The design of the HETG has required ray-trace software in addition to
simple error budgeting.  Tom Markert initiated these efforts by
using Webster Cash's IRT (interactive Ray-Trace) IDL package.  This
IRT framework was also used for the ``error-budget-ray-trace'' work
(to be) described in the previous section.  In addition, IDL lookup table
software ({\tt axaf1.pro}) was developed by the HETG team for effective
area studies and simulations.  With the addition of ASC personel at
MIT these HETG modeling efforts evolved into a pseudo ray-trace
IDL code, {\tt sim}, which was then ported to C ({\tt csim}).  Because
of in-house physics/software expertise (John Davis), {\tt csim} evolved
from an error-budget-efficiency combination into a full up ray-trace
code.  Finally, for release as an ASC product {\tt csim} was renamed \mx.

\mx, therefore, is the main HETG ray-trace code and is a
joint ASC/HETG effort.  It is planned that \mx
embody all relevant effects for HETGS modeling and simulation.
Please see {\tt http://space.mit.edu/ASC/software/software.html} for
more information on \mx.

\subsection{XRCF Simulations with \mx ~}
\label{sec:xrcf_marx}

Custom IDL code was produced to
create modified MARX parameters for a given XRCF measurement.  Some
items of note in this respect are:

\begin{itemize}
\item  The modeled spectrum is used as the SpectrumFile
with SourceFlux=0.0.  
\item  Generally the ExposureTime is set to 0.0 and
the simulation controlled by the NumRays.  
\item  The source distance is set to 537.583 meters;
this is the sum of the source-to-HRMA distance, 527.522 meters, and
the HRMA-to-infinite-source-focus distance in \mx ~of 10.061 meters.  
\item  A finite source size is modeled with SourceType=''DISK''
and S-DiskTheta1=0.0959.  
\item  The nominal in-focus detector position
is offset to -194.872 mm.  
\item   The MARX grating RowlandDiameter is set to the
actual XRCF value minus the 194.872 mm focus offset. 
\item  To simulate the HXDS detectors, the HRC-I 
is used as the detector for its large, planar field.
Additionally HRC-I-BlurSigma=0.0 and DetIdeal=``yes''. 
\item  The IDL s/w post-processes the simulated events
to apply the actual detector quantum efficiency.  
\item  The MARX coordinates are flight coordinates and
hence are rotated by 180 degrees about the X-axis from the XRCF
coordinate system.  
\end{itemize}




\begin{quotation}
{\tt To-do:} \\
Cross-check \mx with SAOSAC simulations for XRCF and flight conditions \\
Compare \mx output with XRCF data \\
Include MEG mis-aligned gratings into \mx. \\
Evaluate the need and method to include HEG scattering in \mx. \\
Generate LRFs for LRF parametrization and RMF creation \\
\end{quotation}