From some correspondence that Wavytone shared with me:
"The most commonly quoted criteria for the resolution of telescopes are the “Rayleigh limit” (¼ wavefront error) and the Dawes criterion. Both of these concern the angular separation of a pair of equally bright point sources, at which they can be reliably distinguished as a pair, not a single point. However… Both limits are not particularly valid for a sharply defined linear edge or a line, such as the lunar limb or the divisions in Saturns rings, nor non-point (extended) objects (ie planets and their moons).
For an edge transition (white to black) such as the lunar limb diffraction theory provides a mathematical formulation of the way the light intensity drops off, and in much the same way as the point spread function, fringes can be observed, eg photometrically during an occultation of a star by the moon.
Similarly, for a bright slit (or a dark line) diffraction theory also provides the slit diffraction function - see for example
http://labman.phys.utk.edu/phys222co...iffraction.htm and the location of the fringes is found from the equation:
w.sin(a) = m(L)
where w is the width of the slit, a is the angle from the centre, m is a positive integer (1 for the first fringe), and L is the wavelength of light.
However this has solutions where the width of the slit (w) is wider than the wavelength of the light (L). When the slit is narrower than the wavelength
the equation has no solution - and no fringes are observed.
This means an extremely narrow linear feature - a bright line or a dark line will not produce fringes -
but this does not necessarily mean it cannot be observed if the contrast is sufficiently high.
In small telescopes the Cassini and Encke divisions are good examples of this. The Cassini division at 4800km wide presents an angular width around 0.66 arc-seconds, yet it is seen with a 60mm refractor. The Encke division is far narrower at 325km, yet is seen on 14” SCTs and has been observed - and photographed - using smaller scopes with superlative optics (7” and 10” maksutov cassegrains)."