First tries using McStas

[artus at obiwan.kri.physik.uni-muenchen.de]

Hello Kristian,

thanks for this huge amount of information! I'll try to use the new
source and the new detector as soon as possible.

Now to the problem with the graphics. I have to confess that I didn't
realize that both PGPLOT packages are necessary! I thought it were just 
a newer and an older version (Sorry!). So I just tried to install 2.11 
but 'make' stops with an error:  

make: *** [PGPLOT.c] Error 139

?? Is the order of installation of the PGPLOT packages important? 'perl 
Makefile.PL' seems to be ok:

obiwan:/usr/local/PGPLOT-2.11 # perl Makefile.PL 
Found compiler g77
ExtUtils::F77: Using system=Linux compiler=G77
Checking for gcc in disguise
Compiler is gcc version 2.7.2.1ExtUtils::F77: Validating -L/usr/lib
 -L/usr/lib -lf2c -lm -L/usr/lib/gcc-lib/i686-pc-linux-gnulibc1/2.8.1 -lgcc   [ok]
ExtUtils::F77: Compiler: g77
ExtUtils::F77: Cflags: -O
Checking if your kit is complete...
Looks good
Writing Makefile for PGPLOT



Back to the guides: Following Murphy's law I interchanged the m's
describing the different reflectivity for the faces within
Guide2.comp! We here used the 'convention', that the side faces are
connected to the HORIZONTAL path of the neutrons and therefor we use
mh for the side faces. You use the terminology vice versa as I now assume?
We ran across this problem earlier here and now we always talk of top
and bottom and left and right to avoid misunderstandings... May be
this is worth noticing in the future documentation of Guide2.  

> It would probably be simpler to place one monitor at the guide entrance
and one at the exit. The transmission is then the ratio between the
intensities in the two monitors, mon2_I/mon1_I. The monitor at the
entrance must have the same dimensions as the guide opening, of
course.

Placing a monitor in front of the guide in fact is a good
idea. Dividing the final sum of weights by (solid angle at source *
4pi) also gives the same as long as you assign xw and yh for
Source_flat to the size of the guide entrance. 

I was a little bit astonished, that your slope alpha to describe the
reflectivity for mirrors is positive?
Beamline uses a slightly simpler way to describe the reflectivity. The 
only(?) difference is, that it doesn't model the sigmoid decay to
reflectivity 0. It just uses a cutoff. But according to the numbers,
this difference is negligible (if I didn't make any mistake):

Guide              Transmissions(%)
           McStas                Beamline
A          1.52                  1.57
B          2.84                  2.91

A: straight guide, 20x60, TB: m=3, LR: m=2, 11.5m long, 2 gaps
B: focussing guide 40x85 -> 20x60, TB: m=3, LR: m=2, 11.5m long, 2 gaps

I have also calculated three guides (straight section + diverging
section, focussing guide, straight guide, m's = 1 and 3)
for Ralph (he was also at the meeting) but I don't have his
transmissions for beamline. But I remember that the numbers were also
very close! Also all profiles looked absolutely the same!

Have a nice evening,

Georg