[mcstas-users] Disk chopper angular definitions

[Andrew Nelson]

Thanks for the clarifications, good to know that I wasn't going crazy.

The chopper axle is at the same height as the beam for the instrument I'm
simulating (i.e. beam at 3 or 9 o'clock). I want to understand whether the
direction of rotation makes a difference for an optically blind system;
leading edge of second chopper lines up with trailing edge of main chopper,
with T0 at the end of the main chopper. The direction of rotation could
matter because of gravity.

Hopefully previous simulations using DiskChopper were careful to check that
their timing was correct.

On Fri, 11 Jun 2021 at 21:45, Peter Kjær Willendrup <pkwi at fysik.dtu.dk>
wrote:

> Dear Andrew,
>
>
> Thanks for raising the issue!
>
> I have had a closer look and also written a small test-instrument myself,
> see attached. This instrument contains a source that fully illuminates a
> chopper disk in the time-interval of [0 1]s, and with exposed parameters to
> set phase, delay, nu etc.
>
> I have the following findings:
>
> 1) Looking at the outputs, setting the default input-parameters it does in
> fact look like the order of rotation is left-handed rather than
> right-handed, see attached plot1.png, run with a 1Hz chopper
>
> 2) Putting phase to 90 degrees and parking the chopper there (nu=0), it
> further looks like the geometry as visualised using mcdisplay is opposite
> to what the resulting simulation output shows, see mcdisplay.png  and
> plot2.png
>
>
>
> I will file a us a GitHub issue so that we remember to do something about
> this, likely the below solution as a number of existing instrument-models
> use the DiskChopper exactly as it looks today:
>
> 1) Fix the documentation so that it is clear that the default “sign” of
> rotation is left-handed
>
> 2) Fix mcdisplay so that it reflects what the neutrons will “see” at t=0
>
> 3) Potentially add a sign-parameter to choose the “handedness”
>
>
> Best and thanks for reporting,
> Peter
>
>
>
>
>
>
>
>
>
> > On 11 Jun 2021, at 04.35, Andrew Nelson <andyfaff at gmail.com> wrote:
> >
> > Dear Markus,
> > thank you for the reply. Perhaps naively I would expect the trace to
> correspond to what happens with the actual simulation.
> >
> > I have done something along the lines you suggest, see end of email. The
> TOF monitor is shifted along the +ve x direction, which is to the right
> when looking from the detector to the source (it'd be roughly where the 'h'
> label is in fig 6.1)
> >
> > When nu is positive the TOF monitor has a pulse distribution whose
> offset is shifted to positive times. I think this means that the midpoint
> of the disk window has already crossed the y-axis before it gets to the
> monitor. This must mean that the direction of rotation is clockwise when
> looking along z from +ve to -ve.
> >
> > When nu is negative the TOF monitor has a pulse distribution whose
> offset is shifted to negative times. This must mean that the direction of
> rotation is anticlockwise when looking along z from +ve to -ve.
> >
> > Is this assessment correct?
> >
> > If so, then this is not what I expect from a right hand coordinate
> system. The direction of rotation from this concept experiment is the
> opposite of what's displayed in figure 6.1 of the component manual.
> >
> >
> >
> >
> > ===============================================================
> > DEFINE INSTRUMENT template_simple(Par1=1)
> >
> > DECLARE
> > %{
> > %}
> >
> > INITIALIZE
> > %{
> > %}
> >
> > TRACE
> >
> > COMPONENT origin = Progress_bar()
> > AT (0, 0, 0) RELATIVE ABSOLUTE
> >
> > // insert components here (e.g. Insert -> Source -> ...)
> > COMPONENT source_div = Source_div(
> >     xwidth=0.02,
> >     yheight=0.05,
> >     focus_aw=1,
> >     focus_ah=1,
> >     lambda0=2.8,
> >     dlambda=0.1)
> > AT (0, 0, 0) RELATIVE PREVIOUS
> >
> > COMPONENT diskchopper = DiskChopper(
> >     theta_0=5,
> >     yheight=0.1,
> >     nu=24,
> > isfirst=1)
> > AT (0, 0, 0) RELATIVE PREVIOUS
> >
> > COMPONENT mon = TOF_monitor(
> >     filename="mon",
> >     xmin=0.005,
> >     xmax=0.01,
> >     ymin=-0.025,
> >     ymax=0.025,
> >     tmin=-500,
> >     tmax=500,
> > restore_neutron=1)
> > AT (0, 0, 0) RELATIVE PREVIOUS
> >
> > FINALLY
> > %{
> > %}
> >
> > END
> > _______________________________________________
> > mcstas-users mailing list
> > mcstas-users at mcstas.org
> > https://mailman2.mcstas.org/mailman/listinfo/mcstas-users
>
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