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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
The SAO AXAF Simulation System
D. Jerius, M. Freeman, T. Gaetz, J. P. Hughes, and W. Podgorski
Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA
02138
Abstract. As part of our efforts to support the AXAF program, the
SAO AXAF Mission Support Team has developed a software suite to
simulate the AXAF telescope. The software traces the fate of photons
through the telescope, from the X­ray source through apertures, baffles,
the telescope optics, and finally to the photons' ultimate interactions with
the focal plane detectors. We model relevant physical processes, including
geometrical reflection, scattering due to surface microroughness, distor­
tions of the optics due to the mirror mounts, attenuation through baffles,
etc. The software is composed of programs and scripts, each specialized to
a given task, which communicate through UNIX pipes. Software tasks are
centered about functional components of the telescope (e.g., apertures,
mirrors, detectors) and provide a comfortable and flexible paradigm for
performing simulations. The use of separate programs and the UNIX pipe
facility allows great flexibility in building different configurations of the
telescope and distilling diagnostics from the photon stream through the
telescope. We are able to transparently use symmetric multi­processing
(e.g., SPARCStation 10s and SGI Challenges) and can easily use sequen­
tial multi­processing (via workstation clusters). Some of the tasks are
amenable to parallel processing and have been implemented using the
MPI standard.
1. Introduction
As part of SAO's support of the design and calibration of the Advanced X­ray
Astrophysical Facility (AXAF ), we have undertaken the task of simulating the
optical performance of the telescope. We seek to create a high fidelity model
which takes into account the major factors which will impact the optics' X­
ray performance. These include distortions of the optics due to mechanical
and thermal stresses in the mounting assembly, manufacturing figure errors in
the optics, and scattering due to mirror surface microroughness and surface
contamination.
We have a far­ranging set of investigations which we are, and will be pur­
suing with this system. These investigations range from studies of requirements
for calibration, to studies of the impact of detector characteristics upon the ob­
servation of astrophysical sources. Among the issues which we are currently
investigating are: (1) the ability of the telescope, as designed, to achieve its
performance goals; (2) the performance of the mirror assembly during testing at
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the Marshall Space Flight Center X­Ray Calibration Facility; (3) the develop­
ment of models for the on­orbit science performance of the telescope based upon
ground calibration data; (4) the performance of flight instruments; and (5) the
efficacy of ground and on­orbit calibration procedures.
2. System Design
The diverse set of studies which we must perform and the goals of the AXAF
calibration effort require of our software system a high degree of flexibility and
efficiency. Because of limited human resources, we have striven for the former,
while keeping open our options for the latter.
AXAF is a complicated system, and the mirror assembly and assorted struc­
tures comprise a highly complex set of obstructions, apertures, and reflective
surfaces. In order to understand how the various components affect each other,
and to diagnose problems (both with our software and the hardware designs),
we must be able to insert or remove routines which model the apertures, baffles,
collimators, and detectors, or perform diagnostics, at any point in the photons'
path. At the same time, we must retain enough efficiency so that we can trace
¸ 10 7 rays through the system on a routine basis with a reasonable turn­around
time. To cap it off, we need to be able to encapsulate existing software which
may not follow our final paradigm. For example, our surface reflection module
is based upon a mature fortran program written in the days of mainframes
and minimal core memory.
One possible approach is to develop a single, monolithic, program, which
follows a ray from generation to collection. This avoids interim output (on
the order of hundreds of MB) and is quite efficient. We rejected this approach
early on, however, because it would require a complicated scheme to control
the various system components and allow us the flexibility we desire. From a
software development point of view, it forces a recompilation and re­verification
each time a new component is added. In an environment composed of scientists,
engineers, and programmers with different levels of programming sophistication,
ensuring the integrity of such a system would be a nightmare.
Instead, we chose to model each optical or structural component as a sepa­
rate program, based on the UNIX filter paradigm. A program reads a ray from
its standard input stream, manipulates it and writes it to its standard output
stream. The rays are encoded as binary data in dynamically extensible records,
allowing routines to add more information to the ray packet, as necessary. The
programs use a common user interface (based upon the IRAF parameter in­
terface) with parameter storage. As programs are completely independent from
each other, validation and verification are quite straightforward. Programs com­
municate with each other via UNIX pipes, which are easily accessible via the
standard UNIX shells. We provide higher level control by embedding the com­
mands in UNIX shell or Perl scripts. We have created a Perl library to easily
interface our Perl scripts to the parameter interface, presenting a uniform envi­
ronment to the user.
In some instances efficiency has won over flexibility, and we have combined
several ``atomic'' operations into one program. One example of this is the pro­
gram which simulates apertures. At the front end of the mirror assembly is a

3
set of thermal baffle plates, which act to control the thermal environment inside
the mirror assembly. It consists of some nine circular plates, with annular aper­
tures to let the photons pass through to the mirrors. Rather than model each
plate with a separate program, our aperture program can deal with any number
of plates. In addition, we have designed and are prototyping an aperture pro­
gram which uses an embedded language and a flexible script­driven approach to
assemble complex apertures from simple building blocks.
3. Components
As an illustration of how the individual programs work together to model the
telescope, we present a short synopsis of the major modules, in the order of their
affect upon an incoming photon.
3.1. Ray Generation
The ray generator spews forth rays from a variety of sources, constraining the
ray directions to fully illuminate the entrance aperture of the telescope. Sources
may be placed at any location in the telescope's field of view, and may have
any of several simple geometrical shapes. They may also be diffuse, simulating
all­sky background radiation or extended X­ray sources.
3.2. Apertures
The aperture module, as noted above, is used to model both optical and thermal
baffles, support structures, and the like. Surfaces may be partially transparent
(via input of an absorption depth), and may be composed of struts and an­
nular plates. Complicated apertures will be possible with our new hierarchical
aperture module, which can use overlapping geometrical shapes and ``cookie­
cutters''.
3.3. Surface Reflection
The surface reflection program is based upon a NASA routine (the ``Optical
Surface Analysis Code'', or OSAC). OSAC can handle deformed optics, with
deformations approximated by Fourier (azimuthal) and Legendre (axial) poly­
nomials. We have built upon the deformed optics module (which we have re­
named as SAOsac), integrating it into our filter­based environment, and have
extended it to use bi­cubic splines as well polynomials. Deformation in the op­
tics is caused by loads at the mirror support points, due either to thermal or
mechanical stresses and (in the case of ground calibration) a one­g gravitational
field. The deformations are modeled by a highly detailed finite­element model of
the mirror assembly. An IDL program (TRANSFIT, Freeman 1993) was writ­
ten which takes output from the finite­element modeling code (ANSYS) and
interactively fits the distortions in the optics.
3.4. Scattering due to Surface Microroughness and Contamination
After a ray's reflection from an optic has been determined, high frequency mirror
roughness (surface microroughness) is modeled by scattering the ray according
to a statistical model of the surface roughness. The model is based upon the

4
measured point­spread distribution (PSD) of high­frequency surface height fluc­
tuations of the optical surfaces. A similar process is used to model scattering
due to surface contamination.
3.5. Ray Accumulation
There is a bevy of detectors which will be used both on orbit and at ground cal­
ibration, including gas proportional counters, CCD's, solid­state spectrometers,
and microchannel plate detectors. We have developed models of these detectors
to provide us with end­to­end predictions of the telescope performance. In one
case (modeling scans of the ray distribution through circular apertures) we were
able to parallelize the aperture­detector combination (using the MPI standard),
allowing us to gang several workstations together (Nguyen & Hillberg 1995).
4. Conclusion
This system permits an essentially unlimited numbers of rays and all of the flex­
ibility that one could desire. At any point between programs we can siphon off
rays for diagnostics or analysis. Manipulating configurations is simple and intu­
itive. Writing detector simulators or diagnostic programs is straightforward. As
an added advantage, when running on multiprocessor computers with symmet­
ric processing (e.g., Solaris 2 or IRIX 5), the system will automatically switch
processes to unused CPU's. For CPU dominated calculations, we can chain
multiple machines together, each operating on one part of our process chain and
passing the result on to the next machine.
The paradigm has proven highly successful, enabling us to provide high pre­
cision results without a complicated setup procedure to construct the particular
configuration of the telescope that we desire. The efficiency (even allowing for
the overhead of the inter­process communications) is reasonable, and on modern
workstations has not a proven a barrier to our need for rapid turn­around.
References
Freeman, M., 1993 TRANSFIT ­ Finite Element Analysis Data Fitting Soft­
ware, Interim Report SAO­AXAF­DR­93­052 (Cambridge, SAO)
Nguyen, D., & Hillberg, B. 1995, this volume, p. ??