Stephen Paul posted:

You can't declare a separation of two point sources who's Airy disks are
touching. This is the idea of the Dawes Limit.

Actually, in a way, you can, as the locations of the centers of each of the
diffraction disks are not overlapping but are separated. You can clearly see
that there are two diffraction disks present so the stars are essentually
"resolved", but you cannot declare the double star as "split", as the Airy
disks are touching or overlapping depending on the angular separation and the
aperture being used. All you can say is that the two point sources are
separated, but their Airy disks are merely resolved and not separated. In the
case of star Airy disks significantly overlapping to the extent that there is
no "notching" at the sides but mere elongation, a separation figure can be
very difficult to arrive at, and the stars are "elongated" but not resolved.
This is where I like the Sparrow limit, as, like the Rayleigh limit, it has
some basis in the optics involved and not on a set of observations in a
limited range of apertures and angular separations. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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