This fall, I have seen it (the Crepe Ring) 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. The B-ring is the brightest
ring, but
shows some interesting variations in brightness across its width.
The inner
half appears somewhat darker, and this year has shown almost a patchy
inside
segment on each ansa, with the degree of darkness being slightly
different
between the leading and following asae.
Cassini's division is another subject. It was discovered in
a 2.5 inch
telescope, but I often wondered exactly what minimum aperture is
required to
actually view it. Two problems crop up when we talk about
actually seeing
this division. The first is familiarity. Most of us
know what the Cassini's
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
A and B rings. The B-ring is brighter than the A-ring, which
tends to
reinforce the perception that there is a dividing line between them,
especially at low power. 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 once again resorted to 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.
This way, I could stop down the scope in well-defined steps to see
at which
point the division would become invisible. Seeing was better
than one arc
second and I started with the widest opening 94mm (3.7 inches),
which, at
141x, showed Saturn nicely and the Cassini Division much of the
way around
the planet. Even the main belt across the planet was easily
visible.
Stopping down caused a drop in the brightness and and the ease of
detail
visibility, but Cassini's Division could still be seen down to 60mm
(2.4
inches), where it was only detectable on the ansae. At 50mm,
the outer edge
of the ring system looked somewhat darker, but I could not say for
certain
that Cassini's was visible as a true division. I tried the
same variable
aperture sequence at 220x, but again, at 50mm, Cassini's division
was not
clearly shown. There just wasn't quite enough light in the
image to make a
firm judgement, although interestingly enough, even the 50mm could
just show
the equatorial band on the planet at 141x. It might be possible
that with
increased ring tilt, the division might just be catchable at 50mm,
but to be
realistic, 60mm seems to be about the minimum to clearly show Cassin's
Division.
The Encke Gap is something else again. It is a much narrower
division,
located in the outer areas of the A-ring. One problem is that
again, an
albedo feature causes a contrast effect which can mimic a true division.
The brightness of the A-ring shows a marked fall off, beginning
about halfway
out from the inner edge of the A-ring and continuing to its outer
edge. Some
people have termed this "the Encke Mimimum", although to my knowledge,
this
darkening has no official name. Many observers prior to the
Voyager probes
mistook the brightness falloff as a division and put the division
nearly in
the middle of the A-ring. Others have had seeing cause doubling
effects
which can make ring edges look like additional divisions.
The true Encke gap is located about 85 percent of the way from the
outer
edge of the Cassini division to the outer edge of the A-ring, or
about
133,800 km from the center of the planet. It therefore hugs
the outermost
edge of the A-ring. To check on the division's true dimensions,
I took a
recent Hubble Space Telescope image of the planet and did some pixel
measurment. Using the known diameter of the visible
ring system, I came up
with an image scale, and then attempted to determine how many pixels
wide the
true Encke gap was. At most, the gap appeared to be possibly
1000 km wide,
and at the least (using the darkest part of the gap), it could have
been
about 530 km in width. A few books have even indicated narrower
values,
although the feature we are seeing might be made up of several closely
space
divisions. In any case, using the largest value of 1000 km,
and the fact
that Cassini's division is around 4200 km in width and apparently
takes 2.4
inches (60mm) of aperture to become visible, I came to the conclusion
that it
would take a telescope of at least 10 inches to have much of a chance
of
seeing the division clearly. 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, and one night where I could
see it
a good way around each side of the rings at 484x, but 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. I have
some doubts that
at this state of ring tilt, apertures significantly under ten inches
in
aperture would be allow viewing of the Encke Gap. Perhaps
a quality 8 inch
refractor might be able to show it when the tilt becomes a bit larger.
David Knisely