Pixel Coordinate Conventions
----------------------------
In Photutils, integer pixel coordinates fall 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, the ``x`` (column) coordinate corresponds to
the second (fast) array index and the ``y`` (row) coordinate corresponds
to the first (slow) index. ``image[y, x]`` gives the value at pixel
coordinates ``(x, y)``.
.. _SourceExtractor: https://sextractor.readthedocs.io/en/latest/
.. _IRAF: http://ast.noao.edu/data/software
.. _ds9: http://ds9.si.edu/