rotate_CD_matrix

grizli.fake_image.rotate_CD_matrix(cd, pa_aper)[source]

Rotate CD matrix

Parameters:
cd : (2,2) array

CD matrix

pa_aper : float

Position angle, in degrees E from N, of y axis of the detector

Returns:
cd_rot : (2,2) array

Rotated CD matrix

Notes

astropy.wcs.WCS.rotateCD doesn’t work for non-square pixels in that it doesn’t preserve the pixel scale! The bug seems to come from the fact that rotateCD assumes a transposed version of its own CD matrix.

For example:

>>> import astropy.wcs as pywcs
>>>
>>> ## Nominal rectangular WFC3/IR pixel
>>> cd_wfc3 = np.array([[  2.35945978e-05,   2.62448998e-05],
>>>                     [  2.93050803e-05,  -2.09858771e-05]])
>>>
>>> ## Square pixel
>>> cd_square = np.array([[0.1/3600., 0], [0, 0.1/3600.]])
>>>
>>> for cd, label in zip([cd_wfc3, cd_square], ['WFC3/IR', 'Square']):
>>>     wcs = pywcs.WCS()
>>>     wcs.wcs.cd = cd
>>>     wcs.rotateCD(45.)
>>>     print '%s pixel: pre=%s, rot=%s' %(label,
>>>                         np.sqrt((cd**2).sum(axis=0))*3600,
>>>                         np.sqrt((wcs.wcs.cd**2).sum(axis=0))*3600)

WFC3/IR pixel: pre=[ 0.1354 0.121 ], rot=[ 0.1282 0.1286] Square pixel: pre=[ 0.1 0.1], rot=[ 0.1 0.1]