Hi Alex,
Quote:
Originally Posted by mental4astro
As for a 50mm high quality refractor showing the Cassini Division, you need to try this yourself - but only derision from you here. Myself I have seen the Cassini Division with a 53mm aperture - masked down an ED80 refractor. David Knisely posted on CN he saw it with the 50mm aperture at 176X ...
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No, sorry, incorrect, no derision from me at all actually! Please re-read what I wrote above last night:
"I have thought about it. Here is the simple arithmetic:
The Cassini division is somewhat larger than 1/4th the Dawes limit for a 50mm telescope and is likely detectable (though not "resolvable") in good conditions. The Cassini division has somewhat better contrast/albedo features compared to Encke."
I regard detection of Cassini using 50mm aperture as very likely credible, as it is
better than 1/4 the Dawes limit for that aperture and that Cassini has somewhat better albedo contrast circumstances than the Encke division. Personally, I have seen it in an 80mm refractor but have not tried in lesser apertures. 1/14 the Dawes limit on a division (Encke) that has lesser albedo contrast, using 18cm (7") is an altogether different bucket of fish -- a very significant (much more than triple) increase in degree of difficulty over the 50mm/Cassini example.
As I've said before many times Alex, I don't doubt your sincerity, but I equally believe that people can be sincerely incorrect, or sincerely mistaken. Over my reading of the material here, there and elsewhere on the net, one thing that is abundantly clear is that people who make entirely credible claims of glimpsing the Encke division also simultaneously record seeing the Encke minima
as well. The Encke minima is a more easily seen feature than the gap, yet so far as I have been able to make out, you see the gap but not the minima and this is at variance with nearly every recent, credible claim. It is for this reason I have to conclude that you have in fact seen the minima and somehow, via a combination of telescope/eye/brain/physiology/psychology mistaken it for the gap.
Your argument regarding the visibility of a few lunar rilles being a "parallel" situation is shown as clearly incorrect.
I think I've said everything that I can usefully contribute on the subject. I stand by my conclusion that the minimum aperture needed, in the absolute best possible circumstances is about 12" (30cm).
If people would like to prove that conclusion incorrect with physics/arithmetic/mathematics please feel free to do so -- I'm all ears.
Like Alex, I invite people to both try and try hard with whatever telescopes they have and report both positives and negatives here -- but be very, very wary of potential false positives.
Best,
L.