Why use ChaRT instead of the PSF libraries?
The usage of the standard PSFs libraries for a detailed spatial/spectral analysis has limitations mainly due to the coarse energy and spatial grids, especially for large off-axis angles. ChaRT retrieves a PSF for any point on the detector and any energy, using no grids at all. The standard PSF library files consist of two dimensional simulated monochromatic PSF images “postage stamps” (only for energies ranging from 0.277 keV to 8.6 keV), made using SAOsac and projected onto ideal detector planes (see the PSF library caveats). They are stored in multi-dimensional FITS images (hypercubes) with azimuth and elevation steps (in telescope fixed system) of either 1 arcminute or 5 arcminutes. The user can extract the desired PSF model image from a library file by interpolating within the energy and off-axis angle grids, using the Chandra Interactive Analysis of Observations package (CIAO) tool mkpsf. The PSF library grids are very coarse: azimuth and elevation angular off-sets of 1′ or 5′,