NorthOffsetFrame

class sunpy.coordinates.NorthOffsetFrame[source] [edit on github]

Bases: object

A frame which is relative to some position and another frame. Based on astropy.coordinates.SkyOffsetFrame

Coordinates in a NorthOffsetFrame are both centered on the position specified by the north keyword and they are oriented in the same manner as the north frame.

Unlike SkyOffsetFrame a NorthOffsetFrame allows you to specify the position of the new north pole rather than the new origin to centre the new frame.

Examples

A common use for this is to create a frame derived from Heliographic, which has the north pole at some point of interest. In this new frame, lines of longitude form great circles radially away from the point, and lines of latitude measure angular distance from the point.

In this example the new frame is shifted so the new north pole is at (20, 20) in the Heliographic Stonyhurst frame. The new grid is overplotted in blue.

import matplotlib.pyplot as plt
import astropy.units as u

from astropy.coordinates import SkyCoord
from sunpy.coordinates import NorthOffsetFrame

import sunpy.map
from sunpy.data.sample import AIA_171_IMAGE

m = sunpy.map.Map(AIA_171_IMAGE)

north = SkyCoord(20*u.deg, 20*u.deg, frame="heliographic_stonyhurst")
new_frame = NorthOffsetFrame(north=north)

ax = plt.subplot(projection=m)
m.plot()

overlay = ax.get_coords_overlay('heliographic_stonyhurst')
overlay[0].set_ticks(spacing=30. * u.deg, color='white')
overlay.grid(ls='-', color='white')

overlay = ax.get_coords_overlay(new_frame)
overlay[0].set_ticks(spacing=30. * u.deg)
overlay.grid(ls='-', color='blue')

(Source code, png, hires.png, pdf)

../_images/sunpy-coordinates-NorthOffsetFrame-1.png

Notes

NorthOffsetFrame is a wrapper around the SkyOffsetFrame factory class. This class will calculate the desired coordianates of the origin from the north keyword argument and then create a SkyOffsetFrame.

Using this frame is equivalent to using SkyOffsetFrame with lat = lat - 90*u.deg for a position of the north pole in the original northern hemisphere.

This class will only work for Heliographic-Stonyhurst and Heliographic Carrington frames, and not helioprojective. If you want to rotate a helioprojective frame, it would be more natural to use the SkyOffsetFrame.