Quote:
Originally Posted by ngcles
This is an important statement -- and I'm sorry, it's not correct. The angular width of these features are not in the same "realm". Here are the angular widths given Saturn at opposition at 10AU distance from Earth and given the Moon's mean orbital distance of approx 380,000km and the formula:
Angular Diameter in arc-seconds = 206265 X (Actual diameter (km) / Distance km).
https://en.wikipedia.org/wiki/Angular_diameter
The two rilles mentioned are about 600m across:
Cassini Division (4,800km): 0.67 arc-seconds
Vallis Schröteri rille and Valles Alpes rille: (~600m): 0.325 arc-seconds
Encke Division: (325km): 0.045 arc-seconds.
Dawes limit various apertures:
Dawes limit (approximation) formula 11.6/D (where D is the aperture in cm)
Dawes limit 50mm: 2.32”
Dawes limit 15cm: 0.77”
Dawes limit 18cm: 0.64”
Dawes limit 23.5cm: 0.49”
Dawes limit 25cm: 0.46”
Dawes limit 30cm: 0.38”
Dawes Limit 46cm: 0.25”
Dawes Limit 63.5cm: 0.18”
I should also add that in one of the other threads, regarding these lunar features you wrote: "At this distance, that 500m detail is 0.0476" in size. Oh, and guess what, the Encke is 0.045"... Hmmm."
You didn't show any working for that calculation, but it appears to me to be incorrect.
Using the formula I cite above: 206,265 x (Actual diameter (km) / Distance km)
206265 x (0.5 / 363766) = 0.28 arc seconds -- ie more than six times the apparent angular width of the Encke division from 10A.U.
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Well, I stand corrected!
I reviewed my working out and I did make a mistake!
Again, I never said the astronomers of the time were dumb. I mentioned that in the post you most kindly misrepresented.
As for "imagined seeing" of the Encke Division, when I first saw it I had no idea about it. I was observing Saturn when I saw that feature. No expectation, no prompting, but there it was. This has been noted many times by others too, no prompting, and using different scope designs. But all these people did see the same thing, and yet you offer not one single plausible explanation for what we all saw other than we were "imagining" it? Difficult to imagine something when you do not expect such a feature.
What is interesting is this following article discusses this very topic, using the notes of the astronomers tracing the history of the detection of the Encke Minima and Division. It also quotes none less than William Dawe's observation and sketch of the Encke Division from 1843 using a 9" refractor (page 5, fig 7 of the document):
http://www.astrosurf.com/re/encke_di...n_2017_PRe.pdf
When it comes to the Encke Minima vs the Encke Division, one is low contrast the other high contrast, respectively. What I have seen is the Divison - the feature's location was exactly where the Division is in the A ring. The same position of the feature that Dawe's himself noted. The Minima is a much broader feature within the A ring, something that I have seen too, and not the same thing as the Division.
Adding to this is the drawing by E. M. Antoniadi of Saturn from 1899 using the 10.25" refractor at Juvisy Observatory showing the Encke Division, in exactly the same position as I have (page 7 of that article).
A photometric diagram (page 9, fig. 12) of the intensity of the rings is on page 9. It labels the position of the Minima (g, h, i) and notes the position of the Division (E). Again, the location of E is where I have seen this feature.
Interesting to note that the apparition of the Encke Division in the 36" Lick refractor was only visible on just the one night, July 3 1899, adding to just how difficult it is to chance upon conditions favourable enough to see it.
So here we have instruments from 9" through to 36" showing it. All professional observations.
This article comes as a complete revelation to me.
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
https://www.cloudynights.com/topic/7...#entry10188216