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Ray generator for SciSim (GSIM) 4.0
M.W. Beijersbergen and H. Siddiqui
January 12, 2005
This is a paper version of the GSIM help. If possible, refer to the HTML version, which contains images
and hypertext links.
GSIM is the ray generator for SciSim. It forms part of the XMM Science Simulator SciSimfor the
XMM-Newtonsatellite.
This document contains information specic to GSIM only; for generic information about SciSim, see
the SciSim User Guide.
1

XMM-Newton Science Simulator Page: 2
Contents
1 Overview of GSIM 3
2 Usage 3
3 Conguration 3
3.1 Coordinate system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Illumination plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4 Parameters 4
4.1 Instrument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2 Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.3 sourcesComment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5 Ray generator tools 5
5.1 Impulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

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1 Overview of GSIM
This document describes version 2 of GSIM. The software makes use of the SSIM raystream module
which provides a uniform interface to pass ray information between various programs. This means that
the output of GSIM or MSIM can be used as input for RSIM or directly for ESIM. It is also possible to
use GSIM as a ray generator for RSIM or ESIM without using MSIM. Furthermore, the set of SciSim
Tools can be directly applied to the output of MSIM for example to obtain plots of the ray paths, spot
diagrams, eective area calculation, ray statistics etc.
2 Usage
The following command illustrates how to execute GSIM from the command line:
gsim --sources sources.dat < spsim.out > gsim.out
In this example, sources.dat is a le containing the source information (see the CSIMdocumentation
for a description of the sourcelist le), spsim.out is the attitude history of the Spacecraft, produced by
SPSIM, and gsim.out is the ray information generated (in binary format).
The output could instead be piped into MSIMand then to one of the instrument simulators, or directly
to an instrument simulator.
3 Conguration
In addition to the sourcelist, the following information needs to be specied:
 the energy range over which to generate rays
 the coordinate system for the rays
 a description of the illumination plane
The energy range allows the user to inhibit highly-energetic rays or rays of too low energy being processed
by another simulator module. The exposure time, combined with the source uxes and illumination area,
determines the total number of rays generated.
These can be specied by using the GSIM GUI, editing the GSIM section of the SciSim conguration
le,scisim.cfg, by using a customised conguration le, which can be specied on the command line using
the -c option (see the SciSim User Guide), or by specifying the parameters explicitly on the command
line using the `--' prex. For example, to set the exposure time from the command line:
gsim --exposureTime 100 --sources sourcelist.dat < spsim.out > gsim.out
To use the GUI, rst invoke SciSimand select the `Congure' drop-down menu. Click on the icon labelled
`RayGen' using the right mouse button.
See also the description of GSIM parameters(Sec. 4).

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3.1 Coordinate system
The coordinate system of GSIM is dened in terms of a rotation (deg), translation (mm) and scale
factor with respect to the spacecraft x, y and z axes and can be modied in addition to the conguration
parameters(Sec. 4).
This is described in detail in the SciSim User Guidedocumentation.
3.2 Illumination plane
The illumination plane is the plane where each ray starts. The point where the ray crosses the illumination
plane is chosen randomly. The direction of the ray is also sampled randomly from a distribution which
is dependant on the spatial property of the source.
The shape of the area covered by the rays in the illumination plane can be specied as a sector. The
origin of the center can also be specied. This allows for example the following illuminations:
 a ring that covers the combined apertures of all the shells
 a sector so that only a quadrant of the shells is illuminated
 a circle that is located o-center to model a pencil beam
 an innitely thin sector so that the rays enter in the yz plane
4 Parameters
This section lists parameters specic to GSIM. All of the parameters listed can be modied on the
command line or via the GUI. Note that the coordinate frame of GSIM can also be modied. See the
SciSim User Guidedocumentation.
4.1 Instrument
If the GUI is used, the following can be modied by selecting the `Instrument' tab:
 Minimum Energy
 Maximum Energy
These specify the energy boundaries, expressed in eV, for all rays produced by GSIM.
4.2 Illumination
If the GUI is used, the following can be modied by selecting the `Illumination' tab:

XMM-Newton Science Simulator Page: 5
 x : The position of the illumination plane along the spacecraft x-axis, with respect to the
position of the mirrors, in mm. A negative value indicates a position away from, and in
front of the focal plane.
 r : The radial position of the illumination region with respect to the x-axis GSIM, in mm.
 phi : The azimuthal position of the illuminated sector measured from the spacecraft y-axis,
in a positive direction towards the GSIM z-axis, in degrees.
 r0 : The inner radius of the illuminated section with respect to the x-axis of GSIM, in mm.
 r1 : The outer radius of the illuminated section with respect to the x-axis of GSIM, in mm.
 phi0 : The lower azimuthal bound for the illuminated section, in degrees. It is dened in
the same sense as phi.
 phi1 : The outer azimuthal bound for the illuminated section, in degrees. It is dened in
the same sense as phi.
4.3 sourcesComment
The parameter sourcesComment can be set to 0 in the conguration le. This prevents GSIM writing
the sourcelist into the output le. This might be used to reduce the size of the output le in the case of
a very long source list.
5 Ray generator tools
5.1 Impulse
A special ray generator impulse is provided to test the impulse of the simulators. impulse generates rays
for a monochromatic point source. It will read an ASCII le line by line, where each line should contain
the energy (in eV), the right ascension (deg) and declination (deg). It will produce a run for each line.
Note that since the rays are produced in separate runs, the tools can be used to analyse these runs
separately. For example,
impulse < impulse.dat | msim | rgasim | ssimraystat
will produce ray statistics for each source specied in the le impulse.dat le.
impulse has the following conguration parameters:
 flux : the source ux, in mm 2 s 1
 sourceX : the position of the source along the x-axis in mm.
 Elow, Ehigh, exposureTime, phi, phi0, phi1, r, r0, r1, seed : see GSIM
 the generic raytracing parameters (rotation, translation)
References