An email from Vinay Kashyap summarizes the reason for this change:
We discovered a problem with how HRC raytraces of point sources are
handled. See attached figure (figure removed because data is not public)
of radial surface brightness
profiles of data from a suspected point source vs different methods of
raytraces. Green is bare HRMA, sans detector; blue is data; black is
current default ChaRT+MARX; and red is ChaRT+MARX with different HRC-I
blur parameters (see below). The curves are normalized to a peak of
1.
As you can see, the default raytrace predicts a significantly larger
wing than is observed. We believe we have tracked this down to a
difference in how MARX renormalizes the amplitude of the 2D Lorentzian
compared to the Sherpa model function.
MARX uses a 2 Gaussian + 1 Lorentzian function to model the HRC
detector blur. Diab tested cases where he manually set the amplitudes
of the second Gaussian and the Lorentzian to zero, and gets a
reasonably good match to the data --
HRC-I-BlurG1FWHM=12.44
HRC-I-BlurG2Amp=0
HRC-I-BlurL1Amp=0
The same changes would also apply to HRC-S-*.
An email from Vinay Kashyap summarizes the reason for this change: We discovered a problem with how HRC raytraces of point sources are handled. See attached figure (figure removed because data is not public) of radial surface brightness profiles of data from a suspected point source vs different methods of raytraces. Green is bare HRMA, sans detector; blue is data; black is current default ChaRT+MARX; and red is ChaRT+MARX with different HRC-I blur parameters (see below). The curves are normalized to a peak of 1.
As you can see, the default raytrace predicts a significantly larger wing than is observed. We believe we have tracked this down to a difference in how MARX renormalizes the amplitude of the 2D Lorentzian compared to the Sherpa model function.
MARX uses a 2 Gaussian + 1 Lorentzian function to model the HRC detector blur. Diab tested cases where he manually set the amplitudes of the second Gaussian and the Lorentzian to zero, and gets a reasonably good match to the data -- HRC-I-BlurG1FWHM=12.44 HRC-I-BlurG2Amp=0 HRC-I-BlurL1Amp=0 The same changes would also apply to HRC-S-*.