Saturn in 20x80 binos

We won a pair of Burgess 20x80 binos at last year's NEAF, and have really
enjoyed them. Tonight we turned them on Saturn, and we all (4 people)
agreed that the ring was clearly distinguished from the ball, with black
visible in between. I have always repeated the 30x rule for splitting
Saturn's rings, and so found this pleasantly surprising. No moons were
detected. The transparency is pretty crumby, and the comet is visible but
nothing to write home about.

Dennis

We also just finished another experiment - how little power to observe the
Cassini division. We used our 6" f/8 flex-mirror newt/dob, the seeing was
pretty good, let's say 8 out of 10, and transparency was mediocre. We all
agreed that Cassini was clearly visible with a 20mm TV plossl (60x), and not
visible with a 26mm Meade plossl (46x). We did not have anything in
between, but I think we could have done a bit better than the 60x.

BTW, the crumby transparency didn't allow very good views of comet Machholz.
I thought I could detect some tail heading up into the Pleiades, but others
could not.

Dennis

Dennis -

I had my 15x70 Barskas on Saturn last night and could see the gap
between the rings and the ball pop in and out occasionally. Yes, this
too surprised me.

Mark Pippin


Dennis Woos wrote:
We won a pair of Burgess 20x80 binos at last year's NEAF, and have
really
enjoyed them. Tonight we turned them on Saturn, and we all (4
people)
agreed that the ring was clearly distinguished from the ball, with
black
visible in between. I have always repeated the 30x rule for
splitting
Saturn's rings, and so found this pleasantly surprising. No moons
were
detected. The transparency is pretty crumby, and the comet is
visible but
nothing to write home about.

Dennis

Fascinating. I could have written your post myself.
Except that I used a 6" f/8 refractor.
*
Chris.B

Here's a link to a whole collection of lowest power observations of the
Cassini division. Your results are not much different than mine, 60x
with a TV85.

http://www.cloudynights.com/ubbthrea...b/5/o/all/vc/1



The space between the rings and the disk is about 6 to 7 arcseconds at
the ansae. While normally it would take between 20x to 25x to see
stars that close, because the space is dark on a light background, it
is easier to see. I can routinely see the space between the rings and
disk with 16x70 and on good nights with 15x70s. I have suspected
seeing it for short moments with 12x50s.

edz

wrote:
Here's a link to a whole collection of lowest power observations of the
Cassini division. Your results are not much different than mine, 60x
with a TV85.

http://www.cloudynights.com/ubbthrea...b/5/o/all/vc/1



The space between the rings and the disk is about 6 to 7 arcseconds at
the ansae. While normally it would take between 20x to 25x to see
stars that close, because the space is dark on a light background, it
is easier to see. I can routinely see the space between the rings and
disk with 16x70 and on good nights with 15x70s. I have suspected
seeing it for short moments with 12x50s.

edz


Admitting to a level of ignorance, and inability to think this out at
the moment, does a larger aperture allow a lower power to reveal these
features, and would this be subjective?

Stephen Paul

Admitting to a level of ignorance, and inability to think this out at
the moment, does a larger aperture allow a lower power to reveal these
features, and would this be subjective?

Larger aperture *should* provide more resolution, but this is in the ideal
case, isn't it? Optical quality is all important, and it is hard to get
aperture and quality. Furthermore, there is always the aperture/seeing
issue. Maybe the "aperture rules" crowd will chime in, even telling us that
the better views through smaller instruments is "aesthetic" only.

What do you mean by "subjective"? Maybe some folks perceive the benefits of
aperture differently, because of eyesight issues? Hmm...

Dennis

This is also very similar to my lowest power to resolve Cassini in my
6" f/8 Dob and TV85. Cassini is clear in the Dob at 50x on a good
night and at 60x in the TV85. Both would probably show it a little
lower, but my next power down for the Dob is 38x and 46x for the TV85,
and while I've thought I've caught glimpses of it in both scopes at
these powers, I'm not comfortable claiming it for certain. I can for
sure see separation between the globe and rings in my 35 Pan in the
TV85, which is 17x.

Clyde

Dennis Woos wrote:
does a larger aperture allow a lower power to reveal these
features, and would this be subjective?

Larger aperture *should* provide more resolution, but this is in the
ideal
case, isn't it? Dennis

You would normally need to consider this approach to resolution if you
were talking about splitting doubles stars, but this is a much
different matter.

Cassini division is actually 2,800 miles across. At an average
distance of 8 A.U. that would make Cassini 0.75 arcseconds wide. Of
course that width is only seen at the ansae. At a maximum tilt of
27=B0, where the rings pass in front of the disk, the width would be
seen as only half that. But we will use the widest gap.

Resolution in scopes is calculated by the Rayleigh Limit, which is
138/Dmm or 5.45/Dinches. BUT, Cassini division is a thin black line on
a brightly lit background. This type of feature can be SEEN at
approximately 1/5th instrument stated resolution or R/5. So an
instrument that has a resolution of 0.75 x 5 =3D 3.75 arcseconds might be
used to detect the Cassini division. That gives 5.45/3.75 =3D approx a
1=2E5" or 40mm scope might be used to detect the Cassini division, as
long as you use sufficient magnification to bring it up to an apparent
size that you can see.

So the question of SEEING the Cassini division is not one how big a
scope needed for resolution, it is one of visual acuity. Some people
can see it with a magnification of 50x to 60x. Many people need 60x to
80x to see it.

Now, for RESOLVING the Cassini division another entire set of criteria
comes into play. Nomally we talk about diffraction limits and
overlapping Airy disks when we discuss resolution limits. In fact our
scopes are defined by the Rayleigh Limit, the value that tells us the
smallest diffraction disk that the scope can create with it's optics.
For instance a 5" scope has a Rayleigh Limit of 5.45/5 or 138.4/127 =3D
1=2E09 arcseconds.

Stars or light point sources are so far away that they would produce
infinitely small points if our instruments were not limited by
diffraction. Any distant point source of light that we look at can
only produce an Airy disk in this 5" scope of a minimum 1.09 arcseconds
radius. The central bright point is smaller than the Airy disk. While
it can be larger than one half, generally it is not less than one half
the diameter of the Airy disk. So the central bright spot in our 5"
scope, at one half of the Airy disk diameter, is about 1.09 arcseconds
wide.

Even extended objects can be considered to be made up of an infinity
number of point sources. And every point source shows the affects of
diffraction. But the Cassini division is not made up of point sources
of light. In fact it is the absence of light that creates the Cassini
division.

The diffraction affects are created by the edges of the bright light of
the A and B rings that border the Cassini division. Each theoretical
point of light on the edges of the rings shows diffraction. That
diffraction intrudes upon the dark Cassini division. The end result is
the thin black line becomes narrowed. If you had a fine enough
instrument to measure hundreths of an arcsecond, you would find the
Cassini division does not show up as a 0.75 arcsecond wide black line.

If we have an instrument, such as our 5" scope that produces a smallest
point sorce image of 1.09 arcseconds, then nothing smaller than 1.09
arcseconds can be resolved by that instrument. Now keep in mind the
Cassini division, due to infringing diffraction from the bright ring
edges, will appear somewhat thinner than 0.75 arcseconds. This is not
so easy to determine, but let's say for example it is narrowed to 0.65
arcseconds. So RESOLVING the Cassini division will require an
instrument that can make the width of the line appear larger than that.
The smallest diameter instrument that can show 0.65 arcseconds
resolved is 138/0.65 =3D 212mm, or something a little larger than 8". So
it reasonable to say, the smallest instrument that could RESOLVE the
Cassini division is 8", maybe 9".

Now that you know what it takes to RESOLVE it, you need to use enough
magnification to see it resolved. While it can be detected at low
minimum magnifications of 50x or 60x, to see it resolved you need to
raise it to your minimum size for your visual acuity. It is a rare few
people who have a visual acuity of 120 arcseconds and a better low
limit for very good visual acuity is 150 arcseconds. Many people are
closer to 180 arcseconds. So enough magnification needs to be employed
to bring this 0.65 arcsecond feature up to your low limits and that
could be 150/0.65 or 180/0.65, or a magnification of 225x to 275x to
see it RESOLVED.

edz

Excellent information - thanks, Ed.

Dennis