CRPIX

CRPIX refers to a set of keywords in the FITS (Flexible Image Transport System) World Coordinate System (WCS) that specify the pixel coordinates of the reference point for each coordinate axis. For each axis i, CRPIXi records the pixel position in the image where the corresponding world coordinate value CRVALi is defined. In a typical 2D image, CRPIX1 and CRPIX2 identify the reference pixel along the x and y axes, respectively.

The reference point serves as the anchor for the WCS transformation from pixel space to world space. The world coordinates at that point are given by CRVALi. The remaining pixel coordinates are mapped to world coordinates by subtracting CRPIXi, multiplying by the scale factors CDELTi, and then applying any rotation or shear described by the CD matrix or the PC factor, followed by the projection specified by the CTYPE value. CUNITi and CTYPEi define the units and the coordinate system or projection (for example, celestial coordinates with RA and Dec, or spectral coordinates).

CRPIX values are typically chosen to place the reference pixel within the image frame, commonly near the center, which minimizes numerical errors in the transformation and aligns the world coordinates with the image content. However, CRPIX can be set outside the image if needed for specific analyses or coordinate conventions. The keywords CRPIX1, CRPIX2 (and CRPIX3 for additional axes) work in concert with CRVAL, CDELT, CD or PC, CTYPE, and CUNIT to define the full WCS mapping for the dataset.