All About Space Astronomer Book - 2014 UK

the system, from one ring tip to the other, is 169,000 miles but the thickness

is less than a mile, so when the rings are almost edgewise on they appear

only as a thin line of light and small telescopes will not show them at all for

several weeks. With a 12.5-inch reflector PM followed them through the

edgewise presentation, though not easily.

The ring system is complex. There are three main rings – two bright and

one semi-transparent. The two bright rings are labelled A and B, separated

[21] Separate Red, Blue and Green (RGB) images taken with a high-frame-rate

camera and combined with a graphics package.

[22] Saturn and its moons. From right to left: Titan, Dione, Mimas, Enceladus,

Tethys, Rhea.

[23] Saturn showing the northern hemisphere storm in March 2011.

[24] Mimas, showing its largest crater, Herschel, at upper right (NASA).

[25] Uranus photographed through a 14-inch reflector, combined

RGB images.

reaction when people see Saturn for the first time. It looks somehow unreal,

as though it is a model put there in the sky especially for our benefit.

The rings move in the same plane as Saturn’s equator, and this means

that over a Saturnian year we see them at various tilt angles. Saturn takes

nearly 30 years to complete one journey around the Sun and for most of

the time the rings are so tilted we can see them in detail. Drawings can be

very instructive, though by no means easy to make, and once again the

by a gap named the Cassini division. B is closer in. Closer in still we come

to ring C, the Crepe or Dusky ring, discovered in the middle of the 20th

century. It is by no means a difficult object, but is semi-transparent and easily

overlooked. One curious fact associated with ring B is the appearance of

strange dark spokes which move around the planet and persist for some

time before dissipating. They are due to particles elevated away from the

main ring plane possibly by electrostatic forces, and are still something of a

mystery. They are confined to ring B; A does not show them.

These are the rings seen through a modest telescope, but there are others

that are much more elusive and beyond the range of the average amateur.

Some move at distances further out than the main rings, and although the

entire ring system is surprisingly extensive it is also very rarefied. Although

Saturn is a giant world, it is also true that its overall density is less than that

of water.

Saturn was the first thing PM saw through a reasonably powerful

telescope and he could not keep back a shout of wonder. This is the general

astronomical photographer has a major part to play. Saturn is the most

beautiful of the planets due to its stunning and very photogenic ring system

but its disc doesn’t typically show a lot of detail in comparison with its

nearer neighbour, Jupiter. Belts and zones can generally be seen, as well

as bright patches which represent storms in the planet’s atmosphere.

Imaging Saturn

The imaging techniques required for Saturn are similar to those described

for Mars and Jupiter although it’s not common to synthesize colour

channels for this world. Capture times using high-frame-rate planetary

cameras should be kept to less than 2 minutes per channel for RGB

images and 4–5 minutes for full-colour captures. The biggest hurdle when

imaging Saturn is the planet’s brightness or rather lack of it. It can be a

struggle getting a signal strong enough on chip to do the planet justice.

The sharpest features on Saturn are where the planet’s disc appears

to interact with the rings and these are regions which are very prone to

21

22

Astronomer Book