Pixel Coordinate ConventionsΒΆ
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)
.