@cdeil - I want to revive this discussion. To answer your question about whether the shape of the aperture depends on the FITS projection of the image - I guess it could, since the world-to-pixel coordinates change. I guess I still don't understand the distinction between sky and world though. The aperture is just defined as a polygon on the sky with the desired properties, and mapped onto the pixel space. So say for the case of a large circular aperture, this aperture is defined by the set of points at a given spherical distance from the position considered. I then map this into pixel coordinates. This will depend on the details of the projection, pixel orientation, etc. So would you call this ``sky`` or ``world``? And what is the other case?