# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""WCS related utility functions."""
import numpy as np
from astropy.wcs import WCS
from astropy.coordinates import Angle
__all__ = [
"linear_wcs_to_arrays",
"linear_arrays_to_wcs",
"get_wcs_ctype",
"get_resampled_wcs",
]
[docs]def get_wcs_ctype(wcs):
"""
Get celestial coordinate type of WCS instance.
Parameters
----------
wcs : `~astropy.wcs.WCS`
WCS transformation instance.
Returns
-------
ctype : {'galatic', 'icrs'}
String specifying the coordinate type, that can be used with
`~astropy.coordinates.SkyCoord`
"""
ctype = wcs.wcs.ctype
if "GLON" in ctype[0] or "GLON" in ctype[1]:
return "galactic"
elif "RA" in ctype[0] or "RA" in ctype[1]:
return "icrs"
else:
raise TypeError("Can't determine WCS coordinate type.")
[docs]def get_resampled_wcs(wcs, factor, downsampled):
"""
Get resampled WCS object.
"""
wcs = wcs.deepcopy()
if not downsampled:
factor = 1.0 / factor
wcs.wcs.cdelt *= factor
wcs.wcs.crpix = (wcs.wcs.crpix - 0.5) / factor + 0.5
return wcs
[docs]def linear_wcs_to_arrays(wcs, nbins_x, nbins_y):
"""Make a 2D linear binning from a WCS object.
This method gives the correct answer only for linear X, Y binning.
The method expects angular quantities in the WCS object.
X is identified with WCS axis 1, Y is identified with WCS axis 2.
The method needs the number of bins as input, since it is not in
the WCS object.
Parameters
----------
wcs : `~astropy.wcs.WCS`
WCS object describing the bin coordinates
nbins_x : int
number of bins in X coordinate
nbins_y : int
number of bins in Y coordinate
Returns
-------
bin_edges_x : `~astropy.coordinates.Angle`
array with the bin edges for the X coordinate
bin_edges_y : `~astropy.coordinates.Angle`
array with the bin edges for the Y coordinate
"""
# check number of dimensions
if wcs.wcs.naxis != 2:
raise ValueError("Expected exactly 2 dimensions, got {}".format(wcs.wcs.naxis))
# check that wcs axes are linear
# TODO: is there an easy way to do this?
# set bins
unit_x, unit_y = wcs.wcs.cunit
delta_x, delta_y = wcs.wcs.cdelt
delta_x = Angle(delta_x, unit_x)
delta_y = Angle(delta_y, unit_y)
bin_edges_x = np.arange(nbins_x + 1) * delta_x
bin_edges_y = np.arange(nbins_y + 1) * delta_y
# translate bins to correct values according to WCS reference
# In FITS, the edge of the image is at pixel coordinate +0.5.
refpix_x, refpix_y = wcs.wcs.crpix
refval_x, refval_y = wcs.wcs.crval
refval_x = Angle(refval_x, unit_x)
refval_y = Angle(refval_y, unit_y)
bin_edges_x += refval_x - (refpix_x - 0.5) * delta_x
bin_edges_y += refval_y - (refpix_y - 0.5) * delta_y
# set small values (compared to delta (i.e. step)) to 0
for i in np.arange(len(bin_edges_x)):
if np.abs(bin_edges_x[i] / delta_x) < 1.0e-10:
bin_edges_x[i] = Angle(0.0, unit_x)
for i in np.arange(len(bin_edges_y)):
if np.abs(bin_edges_y[i] / delta_y) < 1.0e-10:
bin_edges_y[i] = Angle(0.0, unit_y)
return bin_edges_x, bin_edges_y
[docs]def linear_arrays_to_wcs(name_x, name_y, bin_edges_x, bin_edges_y):
"""Make a 2D linear WCS object from arrays of bin edges.
This method gives the correct answer only for linear X, Y binning.
X is identified with WCS axis 1, Y is identified with WCS axis 2.
Parameters
----------
name_x : str
name of X coordinate, to be used as 'CTYPE' value
name_y : str
name of Y coordinate, to be used as 'CTYPE' value
bin_edges_x : `~astropy.coordinates.Angle`
array with the bin edges for the X coordinate
bin_edges_y : `~astropy.coordinates.Angle`
array with the bin edges for the Y coordinate
Returns
-------
wcs : `~astropy.wcs.WCS`
WCS object describing the bin coordinates
"""
# check units
unit_x = bin_edges_x.unit
unit_y = bin_edges_y.unit
if unit_x != unit_y:
ss_error = "Units of X ({}) and Y ({}) bins do not match!".format(
unit_x, unit_y
)
ss_error += " Is this expected?"
raise ValueError(ss_error)
# Create a new WCS object. The number of axes must be set from the start
wcs = WCS(naxis=2)
# Set up DET coordinates in degrees
nbins_x = len(bin_edges_x) - 1
nbins_y = len(bin_edges_y) - 1
range_x = Angle([bin_edges_x[0], bin_edges_x[-1]])
range_y = Angle([bin_edges_y[0], bin_edges_y[-1]])
delta_x = (range_x[1] - range_x[0]) / nbins_x
delta_y = (range_y[1] - range_y[0]) / nbins_y
wcs.wcs.ctype = [name_x, name_y]
wcs.wcs.cunit = [unit_x, unit_y]
wcs.wcs.cdelt = [delta_x.to_value(unit_x), delta_y.to_value(unit_y)]
# ref as lower left corner (start of (X, Y) bin coordinates)
# coordinate start at pix = 0.5
wcs.wcs.crpix = [0.5, 0.5]
wcs.wcs.crval = [
(bin_edges_x[0] + (wcs.wcs.crpix[0] - 0.5) * delta_x).to_value(unit_x),
(bin_edges_y[0] + (wcs.wcs.crpix[1] - 0.5) * delta_y).to_value(unit_y),
]
return wcs