First tries using McStas

[Kristian Nielsen]

> Did I tell you about the difference in the angular profiles between
> beamline and McStas at the end of the tapered guide? Beamline gives me 
> a rather regular profile, while McStas calculates some extra maxima at
> about 11 minutes of arc (looks like Batman). The difference is not
> extreme but large enough to again think about the guide. I should send
> you the files if you want to have a look at them.

Ok, I finally got around to take a look at this (sorry for taking so
long). I think what you are seeing is an effect of the gab you have
between guide section 2 and 3:

    COMPONENT part2 = Guide2(
            w2 = 0.036522,
            l = 1.220,
            ...
      AT (0,0,2.782) RELATIVE a1 

    COMPONENT part3 = Guide2(
            w1 = 0.038644,
            l = 9.500,
            ...
      AT (0,0,4.202) RELATIVE a1 

There is a 20 cm long and about 2 mm wide gab between the two sections
in the horizontal direction. From the divergence detector, this gab is
seen at an angle of about 0.12 degrees, and that is precisely the
location of the two minima on the horizontal beam profile ...

To investigate this, I tried to run a modified version of your
simulation where the gab is closed:

    COMPONENT part2 = Guide2(
	    w2 = 0.036522,
	    l = 1.220,
      AT (0,0,2.782) RELATIVE a1 
    COMPONENT part3 = Guide2(
	    w1 = 0.036522,
	    l = 9.700,
      AT (0,0,4.003) RELATIVE a1 

With the gab closed, the horizontal profile is smooth. So it appears
safe to conclude that the gab is the cause of the strange minima.

If you look closely, you can see a similar effect in the vertical
direction, though it is much less pronounced (probably because of the
larger m value and perhaps the larger guide dimensions in this direction).

The question is then why you 'beamline' program does not show this
effect? I assume that this program also has the gab between the guide
sections?

 - Kristian.