This is the newer version of the article with a big better analysis of the
Encke Division problem (its not just its width, its also its closeness to the
outer edge of the A-ring).
THE RINGS OF SATURN AND THEIR DIVISIONS
(some observational experiments)
Nearly every amateur astronomer has seen Saturn's rings, but not all are
aware of what aperture it takes to view their various details. The rings
themselves can be glimsed in binoculars as an elongation of the image, and at
about 20x, begin to show their true ring form. Even in a small telescope at
30x, the rings are unmistakable. However, there are a few details in the
rings which can be seen with larger apertures, such as the C, or "Crepe"
ring, and the two prominent divisions, the Cassini and Encke Divisions. The
Crepe ring is the innermost and darkest of the three main rings, and can be
glimsed in fairly small telescopes as a narrow band across the planet's disk
when the ring tilt is high. However, it often takes a 3.5 inch or larger
aperture and powers over 150x to begin to easily show that dim ring well
against the black background of space. At high tilt angles, I have seen it
fairly easily in a ten inch, although it isn't very bright. In a 12.5 inch
Portaball, the faintly greyish Crepe ring was rather obvious, hugging the
inside of the B-ring. One problem which newer observers run into is that
with small telescopes, the darker inner band-like edge of the B-ring is
sometimes mistaken for the Crepe ring. The B-ring is the brightest ring
overall, but shows some interesting variations in brightness across its
width. The inner half appears somewhat darker, and occasionally has shown
vague patchyness along the inside of the leading ansa, with the degree of
darkness being slightly different between the leading and following asae.
The A-ring is the outer ring, and while bright, is somewhat darker than the
B-ring.
Cassini's Division, which separates rings A and B is another subject.
It is a black band somewhere around 4220 to 4500 km in overall width with a
diffuse outer edge and a 3000 km wide darker inner segment. The division can
just be glimsed in moderate apertures at powers of around 65x if the seeing
is good, but it best observed at magnifications over 100x. It was discovered
in a 2.5 inch (63.5mm) telescope, but I often wondered exactly what minimum
aperture is required to actually view it. The division's angular width
varies from about 0.59 arc seconds near mean superior conjunction to as large
as 0.73 arc seconds near mean opposition, and its outermost edge is at best
only 2.35 arc seconds from the outer edge of the A-ring (mean opposition).
Simple linear or arc-like features are often visible at apertures smaller
than that needed to actually "resolve" the feature's true width. Thus, the
division is visible even though its angular width is a little less than
1/3rd of the resolution of the 2.5 inch aperture it was first observed with.
However, three problems crop up when we talk about actually seeing the
Cassini Division. The first is familiarity. Most of us know what it looks
like and how far out it is, so we might tend to "think" we see it when we
actually don't. It would be fairer to have someone who has never seen the
rings do the experiment. The second problem is the ring contrast between
the brighter B-ring and the somewhat darker A-ring. The A-ring is between
30% and 50% fainter than the B-ring, and has a dropoff in brightness with
distance from Saturn, mostly in its outer half. This tends to reinforce the
perception that there is a dividing line between the A and B rings,
especially at low power and smaller apertures. A third consideration is the
location of the Cassini Division. The 2.35 arc second separation between its
outer edge and the outer rim of the A-ring at mean opposition means that,
from the limits of diffraction optics, at least a two inch aperture would
probably be required to clearly resolve or separate it from the darkness of
space at the outer edge of the ring system. At smaller apertures, the
division would tend to just blend into the darker color of the A-ring, rather
than forming a distinct dark gap. The division exists of course, but at
what point does it become visible as a curving arc, and not just as a
contrast effect?
To answer this, I used my off-axis variable aperture mask I built for my
ten inch f/5.6 Newtonian to judge double star resolution. This mask provides
me with 94mm, 80mm, 70mm, 60mm, and 50mm clear apertures, and I put on one
additional mask to get 40mm and 30mm apertures. This way, I could stop down
the scope in well-defined steps to see at which point the division would
become invisible. I primarily used 176x and 141x for my tests, although I
did try 235x and 101x as well. I did the tests in October in both 1999 and
2000 on nights when seeing was better than one arc second. I started with
the widest opening 94mm (3.7 inches), which, at 141x and 176x, showed Saturn
nicely and the Cassini Division much of the way around the planet. Even the
main belt across the planet was easily visible, as well as the faint Crepe
ring. Stopping down caused a drop in the brightness and in the ease of
detail visibility, but Cassini's Division could still be seen down to 60mm,
where it was still fairly tracable along a wide arc of each ansa. At 50mm,
the actual division was becoming more difficult and was not very well shown,
detectable mainly at the bend of each ansa. The outer half of the ring
system looked somewhat darker as the division started to blend in a bit with
the A-ring. At 101x, the division was not visible at 50mm aperture. At
40mm, I could no longer see a clear dark division between the rings, although
the A and B-rings could still be seen as separate features with differing
brightness and borders. Interestingly enough, even the 40mm aperture was
still showing the main belt on the planet's disk. At 30mm of aperture, the A
and B rings began to merge somewhat, with no clear signs of any division, and
the only visible ring detail being a somewhat darker outer edge. I tried the
same variable aperture sequence at 235x, but again, at 40mm, Cassini's
division was not visible. To be realistic, while 50mm may allow "detection"
of the division at high ring tilt, in general, 60mm seems to be about the
minimum to clearly and easily show Cassini's Division.
The Encke Division is a considerably more difficult target. It is a
much narrower division, located near the outer edge of the A-ring. One
problem is that again, an albedo feature causes a contrast effect which can
mimic the presence of a true division. The brightness of the A-ring shows a
peak intensity just outside the Cassini division in a sort of brighter
"ringlet". Farther out, the brightness shows a marked fall off, beginning
about halfway out from the inner edge of the A-ring and continuing to its
outer edge. This brightness fall-off combined with the brighter inner
"ringlet" can give the impression that there is a diffuse division about in
the middle of the A-ring, especially at moderate powers where the image scale
is not extremely high. Some amateurs have referred to this illusionary
"feature" as, "the Encke Minimum", although this name is not official. Many
observers prior to the Voyager probes (including Encke himself) apparently
mistook the brightness falloff/contrast effect as a division and repeatedly
reported it nearly in the middle of the A-ring (a few even drew 2 divisions
there!). Others have had seeing cause doubling effects which can make ring
edges look like additional divisions. The Voyager and HST images show one
significant but narrow division in the A-ring near its outer edge *not* near
the middle of that ring. That narrow gap has since been "offically" named
the Encke Division by the IAU.
The true Encke division is located about 80 percent of the way from the
outer edge of the Cassini division to the outer edge of the A-ring, or about
133,706 km from the center of the planet. At mean opposition, this is only
0.5 arc seconds in from the A-ring's outer edge, so if a telescope is to be
able to separate the gap from the edge of the ring, it must have at least
this resolving power. To check on the division's true width, I took a recent
Hubble Space Telescope image of the planet as well as a large-scale Voyager
image print of the entire ring system. Using the known diameter of the
visible ring system, I came up with an image scale, and then determined the
approximate width of the division. At most, the gap appeared to be 400 km
wide, and a few books have even indicated smaller values (325 km from
ASTROPHYSICAL DATA: Planets and Stars, K. R. Lang). In any case, using
the largest value of 400 km, and a 0.5 arc second separation from the outer
edge of the A-ring, I came to the "ball-park" estimate that it would take a
telescope of about 10 inches to have much of a chance of seeing the division
with any certainty. This conclusion is based on the fact that, if the gap
was to be resolved or "separated" from the outer edge of the A-ring, the
telescope would need to have better than a 0.5 arc second resolution, and
this favors a telescope with an aperture greater than 9 inches.
Bearing this in mind, in October, I began to look for the division. I
got two or three outstanding nights when it was just visible with the ten
inch at 440x as a fine low contrast and very narrow arc near the outer edge
of each ansae. One evening in particular, I could glimse it at 353x and
see it a good way around each side of the rings at 440x. However, the fine
narrow appearance of this gap and the powers needed to get the image scale
up to where the gap would be visible makes me think that my estimate of 10
inches minimum aperture for visiblity was fairly accurate. Magnifications
of well over 300x and very steady seeing are necessary to have much of a
chance of seeing the division and not just the illusion of the "Encke
Minimum". I have some doubts that at apertures significantly under ten
inches would allow viewing of the Encke Gap, but perhaps a quality 8 or
9 inch refractor might be able to show hints of it near maximum ring tilt
and at a very close opposition.
David Knisely
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory:
http://www.hydeobservatory.info/
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* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
*
http://www.NebraskaStarParty.org *
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