Graphical Debugging in McStas

This page describes a new facility for doing graphical debugging of instrument simulations created with McStas. A schematic diagram of the instrument is drawn, and the path of each neutron as it passes through the instrument is plotted. The following are two examples (of a triple-axis spectrometer; these pictures clearly show a problem in the monochromator (orange circle)).

The instrument is shown as the red crosshairs connected with red lines. The components appear as circles of different colours. The neutron path is shows in green. A small dot in the same color as a component indicates the point where the neutron leaves that component. A new neutron is shown after a mouseclick or a keypress.

The graphic display is obtained by creating a special instrument simulation that produces debugging output about neutron states etc. The output of this simulation is then fed into the program "display", which reads the information and converts it into graphics using PGPLOT.

The instrument simulation with debugging output is obtained by defining the preprocessor macro DEBUG when compiling. For example

gcc -o instr -DDEBUG instr.c mcstas-r.c -lm

It may be instructive to run the generated simulation instr to see the kind of output produced.

To get the graphics display, the output of instr is fed into using a Unix pipe:

./instr | ./display
./instr | ./display multi

(the latter "multi" option views the instrument from three directions).

A hint when debugging instruments is to use another pipe to avoid the tedious entering of instrument parameters for each run. For example,

echo 0.12 0.45 1.2 0.123 1.5 0.1 | ./instr | ./display

(yes, angles are currently entered in radians, but this will probably change!). This is also useful to set up a whole scan in a simple shell script.

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Kristian Nielsen <kristian.nielsen@risoe.dk> / Oct 2, 1998.

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Last Modified: Tuesday, 22-Jun-2004 14:04:10 CEST