Defining positional information

Positional information is handles internally by astropy.coordinates.SkyCoord. This pages gives a quick overview on how to supply positional information.

Possible way to specify positions

In short:

• astropy.coordinates.SkyCoord instance (with or without a distance)

• A 2-tuple of x and y offsets (in arcsec) relative to the target center

• A dictionary of more specific information (see below), optionally including a distance

Offsets

The position and offset arguments are mutually exclusive. The exception to this is the Binary class, where position implicitly applies to the primary and offset to the secondary. Binaries with explicit positions for both components can be defined as a StarField. If neither is specified, a (relative) position of (0, 0) is assumed as a default.

Radial velocity

If given as a astropy.coordinates.SkyCoord instance (directly in Python or via YAML constructor), it is possible to also include a radial_velocity as a keyword argument (see Astropy docs). If used, it should be supplied as a Quantity and is used to shift the spectrum accordingly. For cosmological distances, the distance attribute is used to apply cosmological redshift to the spectrum. Note that this feature is currently experimental and not all target subclasses fully support it.