Chopcal

Description

Exposes chopper calculations to Python which were otherwise hidden in McStas instruments.

Supported components

source name component type
Instr BIFROST chopcal.bifrost function
a set of choppers, printed as a table chopcal.lib.ChopperSet class
runtime library chopper-lib chopcal.lib submodule
chopper parameters chopcal.lib.Chopper class
transmitted inverse-velocity phase space chopcal.lib.inverse_velocity_windows function
transmitted inverse-velocity extrema chopcal.lib.inverse_velocity_limits function
transmitted wavelength extrema chopcal.lib.wavelength_limits function

Placing the band

BIFROST passes a fixed bandwidth of about 1.77 Å, so one number decides where that band sits. Give it as either the longest wavelength or the lowest energy you want to reach the sample — the band runs from there to about 1.77 Å shorter.

>>> import chopcal
>>> chopcal.bifrost(wavelength_max=3.0)
name  speed [Hz]  delay [ms]  opening [deg]  open [ms]  path [m]
 ps1         196     3.87771            170      2.409     6.342
 ps2         196       6.087            170      2.409     6.362
 fo1          14     6.13892          38.26      7.591      8.53
 fo2          14     9.54467          52.01      10.32    14.973
 bw1          14     42.8605            161      31.94        78
 bw2         -14     42.8605            161      31.94     78.02
>>> chopcal.lib.wavelength_limits(list(chopcal.bifrost(wavelength_max=3.0).values()))
(1, (1.2115739130694068, 2.9831347854552717))

Mind the sense of it: the number you give is the slow end of the band. Asking for energy_min=7 does not cap the incident energy at 7 meV — it puts 7 meV at the bottom and lets everything faster through, up to about 31 meV.

The choppers come back by name in beam order, each with a speed in Hz, a delay in seconds, an opening angle in degrees and a flight path in metres. delay is when an opening is centred on the beam, so a chopper passes neutrons for angle/360/|speed| seconds around it and again every 1/speed after that.

Installation

pip install chopcal

Or, from the source repository to get the latest development version

pip install git+https://github.com/g5t/chopper-calculations.git

Describing a chopper

A Chopper is speed, delay, angle and path: how fast the disk turns in Hz, when one of its openings is on the beam in seconds, how wide that opening is in degrees, and how far the disk sits from the source in metres.

delay is a time, so it means the same thing at any speed and in either direction of rotation — which is how a real chopper is set, and what McStas' DiskChopper acts on. It replaces the phase in degrees that chopper-lib took before its version 2.0.0, and which it converted to a delay at every point of use anyway.

Developing

The build fetches chopper-lib from GitHub. To build against a local checkout instead — which you need when changing both together — point FetchContent at it:

pip install --no-build-isolation -ve . \
  -Ccmake.define.FETCHCONTENT_SOURCE_DIR_CHOPPER_LIB=/path/to/mcstas-chopper-lib
pytest test

Nothing else changes: the same sources are compiled into the extension modules, so a modification to chopper-lib.c shows up in chopcal on the next build. chopper-lib carries a CHOPPER_LIB_VERSION macro and src/choppers.cpp asserts on it, so building against a version that means something else by a chopper's fields fails rather than returning different numbers.

Project links

Data verified by PyPI on Aug 23, 2026
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Avatar for g5t from gravatar.com g5t

Credits

Author: Gregory Tucker

GitHub Statistics

Data verified by PyPI on Aug 23, 2026
The GitHub source repository was provided by the project maintainers and verified by PyPI at the time of upload. Stars, forks, and open issues/PRs are derived from that repository and have not been independently verified.

License

BSD License

Requires

Python >=3.11

Classifiers

Report project as malware

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