Using marx with SAOSAC

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For calibration purposes, the Mission Support Team (MST) has developed an extremely high-fidelity raytrace of the HRMA onboard Chandra called SAOSAC. SAOSAC is the definitive CXC mirror model and 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. Using XRCF and, ultimately, on-orbit calibration data, SAOSAC will be “tuned” to reproduce actual HRMA performance to a high degree of accuracy. SAOSAC serves as baseline for the HRMA model contained within marx. In fact, the marx HRMA model is a simplified version of SAOSAC and includes as many of SAOSAC’s features as can be reasonably be reproduced while still maintaining marx ‘s speed and portability.

Using SAOSAC Rayfiles in marx

For many purposes, the HRMA model within marx will be sufficiently accurate. However, users wishing to study very subtle effects caused by fine details in the HRMA may wish to run simulations utilizing the SAOSAC mirror model. Users involved in analysis of calibration data are one obvious example. In such a situation, the user might wish to use SAOSAC in combination with the detector models or grating models available within marx. The marx package provides this capability by ingesting SAOSAC FITS format rayfiles.

At the current time, it is not planned to distribute the SAOSAC mirror model to the user community. Instead, SAOSAC rayfiles will be pre-computed and distributed as needed. These rayfiles take the form of FITS files containing binary tables of photon or “ray” properties. In order to provide users with the option of using the SAOSAC mirror model, marx is capable of ingesting standard SAOSAC FITS rayfiles.

The interface to SAOSAC rayfiles is implemented in marx by setting the value of the SourceType parameter to SAOSAC. The path to the SAOSAC rayfile to be accessed is specified using the SAOSACFile parameter. For example, if we wanted to run a marx simulation of the HSI first light test at XRCF, we might use a command similar to:

unix% marx SourceType="SAOSAC" SAOSACFile="C-IXH-PI-3.001.fits"

where C-IXH-PI-3.001.fits in this case is the rayfile appropriate for that test.

The distribution of focal plane photon positions for a marx simulation created using an SAOSAC FITS rayfile. This rayfile corresponds to XRCF test ID C-IXH-PI-3.001 and contains 10000 rays. The XRCF test in question was performed with the HRMA 5.0 mm out of focus and this defocus value is reproduced in the simulation.

In marx 4.0, an option for “colorizing” SAOSAC rays has been added. If the parameter SAOSAC_Color_Rays=yes, then photon energies and arrival times for each SAOSAC ray processed will be determined based on the status of the SpectrumType parameter. Note that since SAOSAC rayfiles are typically calculated for monochromatic energies, this option could lead to inaccuracies over large energy ranges. For narrow energy bands, such as in the vicinity of a bright emission line, this scaling should be reasonably accurate. If the SAOSAC rayfile contains event arrival times and SAOSAC_Color_Rays=no, then marx will use the values in the rayfile. In the absence of specified arrival times or colorizing, marx assigns incremental 1 sec arrival times to each detected event.

Using marx Rayfiles in SAOSAC

One of the limitations of the SAOSAC model at present is its inability to model polychromatic sources. Such sources must currently be modeled by forming weighted sums of a set of monochromatic simulations. marx provides an alternative to this approach. Using the marx2dpde tool, marx simulations can be translated into DPDE format suitable for further processing using any tools or pipelines built to handle this SAOSAC output format. The DPDE format is a binary rayfile format which was the precursor to the current FITS standard. A number of tools including the Calibration group’s ACIS simulator were designed to work with these files.

To create a DPDE file from the results of a marx simulation, one might use the following command:

unix% marx2dpde saosac-sim/ saosac-sim.dpde

The resulting DPDE file can then be processed by additional tools as desired. The marx2dpde tool can also be used to examine the contents of an SAOSAC DPDE file with:

unix% marx2dpde --dump saosac-sim.dpde > saosac-sim.out

By default, marx2dpde --dump sends its output to the standard output. Here we have piped that output into an ASCII file.

Note, according to the MST, the SAOSAC DPDE format is being phased out in favor of the FITS rayfile standard discussed in the previous section.