.. auto-generated by script ../../../../utils/nxdl2rst.py from the NXDL source NXdisk_chopper.nxdl.xml

.. index::
    ! NXdisk_chopper (base class)
    ! disk_chopper (base class)
    see: disk_chopper (base class); NXdisk_chopper

.. _NXdisk_chopper:

==============
NXdisk_chopper
==============

**Status**:

  base class, extends :ref:`NXobject`

**Description**:

  A device blocking the beam in a temporal periodic pattern.

  A disk which blocks the beam but has one or more slits to periodically
  let neutrons through as the disk rotates. Often used in pairs, one
  NXdisk_chopper should be defined for each disk.

  The rotation of the disk is commonly monitored by recording a timestamp for
  each full rotation of disk, by having a sensor in the stationary disk housing
  sensing when it is aligned with a feature (such as a magnet) on the disk.
  We refer to this below as the "top-dead-center signal".

  Angles and positive rotation speeds are measured in an anticlockwise
  direction when facing away from the source.

**Symbols**:

  This symbol will be used below to coordinate datasets with the same shape.

  **n**: Number of slits in the disk

**Groups cited**:
  :ref:`NXgeometry`

.. index:: NXgeometry (base class); used in base class

**Structure**:

  .. index:: type (field)

  **type**: (optional) :ref:`NX_CHAR <NX_CHAR>`

    Type of the disk-chopper: only one from the enumerated list (match text exactly)

    Any of these values:

      * ``Chopper type single``

      * ``contra_rotating_pair``

      * ``synchro_pair``


  .. index:: rotation_speed (field)

  **rotation_speed**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_FREQUENCY <NX_FREQUENCY>`}

    Chopper rotation speed. Positive for anticlockwise rotation when
    facing away from the source, negative otherwise.

  .. index:: slits (field)

  **slits**: (optional) :ref:`NX_INT <NX_INT>`

    Number of slits

  .. index:: slit_angle (field)

  **slit_angle**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_ANGLE <NX_ANGLE>`}

    Angular opening

  .. index:: pair_separation (field)

  **pair_separation**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_LENGTH <NX_LENGTH>`}

    Disk spacing in direction of beam

  .. index:: slit_edges (field)

  **slit_edges[2n]**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_ANGLE <NX_ANGLE>`}

    Angle of each edge of every slit from the position of the
    top-dead-center timestamp sensor, anticlockwise when facing
    away from the source.
    The first edge must be the opening edge of a slit, thus the last edge
    may have an angle greater than 360 degrees.

  .. index:: top_dead_center (field)

  **top_dead_center**: (optional) :ref:`NX_NUMBER <NX_NUMBER>` {units=\ :ref:`NX_TIME <NX_TIME>`}

    Timestamps of the top-dead-center signal. The times are relative
    to the "start" attribute and in the units specified in the "units"
    attribute. Please note that absolute
    timestamps under unix are relative to ``1970-01-01T:00:00``.

    .. index:: start (field attribute)

    **@start**: (optional) :ref:`NX_DATE_TIME <NX_DATE_TIME>`


  .. index:: beam_position (field)

  **beam_position**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_ANGLE <NX_ANGLE>`}

    Angular separation of the center of the beam and the
    top-dead-center timestamp sensor, anticlockwise when facing
    away from the source.

  .. index:: radius (field)

  **radius**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_LENGTH <NX_LENGTH>`}

    Radius of the disk

  .. index:: slit_height (field)

  **slit_height**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_LENGTH <NX_LENGTH>`}

    Total slit height

  .. index:: phase (field)

  **phase**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_ANGLE <NX_ANGLE>`}

    Chopper phase angle

  .. index:: delay (field)

  **delay**: (optional) :ref:`NX_NUMBER <NX_NUMBER>` {units=\ :ref:`NX_TIME <NX_TIME>`}

    Time difference between timing system t0 and chopper driving clock signal

  .. index:: ratio (field)

  **ratio**: (optional) :ref:`NX_INT <NX_INT>`

    Pulse reduction factor of this chopper in relation to other
    choppers/fastest pulse in the instrument

  .. index:: distance (field)

  **distance**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_LENGTH <NX_LENGTH>`}

    Effective distance to the origin

  .. index:: wavelength_range (field)

  **wavelength_range[2]**: (optional) :ref:`NX_FLOAT <NX_FLOAT>` {units=\ :ref:`NX_WAVELENGTH <NX_WAVELENGTH>`}

    Low and high values of wavelength range transmitted

  **(geometry)**: (optional) :ref:`NXgeometry`


**NXDL Source**:
  https://github.com/nexusformat/definitions/blob/master/base_classes/NXdisk_chopper.nxdl.xml