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Install and first band

Install

pip install chopcal

Wheels are published for Linux, macOS and Windows on CPython 3.11 and newer. There is nothing to compile and nothing else to install: chopcal has no runtime dependencies.

Two optional packages unlock extras, and only when you call something that needs them:

package what it adds
numpy the grid mask, and drawing samples in bulk
scipp Chopper.quantities, the fields with units attached

Set a train and read its band

BIFROST passes a fixed band about 1.91 Å wide, so a single number decides where it sits. Give it as the longest wavelength you want to reach the sample:

import chopcal

settings = chopcal.bifrost(wavelength_max=3.0)
print(settings)

name  speed [Hz]  delay [ms]  beam [deg]  opening [deg]  open [ms]  aperture [deg]  path [m]
 ps1         196      3.8781           0            170      2.409           5.651     6.342
 ps2         196      6.0874           0            170      2.409           5.651     6.362
 fo1          14     6.13997           0          38.26      7.591           5.651      8.53
 fo2          14     9.54764           0          52.01      10.32           5.651    14.973
 bw1          14     42.8823           0            161      31.94           13.38        78
 bw2         -14     42.8823           0            161      31.94           13.38     78.02

It is a dict in every respect — settings['ps1'], .values(), **settings all work — that prints as a table rather than as six memory addresses.

Now ask what those discs let through:

from chopcal.lib import wavelength_windows

bands = wavelength_windows(list(settings.values()))
low, high = bands[0]
print(f'{low:.4f} to {high:.4f} AA, {high - low:.4f} AA wide')

1.1454 to 3.0515 AA, 1.9062 AA wide

Note the sense of the argument: the number you give is the slow end. 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.

Read one disc

bandwidth = settings['bw1']
print(bandwidth)                 # said out loud
print(bandwidth.delay)           # seconds, not milliseconds
print(list(bandwidth.edges))     # degrees from the disk's zero mark

14 Hz anticlockwise, 161 deg opening centred on the beam on the beam at 42.88 ms, 78 m from the source
0.04288227154711843
[-80.5, 80.5]

delay is in seconds, not the milliseconds the table prints; edges are the disc's own slit angles in degrees. Describe a chopper goes through the rest of the fields.

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