  We present an algorithm implemented in the astroalign Python module for image
registration in astronomy. Our module does not rely on WCS information and
instead matches 3-point asterisms (triangles) on the images to find the most
accurate linear transformation between the two. It is especially useful in the
context of aligning images prior to stacking or performing difference image
analysis. Astroalign can match images of different point-spread functions,
seeing, and atmospheric conditions.
