Hi Alex & All,
Quote:
Originally Posted by mental4astro
But the Encke Division (0.045") is smaller than the angular resolution of my scope!
Actually, no it's not!
The quoted angular resolution given for scopes refers to the scope's ability to resolve two stars - but stars ARE NOT pinpoints of light, but actually disks, the Airy disk, a diffraction pattern in reality - we are not actually seeing the disk of the stars. And the resolution limit is the ability to distinguish between two similarly brilliant stars to be able to make out a pinch between the two Airy disks.
Attachment 258068
When it comes to extended objects, such as the Moon and planets, the actual resolution capability of a scope can be 10 to 20X finer than the Rayleigh or Dawes limit. When it comes to extended objects, there is no diffraction pattern at play, no Airy disk - the possible diffraction pattern is totally disrupted, and the possible resolution limit is much, much finer.
I have seen the Encke (0.045") division in 7" Maks. I have also not even come close to resolving it in 10" scopes. Photos of Saturn using a 16" scope have also not shown it - could also be that the imager didn't know about the gap and then went about eliminating it!!! 
Heck! By strict resolution definition, a 7" Mak SHOULD NOT be able to resolve the Cassini division either!
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Firstly, I'd like to see the source (anecdotal or otherwise) for the statement:
" ... the actual resolution capability of a scope can be 10 to 20X finer than the Rayleigh or Dawes limit."
Secondly, the assertion "When it comes to extended objects, there is no diffraction pattern at play, no Airy disk - the possible diffraction pattern is totally disrupted, and the possible resolution limit is much, much finer." I am sorry to say, is incorrect. The level of detail observable on a planetary disc in an unobstructed telescope is directly proportional to the size of the airy disc it produces. Because the visual image of a planet is a mosaic of cheek-by-jowl airy discs.
Larger telescopes (given consistent quality optics) produce smaller airy discs = more detail (given equal contrast elements). Add a central obstruction (one type of contrast element) to any given aperture and more light is pushed from the central dot of the diffraction pattern and into the surrounding diffraction rings -- contrast drops as the size of the central obstruction increases. Add in other poor contrast elements like internal reflections/scatter and dust on the optics and seeing and the image is further degraded.
Keep the central obstruction small (ie less than 20%) and its diffraction effects (in transferring light from the disc to the rings) are quite small to negligible. Once you pass about 25% (rule of thumb) it begins to become noticeable on nights of very good to excellent seeing -- all other things being equal.
This is quite simple and well established physics.
This is fundamentally why, telescopes with large central obstructions show less
visual contrast in planetary images compared to those with a small, very small or better no central obstruction -- inch-for-inch of aperture. This is why the instrument of choice for dedicated planetary visual observers, inch-for inch will always be a well made refractor that has no central obstruction and inherently very high contrast features.
This is also why, given excellent seeing and thermal stability, there is no rule,
ever; that a good little telescope can beat a good big telescope (assuming equal quality of optics).
The good big telescope will always form smaller airy discs and show more detail visually on planets (given equal contrast elements) than any good small telescope.
Best,
L.