1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
|
.. _initialization:
****************************************
Initializing axes with world coordinates
****************************************
Basic initialization
********************
To make a plot using `~astropy.visualization.wcsaxes.WCSAxes`, we first read in
the data using `astropy.io.fits
<https://docs.astropy.org/en/stable/io/fits/index.html>`_ and parse the WCS
information. In this example, we will use an example FITS file from the
http://data.astropy.org server (the
:func:`~astropy.utils.data.get_pkg_data_filename` function downloads the file
and returns a filename):
.. plot::
:context: reset
:nofigs:
:include-source:
:align: center
from astropy.wcs import WCS
from astropy.io import fits
from astropy.utils.data import get_pkg_data_filename
filename = get_pkg_data_filename('galactic_center/gc_msx_e.fits')
hdu = fits.open(filename)[0]
wcs = WCS(hdu.header)
We then create a figure using Matplotlib and create the axes using the
:class:`~astropy.wcs.WCS` object created above. The following example shows how
to do this with the Matplotlib 'pyplot' interface, keeping a reference to the
axes object:
.. plot::
:context:
:include-source:
:align: center
import matplotlib.pyplot as plt
ax = plt.subplot(projection=wcs)
The ``ax`` object created is an instance of the
:class:`~astropy.visualization.wcsaxes.WCSAxes` class. Note that if no WCS
transformation is specified, the transformation will default to identity,
meaning that the world coordinates will match the pixel coordinates.
The field of view shown is, as for standard matplotlib axes, 0 to 1 in both
directions, in pixel coordinates. As soon as you show an image (see
:doc:`images_contours`), the limits will be adjusted, but if you want you can
also adjust the limits manually. Adjusting the limits is done using the
same functions/methods as for a normal Matplotlib plot:
.. plot::
:context:
:include-source:
:align: center
ax.set_xlim(-0.5, hdu.data.shape[1] - 0.5)
ax.set_ylim(-0.5, hdu.data.shape[0] - 0.5)
.. note:: If you use the pyplot interface, you can also replace ``ax.set_xlim`` and
``ax.set_ylim`` by ``plt.xlim`` and ``plt.ylim``.
Alternative methods
*******************
As in Matplotlib, there are in fact several ways you can initialize the
:class:`~astropy.visualization.wcsaxes.WCSAxes`.
As shown above, the simplest way is to make use of the :class:`~astropy.wcs.WCS`
class and pass this to ``plt.subplot``. If you normally use the (partially)
object-oriented interface of Matplotlib, you can also do::
fig = plt.figure()
ax = fig.add_subplot(1, 1, 1, projection=wcs)
Note that this also works with :meth:`~matplotlib.figure.Figure.add_axes` and
:func:`~matplotlib.pyplot.axes`, e.g.::
ax = fig.add_axes([0.1, 0.1, 0.8, 0.8], projection=wcs)
or::
plt.axes([0.1, 0.1, 0.8, 0.8], projection=wcs)
Any additional arguments passed to
:meth:`~matplotlib.figure.Figure.add_subplot`,
:meth:`~matplotlib.figure.Figure.add_axes`,
:func:`~matplotlib.pyplot.subplot`, or :func:`~matplotlib.pyplot.axes`, such
as ``slices`` or ``frame_class``, will be passed on to the
:class:`~astropy.visualization.wcsaxes.WCSAxes` class.
.. _initialize_alternative:
Directly initializing WCSAxes
*****************************
As an alternative to the above methods of initializing
:class:`~astropy.visualization.wcsaxes.WCSAxes`, you can also instantiate
:class:`~astropy.visualization.wcsaxes.WCSAxes` directly and add it to the
figure::
from astropy.wcs import WCS
from astropy.visualization.wcsaxes import WCSAxes
import matplotlib.pyplot as plt
wcs = WCS(...)
fig = plt.figure()
ax = WCSAxes(fig, [0.1, 0.1, 0.8, 0.8], wcs=wcs)
fig.add_axes(ax) # note that the axes have to be explicitly added to the figure
|
