    Return the background scaling factor for a background data set.

get_bkg_scale(id=None, bkg_id=1, units='counts', group=True, filter=False)
id - int or str, optional
bkg_id - int or str, optional
units - {'counts', 'rate'}, optional
group - bool, optional
filter - bool, optional

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').

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)

PARAMETERS

The parameters for this function are:

Parameter
Definition
id
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
Set to identify which background component to use. The default value is 1.
units
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
Should the values be grouped to match the data?
filter
Should the values be filtered to match the data?

The return value from this function is:

ratio -- The scaling factor. The result can vary per channel, in which case an array is returned.

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 . and areascal_x are the exposure, BACKSCAL, and AREASCAL values for the source ( x=src ) and background ( x=bgnd ) regions, respectively, and nbkg is the number of background datasets associated with the source aperture. When units='rate', the exposure and areascal corrections are not included.

Changes in CIAO

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.

See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.

December 2023