Overlaying coordinate systems

For the example in the following page we start from the example introduced in Initializing axes with world coordinates.

The coordinates shown by default in a plot will be those derived from the WCS or transformation passed to the WCSAxes class. However, it is possible to overlay different coordinate systems using the get_coords_overlay() method:

overlay = ax.get_coords_overlay('fk5')

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The object returned is a CoordinatesMap, the same type of object as ax.coord. It can therefore be used in the same way as ax.coord to set the ticks, tick labels, and axis labels properties:

ax.coords['glon'].set_ticks(color='white')
ax.coords['glat'].set_ticks(color='white')

ax.coords['glon'].set_axislabel('Galactic Longitude')
ax.coords['glat'].set_axislabel('Galactic Latitude')

ax.coords.grid(color='yellow', linestyle='solid', alpha=0.5)

overlay['ra'].set_ticks(color='white')
overlay['dec'].set_ticks(color='white')

overlay['ra'].set_axislabel('Right Ascension')
overlay['dec'].set_axislabel('Declination')

overlay.grid(color='white', linestyle='solid', alpha=0.5)

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Interior ticks and tick labels

The tick labels for an overlay grid can be difficult to associate correctly with gridlines because the default locations at the edges of the rectangular frame may result in multiple gridlines intersecting an edge near the same tick label or too few gridlines intersecting an edge. As with the base grid, it is possible to add interior ticks or tick labels for the overlay grid. Here we add a “tickable” gridline at constant RA (const-ra) and one at constant declination (const-dec). Note that when you use a multi-character string as the name for one of these gridlines, you need to specify that name as a part of a tuple to other methods.

from astropy.coordinates import Angle

overlay['ra'].grid(color='red')
overlay['dec'].grid(color='magenta')

overlay['ra'].add_tickable_gridline('const-ra', Angle('266d20m'))
overlay['dec'].add_tickable_gridline('const-dec', Angle('-29d00m'))

overlay['ra'].set_ticks_position(('const-dec', 't'))
overlay['ra'].set_ticks(color='red')
overlay['ra'].set_ticklabel_position(('const-dec',))
overlay['ra'].set_ticklabel(color='red', size=6)
overlay['ra'].set_axislabel_position('r')
overlay['ra'].set_axislabel('Right Ascension', color='red')

overlay['dec'].set_ticks_position(('const-ra', 'r'))
overlay['dec'].set_ticks(color='magenta')
overlay['dec'].set_ticklabel_position(('const-ra',))
overlay['dec'].set_ticklabel(color='magenta', size=6)
overlay['dec'].set_axislabel_position('t')
overlay['dec'].set_axislabel('Declination', color='magenta')

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