The planets
[17] Amateur astronomer Damian Peach’s prize-winning photograph of Jupiter, Io (left) and Ganymede. The image was taken through Damian’s 14-inch SCT telescope from Barbados.
lower a planet appears in your sky, as the amount of atmosphere its light
has to pass through gets progressively thicker closer to the horizon. This is
where a device known as an atmospheric dispersion corrector (ADC) can
help. An ADC counters the atmospheric dispersion by introducing prisms
of its own to recombine the colours. As dispersion varies with altitude,
one downside of using an ADC is that it needs to be constantly adjusted
throughout a typical imaging session for the best results.
As Jupiter is so bright, imaging it isn’t something limited to the realm
of the high-resolution specialist; some pleasing results can be achieved
with more modest equipment too. A simple camera attached to a tripod
will normally record Jupiter as a dot in the sky as long as the camera’s
sensitivity is set reasonably high. Many camera phones can capture the
planet as a dot too.
For cameras with a zoom or telephoto lens, it is possible for a short
exposure of the planet to show both the planet’s disc and the Galilean moons
- assuming they are visible at the time of the shot of course.
Jupiter’s brilliance also sets it up as a good target for afocal photography,
the technique of pointing a camera with a lens fitted down the eyepiece of
a telescope. Although such images will normally appear small and slightly
blurred, they can show the main belts of Jupiter as well as the four Galilean
moons quite well. Some quite impressive shots of Jupiter with mobile-
phone cameras have been taken using this technique.
17Jupiter's Moons
Jupiter has a whole family of satellites, over 60 in all. Most are small and no
doubt captured asteroids. Of the four large moons, Io is slightly larger than
our Moon, Europa slightly smaller, and Ganymede and Callisto considerably
larger; Ganymede is actually larger than the planet Mercury. All four are
visible via any small telescope and they may pass behind Jupiter and be
occulted, or eclipsed, or in transit across the planet's disc.
They are by no means all alike. Io is the most volcanic world we know,
with violent eruptions going on all the time. It is right in the middle of the
dangerous radiation zones surrounding Jupiter and we are not likely to want
to visit it!
Europa has a smooth icy surface and probably an ocean of liquid water
underneath. There might even be life present in these oceans, perhaps
similar to the life that appears around hydrothermal vents deep in the
darkest parts of Earth's oceans. Ganymede and Callisto are mountainous
and cratered. All the other moons are much smaller.Imaging Jupiter’s Galilean Moons
Jupiter has an ever-growing family of moons (at the time of writing the
current count stands at 64). However, only four of these moons are easily
seen, the rest being quite faint with many in inclined orbits so that they
appear away from Jupiter’s equatorial plane.