ChaRT traces rays through the Chandra X-ray optics to produce a collection of rays. The rays are then projected onto the detector (via MARX), taking into account any detector effects. The result is an event file from which an image of the point spread function may be created.
Since ChaRT runs the same code that is used internally at the CXC for calibration, it gives the best available HRMA PSF for a point source at any off-axis angle and for any energy or spectrum.
With the combination of ChaRT and MARX, users may now easily perform simulations of the Chandra PSF. However, these pieces of software have limitations of which users should be aware. It is very important to read the ChaRT caveats before running ChaRT and before starting any analysis using the resulting PSF simulations.
The following are some technical details on how ChaRT operates. It is not necessary to understand the internals in order to run ChaRT, but advanced users may be interested in reviewing this information.
ChaRT v2 is the Chandra PSF simulator. It is a web interface to the SAOTrace v2 raytrace code which was developed by the CXC for calibration purposes. In addition to the most current mirror model, SAOTrace (and ChaRT) includes many of the details of the HRMA's physical construction such as the stray light baffles and support structures, as well as a detailed model of the reflective properties of the mirror surface. ChaRT uses the trace-nest3 driver script and several other SAOTrace routines. This software includes multi-layer reflectivity and uses the HRMA model, orbit-200809-01f-a. More information on the configurations is available from the Models of the Chandra Optics for SAOTrace. Additionally, ChaRT v2 provides support for the telescope's dithering, allowing for its results be compatible with sub-pixel analysis.
For further details on the SAOTrace code and its history, see the "How to Raytrace" pages.
Historicallly, a standard library of PSFs could be used — via interpolation by the mkpsf tool — to generate a PSF at a specific energy or off-axis angle; however, the standard PSF libraries and mkpsf have been discontinued, and the tool was discontinued in CIAO 4.5.
ChaRT can generate a PSF for a given on- or off-axis position without interpolation. It is the current method of choice to generate Chandra PSFs.