|AHELP for CIAO 4.3 Sherpa v1||
A CCD pileup model developed by John Davis of MIT.
The 1D pileup model can be applied to model the 1D Chandra spectrum obtained in the imaging mode. The pileup model does not work for pileup in dispersed grating spectra or 2D image data. The model was designed for imaging pileup, including pileup by the gratings in zeroth order. It should be used only for energy spectra.
The use of multiple optimization methods is desirable when fitting data with pileup model.
|1||alpha||probability of a good grade when two photons pile together|
|2||g0||probability of grade 0 assignment|
|3||f||fraction of flux falling into the pileup region|
|4||n||number of detection cells|
|5||ftime||frame time [seconds] (keyword EXPTIME in the event file)|
|6||fracexp||fractional exposure that the point source experienced while dithering on the chip (keyword FRACPROB in the ARF file)|
|7||nterms||maximum number of photons considered for pileup in a single frame.|
n, f and g0: The values of n, f and g0 should remain frozen; The full discussion of these parameters is presented in Davis (2001).
alpha: The value of the parameter alpha should be allowed to vary. alpha parameterizes "grade migration" in the detector, and represents the probability, per photon count greater than one, that the piled event is not rejected by the spacecraft software as a "bad event". Specifically, if n photons are piled together in a single frame, the probability of them being retained (as a single photon event with their summed energy) is given by alpha(n-1). In reality, the alpha parameter should be a photon-energy-dependent and detector-chip-dependent matrix; for simplicity, the jdpileup model assumes a constant value.
ftime: The ftime and parameter should be set to the value given in the header keyword EXPTIME of the event file. (Note that EXPTIME is used instead of TIMEDEL because the latter includes the transfer time, which ftime should not.)
fracexp: The fracexp parameter should be set to the value given in the header keyword FRACEXPO of the ARF file.
nterms: This should be left frozen at its maximum value of 30. I.e., the expansion of the model will include terms corresponding to 0, 1, 2, .. 30 photon events landing in the same extraction region during the same frame time.
The integration of models in Sherpa is controlled by an integration flag in each model structure. Refer to "ahelp integrate" for information on integrating model components.
See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.
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