Return the area scaling of a PHA data set.
Return the BACKSCAL setting  for the source or background component of a PHA data set.
- 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 (
None) means that the value is for the source component of the data set.
backscal – The BACKSCAL value, which can be a scalar or a 1D array.
number or ndarray
The BACKSCAL value can be defined as the ratio of the area of the source (or background) extraction region in image pixels to the total number of image pixels. The fact that there is no ironclad definition for this quantity does not matter so long as the value for a source dataset and its associated background dataset are defined in the similar manner, because only the ratio of source and background BACKSCAL values is used. It can be a scalar or be an array.
 “The OGIP Spectral File Format”, Arnaud, K. & George, I. http://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/spectra/ogip_92_007/ogip_92_007.html
>>> get_backscal() 7.8504301607718007e-06 >>> get_backscal(bkg_id=1) 0.00022745132446289