On 2003-09-07, Carlos Moreno wrote:

Any kind soul could help me understand the "physical"
reasons for this phenomenon? (BTW, I'm an Electrical
Engineer, so feel free to get technical and use any
maths or physics needed to explain it)

It's due to the wave nature of light. A textbook on physical optics
will explain it, although the mathematics of computing the diffraction
pattern of a circular aperture can be "interesting". The wave front
is disturbed by the edges of the aperture and you get constructive
and destructive interference that produce a pattern that depends on
the shape of the aperture. For circular apertures you get a circular
pattern that consists of a central disc surrounded by diffraction rings.
The angular dimensions of the diffraction pattern depend on the size of
the aperture and the wavelength of the electromagnetic radiation. The
diffraction blurs the image and puts a limit on the resolution of the
telescope. It's the same thing that limits the resolution of radar
antennas.