Hi Bob & All,
Quote:
Originally Posted by Tropo-Bob
Spoting the discs of Jupiter's Moons depends on both seeing and the quality of the optics. Unfortunately, my seeing is a little below average.
I had several run of the mill newtonians which I used for many years without seeing discs. It was only when I brought a high quality 80mm refractor that the discs just popped out at me. Till then, I did not even realise it was possible to see them.
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Given the angular diameter of Ganymede near opposition is about 1.55 arc-seconds (the other Galilean moons range down to about 1.0 arc-seconds) and the Dawes Limit for an unobstructed 80mm aperture is about 1.45 arc-seconds, I'd suggest that what you are seeing here isn't the actual disc of the Galilean moon(s), but "Airy-discs" formed by the telescope of what are, for all practical purposes in that aperture, point sources (ie like stars).
In order to properly resolve Ganymede into a disc, you will need a high-quality telescope of 20cm aperture and near perfect seeing conditions plus high magnification. The smaller Galilean satellites will require additional aperture. In order to see a properly resolved "disc" of the moon (and no, I'm not talking about a shadow on the Jovian cloud-tops-- that's a slightly different bucket of fish), you'll need enough aperture that the angular diameter of the satellite in question is at least two, better three times -- or more, "Airy-discs" across.
I should add that I've seen Ganymede, Callisto & Io resolved into a disc in my old 31cm telescope at x286 and once or twice seen gross detail on Ganymede (ie one side darker than the other or irregularities in surface brightness across the disc) a few times in my 46cm at x317 and above.
Best,
L.