Hi All,
Quote:
Originally Posted by Saturnine
... I have noted that a lot of the images submitted to the ALPO taken with 14" SCTs' and 12" to 16" newts and DK and Mewlons, in good conditions do show the Encke.
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But do they? I'd argue they don't!
First up take a look at the following two images of Saturn. The first one is with Hubble, the second taken by the renowned amateur planetary imager Chris Go of The Philippines.
Note in the Hubble image how narrow the Encke gap is compared to the Cassini Division, it is truly tiny -- very, very narrow and less than 1/10th the width of Cassini.
Now look at the (top-colour) image by Chris Go. Here, what
appears to be the Encke gap seems to be about half the width of the Cassini Division. Is Chris lying to us? Is this some sort of deception?
Not at all!
The detail portrayed is actually there in the data but it is vastly
over-represented by excessive sharpening during image processing. In other words the broader, subtle undulations within the "A" ring that are now well known and were even commented upon by Encke himself back 200-odd years ago, that are in themselves very, very subtle are vastly over-represented by processing -- particularly over-sharpening.
This is what appears to make an image from a 35cm telescope that is seeing-limited (almost at sea-level) appear to reveal more abundant detail within the rings than even Hubble shows. Look at the two discs of the planet. Chris' image seems to clearly show much, much more banding and more clearly and contrasty than even Hubble -- using a humble C14! Again, I want to emphasise, this is not "lying" or a deception. It is merely displaying the data in a way that vastly over-emphasises and over-represents real but exceedingly subtle detail.
That's the first half of the story.
Here's the second half: Until about 30-odd years ago, back before amateurs were widely publishing these startling images, the Encke gap was claimed as a visual sighting, exceedingly rarely. Nowadays a lot of people are claiming it as being a reasonably regular sighting using what are pretty modest (20-30cm aperture) telescopes. The reason I'd suggest is the psychological effect caused by these images and their wide publication.
People see the image and what seems like the Encke gap so very clearly using moderate apertures and go into the challenge with the mind-set that this really isn't that hard. It just must be visible! On nights of truly good seeing, with that mind-set they catch glimpses of the undulations and their brain interprets that as the actual gap -- a false positive.
Here again I want to very, very clearly emphasise, most people aren't lying when reporting seeing the gap in moderate or smallish instruments at
"relatively" low magnifications. They are either reporting a false positive or their brain is fooling them into believing they are actually seeing the gap based on the above. They truly want to join that "club" of people who have ticked it off the list and that desire more easily induces the false positive. Almsot exactly the same phenomena as the "canals" on Mars. Others "see" them, report them, others believe that they must similarly be able to see them and then suddenly they do. Even Percival Lowell fell into error this way!
For the record again (I might be wrong but I suspect not) even in a very high quality, exceptionally sharp visually optimised telescope, in exceptional seeing conditions the lower limit is going to be about the 30cm aperture mark at more than x350 -- better x500. Even then, the concurrence of quality instrument with very high quality conditions is going to occur maybe a few nights per five years of trying -- at best. Your chances are improved with increasing aperture over that 12" mark assuming it is quality aperture.
I remember a night about this time three years ago when I visited Lowell Observatory at Flagstaff Arizona using one of the highest quality planetary telescopes ever made, the 61cm f/11.3 Alvin Clark refractor. We spent about an hour on Saturn at about x400 with a 16mm Nagler. It was for me a highly memorable experience because looking through that particular telescope was a childhood dream. I also got to observe for the first time with a long-time idol and friend Brian Skiff.
The seeing was very good that night, between perhaps 8 and 9/10. Several seconds often passed between the image quivering. But, I still didn't see the Encke gap. The undulations in surface brightness within the "A" ring were certainly there, but no gap was apparent. Yes, it
is that hard to see. It is no accident at all that the first telescope to show it was the mighty 36" Lick refractor at x910.
Despite what I've written above, I ask you (as I will) to keep and open mind and keep trying, but be very wary of false positives.
Best,
L.
). One night we were playing with the resident ETX125 at over 500x, and the seeing was the best I have ever seen. Saturn was rock solid and sharp even at that power, and the Encke gap was clearly visible...
