#### Description:

This attribute controls how a SpecFrame behaves when it is used (by astFindFrame or astConvert) as a template to match another (target) SpecFrame. It identifies the standard of rest in which alignment is to occur. See the StdOfRest attribute for a desription of the values which may be assigned to this attribute. The default AlignStdOfRest value is `"` Helio`"` (heliocentric).

When astFindFrame or astConvert is used on two SpecFrames (potentially describing different spectral coordinate systems), it returns a Mapping which can be used to transform a position in one SpecFrame into the corresponding position in the other. The Mapping is made up of the following steps in the indicated order:

- Map values from the system used by the target (wavelength, apparent radial velocity, etc) to the system specified by the AlignSystem attribute, using the target`’` s rest frequency if necessary.

- Map these values from the target`’` s standard of rest to the standard of rest specified by the AlignStdOfRest attribute, using the Epoch, ObsLat, ObsLon, ObsAlt, RefDec and RefRA attributes of the target to define the two standards of rest.

- Map these values from the standard of rest specified by the AlignStdOfRest attribute, to the template`’` s standard of rest, using the Epoch, ObsLat, ObsLon, ObsAlt, RefDec and RefRA attributes of the template to define the two standards of rest.

- Map these values from the system specified by the AlignSystem attribute, to the system used by the template, using the template`’` s rest frequency if necessary.

Type:

  String.

#### Applicability

##### SpecFrame

All SpecFrames have this attribute.
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