Hi Rainmaker, John & All,
Quote:
Originally Posted by Rainmaker
I expect that many are confusing the Encke Minima with the Encke Division.
I have seen the Minima in several scopes and thought I saw a hint of the Encke Division in the TEC 200 Maksutov but couldn't really claim it .....
I have seen it for certain just once in each of my 18" Dobs from the rural skies near Coonabarabran. It really is very elusive. Trying again tonight....  |
Yes this issue was the subject of some discussion here during the last apparition of Saturn in 2018:
http://www.iceinspace.com.au/forum/s...d.php?t=168492
Let's remember the gap was first observed by James Keeler (for whom the even thinner Keeler gap is named) using the 36" refractor at Lick Observatory. It is only 325km wide (less than 1/10th the size of the Cassini Division)and has an angular diameter of about 0.05 arc-seconds when viewed from Earth. The Cassini Divison is about 0.8 arc-seconds
I feel that many claiming to see the Encke gap through relatively modest telescopes (ie 25cm and less) are mistaking the Encke minima (a broader trough in surface brightness of Saturn's "A" ring) for the actual gap that is very close to the edge of the A ring.
While I'm happy to concede it is possible to observe high-contrast features like the Encke gap in instruments where the angular diameter of the gap is less than the Dawes Limit (see my "Observable -v- Resolvable" post in the other thread), seeing features approximately 1/16th the Dawes limit (or less) just isn't possible. I would be very sceptical of claims it is truly observable in telescopes less than about 25-30cm aperture. This is consistent with my own experience, yours and John's -- it is a very, very difficult feature to see, only occasionally observable in large amateur instruments. As I said elsewhere, I've seen it with certainty twice -- once with 31cm at x367 on a night of exceptional seeing in 2000 and again about 3 years ago with 46cm at x464 on a night of almost singular seeing at Mudgee. On that night, this dedicated deep sky observer and a number of others spent about 4 hours going from Mars to Jupiter to Saturn over and over at powers of x462 to over x550 with 46 and 50cm telescopes and sometimes not seeing the merest quiver in the eyepiece image for 30-40 seconds at a time.
That's not for one minute saying people are lying or falsifying their observations, but simply characterise such claims as probably spurious observations or mistakes. The so called "canals" on Mars are an excellent example.
We all make mistakes. I know, I've certainly made my share of 'em.
Best,
L.