McStas

[Kristian Nielsen]

> Date: Mon, 06 Sep 1999 15:24:24 +0200
> From: "Pay H. Schulze Horn" <horn at ill.fr>

> After a more a week now here is the Component, which gives the
> Divergence in relation to the energy
> out. Change it for distribution like you want.

Ok, thanks, I will put it on the web page and probably later in the
distribution.

> The other monitor, which gives out everything, has small syntax-error
> which i cant't find.

Hm, if you want you can send it to me and I will have a look at it.

> Are there any know problems with the ROTATED-command?

> I had this output:
> Phi:4.19437 theta0:-60

> Why do i have a value of 4.19 degree as an divergence ?
> 
> Why not 2.1 ?
> 
> Why is it nearly the double value i was awaiting ?

> COMPONENT a2=Arm()
> 	AT (0,0,1) RELATIVE a1
> 	ROTATED (2.1,60,0) RELATIVE a1

I think you are being confused by the double rotation here. What happens
is that the axes of a2 are obtained from the axes of a1 as follows:
First rotate 2,1 degrees around the X axis of a1. Then rotate 60 degrees
around the _new_ Y axis. Note that order is important here; rotation in
the opposite order (first around Y, then around X) would be different.

You should not expect a value for phi of 2.1. For example, if the second
rotation is set to 90 degrees, phi will also be 90 degrees (the neutron
beam is then perpendicular to the Z axis of a2). At a rotation of 180
degrees, phi would be 180 - 2.1.

Clearly what you meant was to set up the coordinate system of a2 such
that the Z axis pointed in the direction (tan 60, tan 2.1, 1) in the
coordinate system of a1 (but where would X and Y then point?). I am not
sure that there is an easy way to do that in McStas currently, but I
will think about it when I start adding other ways to specify rotations.
The current way is somewhat stupid anyway, since it is almost never
useful to specify multiple rotations (because the outcome is really hard
to understand).

I hope this helps, otherwise just ask again.

 - Kristian.