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Astronomical Data Analysis Software and Systems VI ASP Conference Series, Vol. 125, 1997 Gareth Hunt and H. E. Payne, eds.

The AXAF Science Center Performance Prediction and Calibration Simulator
R. A. Zacher, A. H. MacKay, B. R. McNamara, and L. P. David SAO/ASC, Cambridge, MA 02138 Abstract. We are developing and integrating software to simulate the focal plane detectors, shutters, and gratings for the Advanced X-ray Astrophysical Facility (AXAF). AXAF is one of four observatories in the NASA "Great Observatory" series, scheduled for launch in 1998. AXAF will offer unprecedented spatial and sp ectral resolution in the X-ray band ranging from 0.1­10 keV. The path of X-ray photons is simulated from the exit of the telescop e mirrors to the focal plane. Each ma jor functional element of the simulation is represented by an indep endent module. Module execution and inter-module communication is accomplished within a pip eline architecture. The software is written in C/UNIX and utilizes a numb er of existing astronomical software libraries. Detailed models are b eing develop ed for the two focal plane instruments. These instruments are ACIS, which is a CCD camera, and HRC, which is a microchannel plate detector. Realistic detector output files are generated in a variety of formats. The simulations are currently b eing used for planning calibration activities, on-orbit p erformance prediction and for testing the analysis and telemetry software.

1.

Introduction

We are developing computer models to simulate the focal plane detectors of the Advanced X-ray Astrophysical Facility (AXAF). The models, b eing develop ed as part of the AXAF Science Center, are b eing used to aid in calibration planning and to characterize the p erformance of the AXAF observatory. Scripts have b een develop ed to configure and run the simulations automatically from a test database. Dep ending on the output mode selected, the results can b e viewed directly, sent through telemetry processing, or fed into higher-level analysis pip elines in the Data System. Much of our work has b een focused on developing high-fidelity simulations of the two main Scientific Instruments located in the focal plane of the telescop e. These are the AXAF CCD Imaging Sp ectrometer (ACIS) and the High Resolution Camera (HRC). In addition to these, we have integrated gratings modules and we use output from other simulators which model the sources and telescop e mirrors (see Figure 1). 1.1. AXAF CCD Imaging Sp ectrometer (ACIS)

ACIS is a charge-coupled device optimized for X-ray detection. Its 24µm pixel size offers 1/2 resolution in the AXAF focal plane. The field of view is 16в16 for 481

© Copyright 1997 Astronomical Society of the Pacific. All rights reserved.


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SHUTTERS

GRATINGS

FILTERS

DETECTORS

SOURCE + MIRROR RAYTRACE FILES

FITS

QPOE

ASCII

TELEMETRY

ASCDS ANALYSIS PIPELINES

Figure 1. Simulation Schematic. The SHUTTERS module simulates 16 separately configurable shutters b ehind the mirror assembly. The GRATINGS module simulates the High, Medium, and Low Energy Transmission Gratings. The FILTERS module simulates optical blocking filters in front of the detectors. The detectors are ACIS ( a charge coupled device) and HRC ( a microchannel plate).

the imaging array and 8в48 for the sp ectroscopic array. The chip is modeled as a multilayer structure. The incident X-ray photons create a charge cloud whose p osition and size are determined by the silicon absorption depth, the photon energy, the photon p osition and the dopant concentration. The charge cloud then drifts to the surface of the chip under the influence of a layer dep endent electric field, which we model using a Monte Carlo method. At the surface, the charge is mapp ed onto a 3в3 pixel array. The functional dep endence for the numb er of electrons (ne ) created by an incident X-ray of energy Ex is given by ne nx (Ex /E ). Here, nx is a function calculated by the Monte Carlo program and =3.65 eV is the energy required to lib erate a charge carrier. Additional features modeled include read noise, charge transfer inefficiency, bias, gain, and layer thicknesses. The algorithms used in the CCD simulation are based on the program XRAYSIM develop ed by Lumb et al. (1994), which in turn was based on analytical calculations by Janesick (1987, 1988) and Hopkinson (1987). The simulator can b e op erated in two modes. The Event List Mode outputs the ab ovementioned 3в3 pixel array to a FITS event list. This mode is designed for high throughput and does not model effects which arise when two photons hit the same location on the chip in a given integration p eriod. The Ful l Frame Mode emb eds the 3в3 pixel array in a much larger rectangular array in memory. This mode includes the effects which arise when multiple photons hit the same location in a given integration p eriod. The Ful l Frame Mode has two output formats available. The events can b e extracted from the array in memory using the same algorithm used to detect events in real ACIS frames. The extracted events are output to a FITS event list. Alternatively, the full arrays can b e written to FITS image files, one for each integration p eriod. These image files are similar to those produced by the physical chips b efore event extraction. Event detection in the array in memory yields substantial p erformance gains over detection of the events in the FITS image files.


The ASC Performance Prediction and Calibration Simulator 1.2. High Resolution Camera (HRC)

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The HRC is a microchannel plate (MCP) detector that provides a spatial resolution of less than 1/2 . The field of view is 31в31 for the imaging array and 7в97 for the sp ectroscopic array. Resolving p ower is limited, with E /E 1. The simulation models the UV Ion Shield (UVIS), the MCP itself, and the wire charge grid. The UVIS is modeled using a generalized filter program that statistically simulates photon absorption by applying a transmission curve to the input photon energy. The MCP surface is modeled as a surface of circular p ores with a diameter of 0.0125 mm and spacing of 0.015 mm. A model of quantum efficiency as a function of incident angle is also applied. The wire grid charge resulting from the charge cloud produced by the MCP is modeled by a scaled Lorentz function. Events are passed into a telemetry simulator that models dead time induced by telemetry bandwidth limitations. Output modes are raw telemetry, FITS event list, and QPOE image formats. The HRC simulator has also b een adapted to simulate a similar instrument called the High Sp eed Imager (HSI) which is used for telescop e mirror calibration. 1.3. Mirror and Grating Mo dules

The mirror simulation's raytrace output can b e pro jected directly on to the model detectors, or diffracted by the gratings module b efore pro jection on to the model detectors. The disp ersed gratings sp ectrum provides a resolving p ower of E/E 1023 . The High Energy Transmission Grating is typically used in conjunction with the ACIS detector and the Low Energy Transmission Grating with the HRC. 2. Architecture

The simulator control hierarchy is depicted in Figure 2. The simulators run as a set of UNIX processes, each of which represents a physical comp onent b eing modeled. These processes are started, monitored, and stopp ed by the ASCDS Pip eline Controller. The simulators utilize common ASCDS libraries where p ossible, such as the IRAF parameter interface. Events (photons) are passed from one process in the pip eline to the next with each process p erforming some necessary action on an event b efore passing it along. The action may b e to alter the event or to decide not to propagate it. The simulators are implemented primarily in C, with some supp orting code written in Perl. 3. Conclusions

The software is designed to easily accommodate modifications and enhancements. The modular pip eline approach has facilitated the interchange of modules as we continue to upgrade the fidelity and capabilities of the simulations. In order to further improve flexibility and provide access to data in the simulator pip elines, a C++ Application Program Interface to the simulator data stream is b eing prototyp ed. Acknowledgments. This pro ject is supp orted by NASA contract NAS839073 (ASC). We would like to thank the following individuals for their contributions to the simulations. Terry Gaetz, Diab Jerius, and Dan Nguyen develop ed


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SIMULATION TEST DATABASE ENTRY CONTROL SCRIPT OUTPUT FILES

SET PARAMETERS

COLLECT PIPELINES

PIPELINE CONTROLLER

MODULE 1

MODULE 2

MODULE N

PARAMETERS 1

PARAMETERS 2

PARAMETERS N

Figure 2. Simulation Control Hierarchy. The hierarchy of program control is depicted. Using database entries describing the test to b e p erformed, parameters describing the configuration are set. The ASCDS pip eline controller initiates the raytrace and monitors program execution. the mirror and source models. John Davis, Dan Nguyen, and Mike Wise develop ed the gratings model. Diab Jerius develop ed the HRC p ore surface model. Dave Plummer develop ed the HRC Telemetry dead time simulation. Adam Dobrzycki develop ed the HRC charge grid algorithm. References Janesick, J. R., Elliot, T., Collins, S., Taher, 1987, Optical Engineering, 26, 2, 156 Janesick, J. R., Elliot, T., Bredthauer, R., SPIE, 982, 70 Hopkinson, G. R. 1987, Optical Engineering, Lumb, D., Townsley, L., Nousek, J., Burrows communication D., Campb ell, D., & Garmire, G. Chandler, C., & Burke, B. 1988, 26, 8, 766 , D., & Corb et, R., 1994, p ersonal