Chandra X-Ray Observatory
Skip to the navigation links
Last modified: December 2013

Jump to: Description ยท Examples

AHELP for CIAO 4.6 Sherpa v1


Context: info


Display PHA background data set information


show_bkg([id=None, bkg_id=None, outfile=None, clobber=False] )


The show_bkg() function displays the details of the PHA background data sets which have been loaded into Sherpa, by data set ID. Numerous pieces of information are returned, including any filters which may have been applied to the data, the grouping settings, mission-specific header keywords, the details of any associated instrument responses files (ARF, RMF), to name a few. This information can be saved to a text file by specifying a filename in the 'outfile' argument.

  • id - the id of the source data set to use; default is to show information for all available data sets.
  • bkg_id - the id of the background data set to use (e.g., bkg_id=1 represents the first background data set loaded in association with the source data set specified in the 'id' argument); default is bkg_id=1 for id=1.
  • outfile - name of the file to which to save the output of the function
  • clobber - a Boolean switch which indicates whether or not the 'outfile' should be overwritten, if the file already exists; default=False

The command output looks like:

Background Data Set: 1:1
Filter: 0.1248-12.4100 Energy (keV)
Noticed Channels: 1-1024
meta           = 
 object        = 3C 273
 obs_id        = 459
 mission       = AXAF
 telescope     = CHANDRA
 instrument    = ACIS
 detector      = ACIS-56789
 grating       = HETG
name           = 3c273_bg.pi
channel        = Float64[1024]
counts         = Float64[1024]
staterror      = None
syserror       = None
bin_lo         = None
bin_hi         = None
grouping       = Int16[1024]
quality        = Int16[1024]
exposure       = 38564.6089269
backscal       = 1.87253514146e-05
areascal       = 1.0
grouped        = True
subtracted     = False
units          = energy
rate           = True
plot_fac       = 0
response_ids   = [1]
background_ids = []

Background RMF Data Set: 1:1
meta     = 
 telescope     = CHANDRA
 instrument    = ACIS
name     = 3c273.rmf
detchans = 1024
energ_lo = Float64[1090]
energ_hi = Float64[1090]
n_grp    = Int16[1090]
f_chan   = UInt32[2002]
n_chan   = UInt32[2002]
matrix   = Float64[61834]
offset   = 1
e_min    = Float64[1024]
e_max    = Float64[1024]

Background ARF Data Set: 1:1
meta     = 
 telescope     = CHANDRA
 instrument    = ACIS
 grating       = HETG
 detector      = ACIS-7
name     = 3c273.arf
energ_lo = Float64[1090]
energ_hi = Float64[1090]
specresp = Float64[1090]
bin_lo   = None
bin_hi   = None
exposure = 38564.1414549

The behavior of the screen output is determined by the user's $PAGER environment variable setting, which is usually a variant of "more." If PAGER is not found, '/usr/bin/more' is attempted before error. To disable paging within show_bkg() - i.e. print the entire contents, even if it scrolls offscreen - change this setting to "cat" before starting Sherpa:

unix% setenv PAGER cat

unix% sherpa

Example 1

sherpa> show_bkg()

The function show_bkg() can be called with no arguments, in which case the details of all available background data sets will be displayed.

Example 2

sherpa> show_bkg(3, 1)

The data set id arguments are used to specify that the first background data set loaded in association with source data set 3 should be displayed (id=3, bkg_id=1). If only the source data set id "3" had been entered, without specifying a background id, all background data sets associated with data set 3 would be returned.

Example 3

sherpa> show_bkg(3, "sherpa.bkg", True)

Here, the outfile argument is used to store information pertaining to all background data sets loaded for source data set 3 in the file 'sherpa.bkg'. This file will be overwritten if it already exists in the current working directory, since 'clobber' is set to True.

Last modified: December 2013
Smithsonian Institute Smithsonian Institute

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email: Smithsonian Institution, Copyright © 1998-2014. All rights reserved.