Getting started

Initialization

To make a plot using WCSAxes, we first read in the data using astropy.io.fits and parse the WCS information. In this example, we will use a FITS file from the wcsaxes.datasets module:

from astropy.wcs import WCS
from wcsaxes import datasets

hdu = datasets.fetch_msx_hdu()
wcs = WCS(hdu.header)

If you have the original FITS file, this is equivalent to doing:

from astropy.io import fits

hdu = fits.open('msx.fits')[0]
wcs = WCS(hdu.header)

We then create a figure using Matplotlib and create the axes using the WCS object created above:

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_axes([0.15, 0.1, 0.8, 0.8], projection=wcs)
_images/getting_started-2.png

The ax object created is an instance of the WCSAxes class. For more information about the different ways of initializing axes, see Initializing axes.

The field of view shown is, as for standard matplotlib axes, 0 to 1 in both directions, in pixel coordinates. The set_xlim() and set_ylim() methods can be used to re-set the pixel coordinates. For example, we can set the limits to the edge of the FITS image in pixel coordinates:

ax.set_xlim(-0.5, hdu.data.shape[1] - 0.5)
ax.set_ylim(-0.5, hdu.data.shape[0] - 0.5)
_images/getting_started-3.png

If no WCS transformation is specified, the transformation will default to identity, meaning that the world coordinates will match the pixel coordinates.

Plotting images and contours

Plotting images as bitmaps or contours should be done via the usual matplotlib methods such as imshow() or contour(). For example, continuing from the example in Initialization, you can do:

ax.imshow(hdu.data, vmin=-2.e-5, vmax=2.e-4, cmap=plt.cm.gist_heat,
          origin='lower')
_images/getting_started-4.png

and we can also add contours corresponding to the same image using:

import numpy as np
ax.contour(hdu.data, levels=np.logspace(-4.7, -3., 10), colors='white', alpha=0.5)
_images/getting_started-5.png

To show contours for an image in a different coordinate system, see Overplotting markers and artists.