# 3.3.1.27. NXguide¶

**Status**:

base class, extends NXobject

**Description**:

## A neutron optical element to direct the path of the beam. ...

A neutron optical element to direct the path of the beam.

NXguide is used by neutron instruments to describe a guide consists of several mirrors building a shape through which neutrons can be guided or directed. The simplest such form is box shaped although elliptical guides are gaining in popularity. The individual parts of a guide usually have common characteristics but there are cases where they are different. For example, a neutron guide might consist of 2 or 4 coated walls or a supermirror bender with multiple, coated vanes.

To describe polarizing supermirrors such as used in neutron reflection, it may be necessary to revise this definition of NXguide to include NXpolarizer and/or NXmirror.

When even greater complexity exists in the definition of what constitutes a

guide, it has been suggested that NXguide be redefined as a NXcollection of NXmirror each having their own NXgeometry describing their location(s).For the more general case when describing mirrors, consider using NXmirror.

NOTE: The NeXus International Advisory Committee welcomes comments for revision and improvement of this definition of NXguide.

**Symbols**:

nsurf: number of reflecting surfaces

nwl: number of wavelengths

**Groups cited**:

**Structure**:

@default: (optional) NX_CHAR## Declares which child group contains a path leading ...

Declares which child group contains a path leading to a NXdata group.

It is recommended (as of NIAC2014) to use this attribute to help define the path to the default dataset to be plotted. See https://www.nexusformat.org/2014_How_to_find_default_data.html for a summary of the discussion.

description: (optional) NX_CHARA description of this particular instance of

`NXguide`

.

incident_angle: (optional) NX_FLOAT {units=NX_ANGLE}TODO: documentation needed

bend_angle_x: (optional) NX_FLOAT {units=NX_ANGLE}TODO: documentation needed

bend_angle_y: (optional) NX_FLOAT {units=NX_ANGLE}TODO: documentation needed

interior_atmosphere: (optional) NX_CHARAny of these values:

`vacuum`

|`helium`

|`argon`

external_material: (optional) NX_CHARexternal material outside substrate

m_value: (optional) NX_FLOAT (Rank: 1, Dimensions: [nsurf])## The ``m`` value for a supermirror, which defines the supermirror ...

The

`m`

value for a supermirror, which defines the supermirror regime in multiples of the critical angle of Nickel.

substrate_material: (optional) NX_FLOAT (Rank: 1, Dimensions: [nsurf])TODO: documentation needed

substrate_thickness: (optional) NX_FLOAT (Rank: 1, Dimensions: [nsurf]) {units=NX_LENGTH}TODO: documentation needed

coating_material: (optional) NX_FLOAT (Rank: 1, Dimensions: [nsurf])TODO: documentation needed

substrate_roughness: (optional) NX_FLOAT (Rank: 1, Dimensions: [nsurf]) {units=NX_LENGTH}TODO: documentation needed

coating_roughness: (optional) NX_FLOAT (Rank: 1, Dimensions: [nsurf]) {units=NX_LENGTH}TODO: documentation needed

number_sections: (optional) NX_INT {units=NX_UNITLESS}## number of substrate sections (also called ``nsurf`` as an ...

number of substrate sections (also called

`nsurf`

as an index in the`NXguide`

specification)

depends_on: (optional) NX_CHAR

GEOMETRY: (optional) NXgeometry

DEPRECATED: Use the field depends_on and NXtransformations to position the guid and NXoff_geometry to describe its shape insteadTODO: Explain what this NXgeometry group means. What is intended here?

reflectivity: (optional) NXdataReflectivity as function of reflecting surface and wavelength

Obligatory value:

`data`

Obligatory value:

`surface wavelength`

@surface_indices: (optional) NX_CHARObligatory value:

`0`

@wavelength_indices: (optional) NX_CHARObligatory value:

`1`

data: (optional) NX_NUMBER (Rank: 2, Dimensions: [nsurf, nwl]) ⤆reflectivity of each surface as a function of wavelength

surface: (optional) NX_NUMBER (Rank: 1, Dimensions: [nsurf]) {units=NX_ANY}## List of surfaces. Probably best to use index ...

List of surfaces. Probably best to use index numbers but the specification is very loose.

wavelength: (optional) NX_NUMBER (Rank: 1, Dimensions: [nwl]) {units=NX_WAVELENGTH}wavelengths at which reflectivity was measured

OFF_GEOMETRY: (optional) NXoff_geometryThis group describes the shape of the beam line component

TRANSFORMATIONS: (optional) NXtransformations## This is the group recommended for holding the chain of translation ...

This is the group recommended for holding the chain of translation and rotation operations necessary to position the component within the instrument. The dependency chain may however traverse similar groups in other component groups.

## Hypertext Anchors¶

List of hypertext anchors for all groups, fields, attributes, and links defined in this class.