[mcstas-users] Disk chopper angular definitions

[Andrew Nelson]

I'm trying to simulate up a 4 disk chopper system, as used in Platypus
(ANSTO) or FIGARO (ILL). For those disks the window opening is at 3
o'clock, rather than 12 o'clock.

To account for this I rotate the DiskChopper component as "ROTATED (0, 0,
+90) ABSOLUTE". When I run a trace the disk window is rotated clockwise to
3 o'clock, which is what I wanted, and which indicates that a positive
angle refers to a clockwise rotation (i.e. a right hand coordinate system).
I therefore assume that a positive rotation frequency (`nu > 0` corresponds
to a clockwise rotation when looking from the source to the detector (from
-z to +z). Is this correct?

However, when I set the `phase` of the DiskChopper to +30 the window of the
disk in the trace appears to have moved anticlockwise when looking down the
beamline. This is the opposite to what I would have expected, +ve angle
moving things clockwise. It's not clear to me what phase means in this
situation, can someone clear up my confusion please?

Andrew.
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