Controlling Axes

Changing Axis Units

WCSAxes also allows users to change the units of the axes of an image. In the example in Slicing Multidimensional Data, the x axis represents velocity in m/s. We can change the unit to an equivalent one by:

import astropy.units as u
ax.coords[2].set_major_formatter('x.x') # Otherwise values round to the nearest whole number
ax.coords[2].set_format_unit(u.km / u.s)
_images/controlling_axes-2.png

This feature is only for non-angular coordinate axes. To change the format of angles, refer to Tick label format.

Changing Axis Directions

Sometimes astronomy FITS files don’t follow the convention of having the longitude increase to the left, so we want to flip an axis so that it goes in the opposite direction. To do this on our example image:

ax.invert_xaxis()
_images/controlling_axes-3.png