get_bkg_scale
- sherpa.astro.ui.get_bkg_scale(id=None, bkg_id=1, units='counts', group=True, filter=False)
Return the background scaling factor for a background data set.
Return the factor applied to the background component to scale it to match it to the source, either when subtracting the background (units=’counts’), or fitting it simultaneously (units=’rate’).
Changed in version 4.12.2: The bkg_id, counts, group, and filter parameters have been added and the routine no-longer calculates the average scaling for all the background components but just for the given component.
- Parameters
id (int or str, optional) – The identifier for the data set to use. If not given then the default identifier is used, as returned by
get_default_id
.bkg_id (int or str, optional) – Set to identify which background component to use. The default value is 1.
units ({'counts', 'rate'}, optional) – The correction is applied to a model defined as counts, the default, or a rate. The latter should be used when calculating the correction factor for adding the background data to the source aperture.
group (bool, optional) – Should the values be grouped to match the data?
filter (bool, optional) – Should the values be filtered to match the data?
- Returns
ratio – The scaling factor. The result can vary per channel, in which case an array is returned.
- Return type
number or array
See also
get_areascal
Return the fractional area factor of a PHA data set.
get_backscal
Return the area scaling factor for a PHA data set.
set_backscal
Change the area scaling of a PHA data set.
set_full_model
Define the convolved model expression for a data set.
set_bkg_full_model
Define the convolved background model expression for a PHA data set.
Notes
The scale factor when units=’counts’ is:
exp_src * bscale_src * areascal_src / (exp_bgnd * bscale_bgnd * areascal_ngnd) / nbkg
where
exp_x
,bscale_x
. andareascal_x
are the exposure, BACKSCAL, and AREASCAL values for the source (x=src
) and background (x=bgnd
) regions, respectively, andnbkg
is the number of background datasets associated with the source aperture. When units=’rate’, the exposure and areascal corrections are not included.Examples
Return the background-scaling factor for the default dataset (this assumes there’s only one background component).
>>> get_bkg_scale() 0.034514770047217924
Return the factor for dataset “pi”:
>>> get_bkg_scale('pi') 0.034514770047217924
Calculate the factors for the first two background components of the default dataset, valid for combining the source and background models to fit the source aperture:
>>> scale1 = get_bkg_scale(units='rate') >>> scale2 = get_bkg_scale(units='rate', bkg_id=2)