docs: def derive_identity_arf(name, arf):
      """Create an "identity" ARF that has uniform sensitivity.
  
      *name*
        The name of the ARF object to be created; passed to Sherpa.
      *arf*
        An existing ARF object on which to base this one.
      Returns:
        A new ARF1D object that has a uniform spectral response vector.
  
      In many X-ray observations, the relevant background signal does not behave
      like an astrophysical source that is filtered through the telescope's
      response functions. However, I have been unable to get current Sherpa
      (version 4.9) to behave how I want when working with backround models that
      are *not* filtered through these response functions. This function
      constructs an "identity" ARF response function that has uniform sensitivity
      as a function of detector channel.
  
      """
      from sherpa.astro.data import DataARF
      from sherpa.astro.instrument import ARF1D
  
      darf = DataARF(
          name,
          arf.energ_lo,
          arf.energ_hi,
          np.ones(arf.specresp.shape),
          arf.bin_lo,
          arf.bin_hi,
          arf.exposure,
          header = None,
      )
      return ARF1D(darf, pha=arf._pha)