# SunPy wcs¶

## sunpy.wcs Package¶

The WCS package provides functions to parse World Coordinate System (WCS) coordinates for solar images as well as convert between various solar coordinate systems. The solar coordinates supported are

• Helioprojective-Cartesian (HPC): The most often used solar coordinate
system. Describes positions on the Sun as angles measured from the center of the solar disk (usually in arcseconds) using cartesian coordinates (X, Y)
• Helioprojective-Radial (HPR): Describes positions on the Sun using angles,
similar to HPC, but uses a radial coordinate (rho, psi) system centered on solar disk where psi is measured in the counter clock wise direction.
• Heliocentric-Cartesian (HCC): The same as HPC but with positions expressed
in true (deprojected) physical distances instead of angles on the celestial sphere.
• Heliocentric-Radial (HCR): The same as HPR but with rho expressed in
true (deprojected) physical distances instead of angles on the celestial sphere.
• Stonyhurst-Heliographic (HG): Expressed positions on the Sun using
longitude and latitude on the solar sphere but with the origin which is at the intersection of the solar equator and the central meridian as seen from Earth. This means that the coordinate system remains fixed with respect to Earth while the Sun rotates underneath it.
• Carrington-Heliographic (HG): Carrington longitude is offset
from Stonyhurst longitude by a time-dependent scalar value, L0. At the start of each Carrington rotation, L0 = 360, and steadily decreases until it reaches L0 = 0, at which point the next Carrington rotation starts.

Some definitions

• b0: Tilt of the solar North rotational axis toward the observer
(helio- graphic latitude of the observer). Note that SOLAR_B0, HGLT_OBS, and CRLT_OBS are all synonyms.
• l0: Carrington longitude of central meridian as seen from Earth.
• dsun_meters: Distance between observer and the Sun. Default is 1 AU.
• rsun_meters: Radius of the Sun in meters. Default is 6.955e8 meters. This valued is stored locally in this module and can be modified if necessary.

### References¶

Thompson (2006), A&A, 449, 791 <http://dx.doi.org/10.1051/0004-6361:20054262>

### Functions¶

 convert_data_to_pixel(x, y, scale, ...) Calculate the pixel indices for a given data coordinate. convert_hcc_hg(x, y[, z, b0_deg, l0_deg, radius]) Convert from Heliocentric-Cartesian (HCC) (given in meters) to Stonyhurst Heliographic coordinates (HG) given in degrees, with radial output in meters. convert_hcc_hpc(x, y[, dsun_meters, angle_units]) Convert Heliocentric-Cartesian (HCC) to angular Helioprojective-Cartesian (HPC) coordinates (in degrees). convert_hg_hcc(hglon_deg, hglat_deg[, ...]) Convert from Stonyhurst Heliographic coordinates (given in degrees) to Heliocentric-Cartesian coordinates (given in meters). convert_hg_hpc(hglon_deg, hglat_deg[, ...]) Convert from Heliographic coordinates (HG) to Helioprojective-Cartesian (HPC). convert_hpc_hcc(x, y[, dsun_meters, ...]) Converts from Helioprojective-Cartesian (HPC) coordinates into Heliocentric-Cartesian (HCC) coordinates. convert_hpc_hg(x, y[, b0_deg, l0_deg, ...]) Convert from Helioprojective-Cartesian (HPC) to Heliographic coordinates (HG) in degrees. convert_pixel_to_data(size, scale, ...[, x, y]) Calculate the data coordinate for particular pixel indices. convert_to_coord(x, y, from_coord, to_coord) Apply a coordinate transform to coordinates. get_center(size, scale, reference_pixel, ...) Returns the center of the image in data coordinates. proj_tan(x, y[, force]) Applies the gnomonic (TAN) projection to intermediate relative coordinates.