For a WCS returns pixel scales along each axis of the image pixel at the CRPIX location once it is projected onto the “plane of intermediate world coordinates” as defined in Greisen & Calabretta 2002, A&A, 395, 1061.


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


In order to compute the scales corresponding to celestial axes only, make sure that the input WCS object contains celestial axes only, e.g., by passing in the celestial WCS object.

wcs : WCS

A world coordinate system object.

scale : ndarray

A vector (ndarray) of projection plane increments corresponding to each pixel side (axis). The units of the returned results are the same as the units of cdelt, crval, and cd for the celestial WCS and can be obtained by inquiring the value of cunit property of the input WCS WCS object.