Cutout2D behaviour for non-rectangular pixel sizes

Description

When specifying a non-rectangular pixel size for the WCS provided to the Cutout2D class, the cutout does not return the expected cutout size and data shape.

Steps to Reproduce

from astropy.nddata.utils import Cutout2D
from astropy.wcs.utils import celestial_frame_to_wcs, proj_plane_pixel_scales
from astropy.coordinates import SkyCoord
import numpy as np

center = SkyCoord("0d", "0d", frame="icrs")
wcs = celestial_frame_to_wcs(center.frame, projection="CAR")

wcs.wcs.crval = (0, 0)
wcs.wcs.cdelt = (-5, 5)
wcs.wcs.crpix = (36.5, 18.5)

data = np.ones((36, 72))
print(data.shape)

size = [90, 180] * u.deg
c2d = Cutout2D(data=data, position=center, size=size, wcs=wcs)
print(c2d.data.shape)

Prints:

(36, 72)
(18, 36)

So it behaves as expected and returns a data array with half the input shape in all axes.

In case of specifying an un-even pixel size:

from astropy.nddata.utils import Cutout2D
from astropy.wcs.utils import celestial_frame_to_wcs, proj_plane_pixel_scales
from astropy.coordinates import SkyCoord
import numpy as np

center = SkyCoord("0d", "0d", frame="icrs")
wcs = celestial_frame_to_wcs(center.frame, projection="CAR")

wcs.wcs.crval = (0, 0)
wcs.wcs.cdelt = (-10, 5)
wcs.wcs.crpix = (18.5, 18.5)

data = np.ones((36, 36))
print(data.shape)

size = [90, 180] * u.deg
c2d = Cutout2D(data=data, position=center, size=size, wcs=wcs)
print(c2d.data.shape)

It prints:

(36, 36)
(9, 36)

While I would have expected (18, 18). Maybe it seems the lon and lat axes are switched in this case? cc @LauraOlivera

System Details

macOS-10.14.6-x86_64-i386-64bit
Python 3.8.6 | packaged by conda-forge | (default, Jan 25 2021, 23:22:12) 
[Clang 11.0.1 ]
Numpy 1.20.0
astropy 4.1
Scipy 1.6.0
Matplotlib 3.3.4

[adonath]

From a quick look at the code, I think I already found the bug:

• The size argument of the Cutout2D class is given in (lat, lon) order, as documented in the docstring
• When converting the size to pixel in the loop here proj_plane_pixel_scales(wcs) returns the pixel size in (lon, lat) order
• So it seems like the incorrect pixel size is used to convert the size to pixels…lon for lat and the other way around.

If someone confirms the bug, I could possibly implement a fix myself.

[larrybradley]

“axis of the image” refers to the axis of data, e.g. (x, y) not WCS axis. In general (lat, lon) (in whichever order) aren’t going to be aligned along the (x, y) axes, so I don’t know another way to do this other than to use proj_plane_pixel_scales (the data cutouts are always rectangles in (x, y) space). We should update the documentation if that is confusing. Perhaps even explicitly by stating the cutout pixel size will be:

• nx_cutout = x_size (ang) / x_pixscale
• ny_cutout = y_size (ang) / y_pixscale

where the x and y pixel scales are computed using proj_plane_pixel_scales

Would that help?