Using the WCS object¶
Let’s expand the WCS created in Getting Started by adding a distortion.
First create polynomial transform to represent distortion:
>>> import numpy as np
>>> from astropy.modeling.models import (Polynomial2D, Shift, Scale, Rotation2D,
... Pix2Sky_TAN, RotateNative2Celestial, Mapping)
>>> polyx = Polynomial2D(4)
>>> polyx.parameters = np.arange(15) * .1
>>> polyy = Polynomial2D(4)
>>> polyy.parameters = np.arange(15) * .2
>>> distortion = (Mapping((0, 1, 0, 1)) | polyx & polyy).rename("distortion")
>>> det2sky = (Shift(-10.5) & Shift(-13.2) | Rotation2D(0.0023) | \
... Scale(.01) & Scale(.04) | Pix2Sky_TAN() | \
... RotateNative2Celestial(5.6, -72.05, 180)).rename("det2sky")
Create an intermediate frame. The distortion transforms positions on the detector into this frame.
>>> from astropy import units as u
>>> from astropy import coordinates as coord
>>> from gwcs import coordinate_frames as cf
>>> focal_frame = cf.Frame2D(name="focal_frame", unit=(u.arcsec, u.arcsec))
>>> detector_frame = cf.Frame2D(name="detector", unit=(u.pix, u.pix))
>>> sky_frame = cf.CelestialFrame(reference_frame=coord.ICRS(), name='icrs')
Create the WCS pipeline and initialize the WCS:
>>> from gwcs import wcs
>>> pipeline = [(detector_frame, distortion),
... (focal_frame, det2sky),
... (sky_frame, None)
... ]
>>> wcsobj = wcs.WCS(pipeline)
>>> print(wcsobj)
From Transform
----------- ----------
detector distortion
focal_frame det2sky
icrs None
To see what frames are defined:
>>> print(wcsobj.available_frames)
['detector', 'focal_frame', 'icrs']
>>> wcsobj.input_frame
<Frame2D(name="detector", unit=(Unit("pix"), Unit("pix")), axes_names=('x', 'y'),
axes_order=(0, 1))>
>>> wcsobj.output_frame
<CelestialFrame(name="icrs", unit=(Unit("deg"), Unit("deg")), axes_names=('lon', 'lat'),
axes_order=(0, 1), reference_frame=<ICRS Frame>)>
Because the output_frame
is a CoordinateFrame
object we can get
the result of the WCS transform as an astropy.coordinates.SkyCoord
object and transform
them to other standard coordinate frames supported by astropy.coordinates
.
>>> skycoord = wcsobj(1, 2, with_units=True)
>>> print(skycoord)
<SkyCoord (ICRS): (ra, dec) in deg
(6.62759055, -68.75445668)>
>>> print(skycoord.transform_to("galactic"))
<SkyCoord (Galactic): (l, b) in deg
(306.31586901, -48.20968112)>
Some methods allow managing the transforms in a more detailed manner.
Transforms between frames can be retrieved and evaluated separately.
>>> distortion = wcsobj.get_transform('detector', 'focal_frame')
>>> distortion(1, 2)
(47.8, 95.60)
Transforms in the pipeline can be replaced by new transforms.
>>> from astropy.modeling.models import Shift
>>> new_transform = Shift(1) & Shift(1.5) | distortion
>>> wcsobj.set_transform('detector', 'focal_frame', new_transform)
>>> wcsobj(1, 2)
(10.338562883899195, -42.33182878519405)
A transform can be inserted before or after a frame in the pipeline.
>>> from astropy.modeling.models import Scale
>>> scale = Scale(2) & Scale(1)
>>> wcsobj.insert_transform('icrs', scale, after=False)
>>> wcsobj(1, 2)
(20.67712576779839, -42.33182878519405)
The WCS object has an attribute domain
which describes the range of
acceptable values for each input axis.
>>> wcsobj.bounding_box = ((0, 2048), (0, 1000))