is_proj_plane_distorted

astropy.wcs.utils.is_proj_plane_distorted(wcs, maxerr=1e-05)[source] [edit on github]

For a WCS returns False if square image (detector) pixels stay square when projected onto the “plane of intermediate world coordinates” as defined in Greisen & Calabretta 2002, A&A, 395, 1061. It will return True if transformation from image (detector) coordinates to the focal plane coordinates is non-orthogonal or if WCS contains non-linear (e.g., SIP) distortions.

Note

Since this function is concerned only about the transformation “image plane”->”focal plane” and not about the transformation “celestial sphere”->”focal plane”->”image plane”, this function ignores distortions arising due to non-linear nature of most projections.

Let’s denote by C either the original or the reconstructed (from PC and CDELT) CD matrix. is_proj_plane_distorted verifies that the transformation from image (detector) coordinates to the focal plane coordinates is orthogonal using the following check:

\[\left \| \frac{C \cdot C^{\mathrm{T}}} {| det(C)|} - I \right \|_{\mathrm{max}} < \epsilon .\]
Parameters:

wcs : WCS

World coordinate system object

maxerr : float, optional

Accuracy to which the CD matrix, normalized such that \(|det(CD)|=1\), should be close to being an orthogonal matrix as described in the above equation (see \(\epsilon\)).

Returns:

distorted : bool

Returns True if focal (projection) plane is distorted and False otherwise.