WCS handling¶

FITS output¶

FITS cutouts keep the full WCS of their parent tile, with accurate astrometric calibration:

Parent WCS reproduction: the cutout reproduces the parent tile's mapping exactly. CRVAL, CTYPE and the CD/PC orientation are inherited unchanged, and only CRPIX is shifted to the extraction origin. When the parent tile carries no usable WCS, Cutana synthesises a minimal TAN header instead, with CRPIX at the cutout centre and CRVAL at the source position.
Pixel scale correction: when a cutout is resized from its extracted size to target_resolution, the WCS pixel scale is adjusted so coordinates still map correctly.
Sky area preservation: the sky area a cutout covers stays the same; only the pixel scale changes with the resize.
Format compatibility: CD matrix, CDELT and PC+CDELT WCS formats in the parent files are all supported.

Pixel units are separate from the WCS; see pixel units in FITS headers.

Why the parent mapping is inherited

Re-projecting a tangent plane at each source's position rotates the cutout frame by the meridian convergence between tile centre and source. The error is zero at the cutout centre and grows towards the edges, and it is worst far from the tile centre and at high declination. Inheriting the parent's mapping avoids it. Each release checks cutout WCS against the parent tiles to keep it that way.

Zarr output¶

Warning

Zarr archives do not contain WCS information. The metadata records each source's position and size, and where in the parent tile the cutout was extracted; see the Zarr metadata columns.