL>-D to exit Sherpa.

Command-line options for Sherpa

The Sherpa command-line options can be listed with the -h flag:

unix% sherpa -h
-- sherpa
  usage: sherpa [-x] [-n] [-b] [-rcfile file] [norcfile] <file>
     -x       launch sherpa shell in separate display terminal
     -n       do not print banner
     -b       batch mode
     -rcfile  load a user specifiable preference file
     -norcfile do not load any user specifiable preference file (overrides -rcfile)
     <file> Python command file to execute

-x

This will create a new Xterm and start Sherpa in it.

-n

This switch causes Sherpa to start without a banner:

unix% sherpa -n
sherpa-1>

-b

Executes Python command file as a non-interactive Sherpa session.

-rcfile

This option allows the user to specify a specific .sherpa.rc file to use; refer to "ahelp sherparc" for details.

-norcfile

This option specifies that a .sherpa.rc file should not be loaded; overrides "-rcfile" if both are set.

<file>

The contents of the file are executed before the user is prompted for further commands. The command must be in the same language as used at the interactive prompt. So, if simple.py contained

 
load_pha("src.pi")
plot_data()
print("Loaded and plotted data")

then we will load and plot the data if Sherpa is started by saying:

unix% sherpa simple.py
-----------------------------------------------------
Welcome to Sherpa: CXC's Modeling and Fitting Package
-----------------------------------------------------
Sherpa 4.17.0

Python 3.11.6 | packaged by conda-forge | (main, Oct  3 2023, 10:40:35) [GCC 12.3.0]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.30.0 -- An enhanced Interactive Python. Type '?' for help.

IPython profile: sherpa
Using matplotlib backend: tkagg

read ARF file src.arf
read RMF file src.rmf
read background file bgnd.pi
Loaded and plotted data

sherpa>

Running Sherpa Scripts from the Sherpa Prompt

Given the script of Sherpa (and putative Python) commands in the simple.py script described above, the script can be executed from the Sherpa command-line using the IPython %run magic command:

unix% sherpa
-----------------------------------------------------
Welcome to Sherpa: CXC's Modeling and Fitting Package
-----------------------------------------------------
Sherpa 4.17.0

Python 3.11.6 | packaged by conda-forge | (main, Oct  3 2023, 10:40:35) [GCC 12.3.0]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.30.0 -- An enhanced Interactive Python. Type '?' for help.

IPython profile: sherpa
Using matplotlib backend: tkagg

sherpa> %run -i simple.py
sherpa>

Using Sherpa from Python

The Sherpa and Crates modules which come packaged with CIAO can be loaded into Python as follows, enabling the user to access Sherpa functionality on the Python command line.

unix% ipython
Python 3.11.6 | packaged by conda-forge | (main, Oct  3 2023, 10:40:35) [GCC 12.3.0]
Type 'copyright', 'credits' or 'license' for more information
IPython 8.30.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from sherpa.astro.ui import *

In [2]: from pycrates import *

In [3]: import matplotlib.pyplot as plt

In [4]: %matplotlib

Using matplotlib backend: tkagg

Use the --matplotlib flag

IPython accepts the --matplotlib command-line flag which removes the need to say %matplotlib. The latter is useful when you forget to start IPython with the former!

Sherpa is also available for Python users as a standalone application, independent of CIAO; download instructions and user documentation are available at the Contributed CIAO Sherpa (Python) Extension Packages page.

Customizing the Sherpa Environment

When Sherpa is started, it processes the $HOME/.sherpa.rc resource file, which defines the default behavior for Sherpa commands. You can modify the settings in this file in any text editor to suit your preferences; see the .sherpa.rc ahelp file for detailed descriptions of the available settings.