Coordinates

The sunpy.coordinates sub-package contains:

• A robust framework for working with solar-physics coordinate systems

• Functions to obtain the locations of solar-system bodies (sunpy.coordinates.ephemeris)

• Functions to calculate Sun-specific coordinate information (sunpy.coordinates.sun)

The sunpy coordinate framework extends the Astropy coordinates framework.

This page contains an overview of how coordinates work in sunpy. See the following pages for detailed discussion of specific topics:

• Calculating Carrington longitude
• Differential rotation using coordinate frames
• Coordinates with velocity information
• Coordinates and WCS
• The role of rsun in coordinate transformations

Design of the Coordinates Sub-Package

This sub-package works by defining a collection of “Frames” (sunpy.coordinates.frames), which exists on a transformation graph, where the transformations between the coordinate frames are then defined and registered with the transformation graph (sunpy.coordinates). It is also possible to transform SunPy frames to Astropy frames.

Positions within these “Frames” are stored as a “Representation” of a coordinate, a representation being a description of a point in a Cartesian, spherical or cylindrical system (see Using and Designing Coordinate Representations). A frame that contains a representation of one or many points is said to have been ‘realized’.

For a more in depth look at the design and concepts of the Astropy coordinates system see Overview of astropy.coordinates Concepts

Frames and SkyCoord

The SkyCoord class is a high level wrapper around the astropy.coordinates sub-package. It provides an easier way to create and transform coordinates, by using string representations for frames rather than the classes themselves and some other usability improvements. For more information see the SkyCoord documentation.

The main advantage provided by SkyCoord is the support it provides for caching Frame attributes. Frame attributes are extra data specified with a frame, some examples in sunpy.coordinates are obstime or observer for observer location. Only the frames where this data is meaningful have these attributes, i.e., only the Helioprojective frames have observer. However, when you transform into another frame and then back to a projective frame using SkyCoord it will remember the attributes previously provided, and repopulate the final frame with them. If you were to do transformations using the Frames alone this would not happen.