The `NXdisk_chopper` Component

A disc chopper with arbitrary openings,

Identification

Site:
Author: Gregory Tucker
Origin: European Spallation Source ERIC
Date: 2026

Description

One component per physical disc, however many openings it has.

`DiskChopper` describes `nslit` *identical, evenly spaced* openings
so a chopper whose openings are neither identical nor evenly spaced
has to be emitted as a GROUP of several components.
This component takes the edges directly, so one disc is one component
and corresponds to one NeXus group.

It also stops. `DiskChopper` replaces a zero frequency with `omega = 1e-15`,
which leaves the disc permanently *open* rather than parked; here `nu = 0`
means the disc is stationary with its zero mark at `park_angle`, and a ray
passes only if an opening happens to be in front of the beam. That is what a
commissioning run with a parked chopper actually does.

Geometry follows `DiskChopper`: the beam crosses the disc a distance
`radius - yheight/2` from the spindle, and rays are located by
`atan2(x, y + delta_y)` about that crossing.

Where the beam crosses the disc is `beam_angle`, measured from the zero mark in
the same sense as `slit_edges` and `park_angle`. The default of 0 puts the beam
at the top of the disc, which is the only thing `DiskChopper` can express -- so
a caller that used to turn the whole component about its own z to bring the
right part of the disc to the top can stop, and say the angle instead. A disc
hanging above its beam is `beam_angle = 180`.

Examples

(Test cases in bold)

Input parameters

Parameters in boldface are required; the others are optional.

Name	Unit	Description	Default
slit_edges	deg	opening and closing edge of each slit, from the disc's zero mark, in pairs
n_edges	1	number of entries in slit_edges; must be even and non-zero	0
radius	m	outer radius of the disc	0.35
yheight	m	radial clearance of the openings across the whole window; 0 means they reach the spindle	0
xwidth	m	chord width of the beam window; 0 leaves the opening unrestricted	0
nu	Hz	signed rotation frequency; 0 parks the disc at park_angle	0
delay	s	when the disc's zero mark is on the beam	0
park_angle	deg	where the zero mark stands while parked; only read when nu is 0	0
beam_angle	deg	where the beam crosses the disc, from the zero mark, in the same sense as slit_edges; 0 puts the beam at the top of the disc	0
zero_angle	deg	from the component's +y axis to the top-dead-centre pickup; with beam_angle it says which way the spindle lies	0
jitter	s	timing jitter applied to each ray	0
abs_out	1	absorb rays that miss the disc entirely	0
verbose	1	-1: silent, 0: errors, 1: warnings, 2: info, 3: details	0

AT (	,	,		) RELATIVE
ROTATED (	,	,		) RELATIVE

The NXdisk_chopper Component

Identification

Description

Examples

Input parameters

Links

The `NXdisk_chopper` Component