[mcstas-users] Disk chopper angular definitions

[Markus APPEL]

Hi Andrew,

looking into the source code of DiskChopper, I think that the mcdisplay 
visualization of the DiskChopper might not always correspond to what it 
is really doing to the neutron beam in terms of the phase setting and 
rotation direction (visualization of the component is programmed 
independently of its TRACE section).

I suggest to check "experimentally" what the chopper is doing instead of 
relying on the component visualization, e.g. by putting a Monitor_nD 
right behind the chopper and do a 2D intensity image plot with TOF vs. 
y-position. Then it should be clear if the slit crosses the guide from 
top to bottom or vice versa, and how a different phase setting shifts 
the pulse.

If the goal of the non-zero phase setting is to account for the TOF 
delay of the neutrons between different choppers, it might be more 
convenient to use the 'delay' parameter.

Cheers,
Markus


On 10/06/2021 08:19, Andrew Nelson wrote:
> 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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