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by the time you add in any sort of atmosphere, you will have stars of at least 2 arcsec FWHM and the atmosphere will dominate. You will need ~3 pixels across for best resolution, so you could use down to about 0.7 arcsec - mind you it will be as slow as a wet week at that resolution and my guess is that ~1 arcsec should be a good compromise.
In the average Australian atmosphere, the resolution of a good 4 inch scope will be almost the same as that of a 20inch RCOS or the AAT. It will just take a looong time to get many photons.
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