CDELT

CDELT is a FITS header keyword used in the description of the World Coordinate System (WCS) for astronomical images. For each image axis n, CDELTn specifies the increment in world-coordinate units that corresponds to a one-pixel step along that axis. The values are interpreted in the units given by CUNITn, and the sign indicates whether the world coordinate increases or decreases with increasing pixel index.

Typically, in a two-dimensional image, CDELT1 gives the scale along axis 1 and CDELT2 along axis 2. These scales work together with CRVALn (the world-coordinate value at the reference pixel CRPIXn) and CTYPEn (the axis type and projection) to map pixel coordinates to world coordinates. A negative CDELT value means the axis is reversed relative to the pixel coordinate.

In modern FITS WCS conventions, more general transformations are supported by matrices such as CDi_j or PCi_j. CDELTn provides a diagonal scaling component and may be used alongside a rotation or other linear transformation, or in some implementations may be superseded by the CD/PC representation. Software compatibility varies, and some packages may ignore CDELT if a full CDi_j or PCi_j matrix is present.

Example: with CTYPE1 = "RA---TAN", CUNIT1 = "deg", CRPIX1 = 1024.5, CRVAL1 = 180.0, CDELT1 = -0.0002777778 would map pixel increments along axis 1 to small decreases in Right Ascension in degrees per pixel, while CDELT2 = 0.0002777778 with CUNIT2 = "deg" maps vertical pixel steps to increases in Declination.