Pixel Coordinate Conventions

In Photutils, integer pixel coordinates are located at the center of pixels, and they are 0-indexed, matching the Python 0-based indexing. That means the first pixel is considered pixel 0, but pixel coordinate 0 is the center of that pixel. Hence, the first pixel spans pixel values -0.5 to 0.5.

For a 2-dimensional array, (x, y) = (0, 0) corresponds to the center of the bottom, leftmost array element. That means the first pixel spans the x and y pixel values from -0.5 to 0.5. Note that this differs from the IRAF, FITS WCS, ds9, and SourceExtractor conventions, in which the center of the bottom, leftmost array element is (x, y) = (1, 1).

Following Python indexing, two-dimensional arrays are indexed as image[yi, xi], with 0 being the first index. The xi (column) index corresponds to the second (fast) array index and the yi (row) index corresponds to the first (slow) index.