Parameterization of the Chandra point spread function

The Chandra X-ray Observatory point spread function (PSF) is a complex function of source position and energy. On-orbit calibration observations with sufficient S/N sample only a small fract ion of the possible parameter space, and are complicated by detector systematics. Thus, the standard method of analyzing Chandra data uses the standard Chandra optics model as a reference. The optics model accurately simulates the telescope's PSF, but as it is a raytrace based technique, it can be time-consuming to run and is not always appropriate for a given analysis task. A simple parameterization of the PSF would be useful for many analysis purposes, in many cases obviating the need for users to run lengthy raytraces. We present an approach to a simple PSF parameterization of off-axis point sources, discussing its applicability to analysis of Chandra observations in light of the complicated PSF structure. We also present some results of our PSF parameterization and discuss its accuracy. Previous parameterizations of the Chandra PSF have used the encircled energy function, characterizing the PSF with the fractional energy measured within a circular aperture as a function of radius. Chandra's PSF in reality is not circular, it becomes elongated with increasing off-axis angle. Elliptical apertures would thus be better representatives of the underlying PSF. We have developed a technique of deriving a series of elliptical regions that have been fit to the PSF, and comparing these to the total number of counts incident on the detector in the raytrace. This enclosed count fraction (ECF) is analogous to the encircled energy. The ECF and the properties of the elliptical regions from which it was derived provide a parameterization of the Chandra PSF that can be compared to real observations. The determined ellipses are representative of the large scale PSF. However, our method produces nonsensical results in the PSF core, which devolves into multiple components at sufficiently high off-axis source positions. Figure 1 illustrates the complexity of the off-axis core, When completed, the parameterization of the Chandra PSF described in this paper will be presented to users as a standard calibration product.