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Nik,
Diffraction limited optics are a way of expressing the telescopes resolution, i.e. the smallest dot of light it can reveal in arcsecs.
So if you see a telescope advertised as diffraction limited it means that it complies with the resolution formula of 4.56 / aperture (inches).
So your 80mm ED refractor has a resolution of 1.45 arcsecs.
Nyquest theory says we should sample the smallest resolution by 2x or for your ED80 use a chip with that can sample at 0.7 arcsecs per pixel.
But as most of us are seeing limited where the smallest resolution gets blurred out by the seeing. We only need to sample for 2 arcsecs (good seeing) or up to 4 arcsecs (bad seeing).
So a chip which can provide 1 arcsec/pixel sampling is fine for us amateurs.
arcsec / pixel = pix size(microns) x 206.3 / focal length(mm)
Hope this helps
Bill
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