SkyandTelescope.com July 2014 73high-resolution plan-
etary images has become nearly commonplace. Today’s
sensitive, high-speed video cameras, coupled with
excellent optics, enable subarcsecond detail to routinely
appear in amateur images. But amateurs are capable of
much more than “pretty pictures.” Monochrome cameras
combined with specialized fi lters can deliver a wealth
of information about planetary activity. Exploring our
solar system beyond the visible spectrum allows ama-
teurs to produce scientifi cally useful data. And because
few professional observatories are dedicated to planetary
study, amateur observations are extremely useful. So let’s
explore what’s within our grasp.Look Deeper with Near-Infrared
Filters that block visual wavelengths and allow near-
infrared (near-IR) light to pass through have been popular
for many years. CCD cameras are sensitive to light
beyond 700 nm — the limit of the human eye’s sensitiv-
ity to light. Companies including Astronomik, Astrodon,
Baader Planetarium, Chroma Technology, and others
off er a range of fi lters passing light longer than 685 nm
suitable for astronomy.
Under our often-turbulent skies, images captured in
visible wavelengths are sometimes soft and blurry. But
due to the nature of light, longer wavelengths in red to
near-IR wavelengths are less aff ected by atmospheric
turbulence. Near-IR images are also less aff ected by dust
scattering within our atmosphere, which is one reason
why near-IR images have higher contrast than their
visible-light counterparts. But near-IR imaging does have
one signifi cant drawback: these longer wavelengths are
inherently lower resolution than light in the visible spec-
trum. In many instances though, the benefi ts outweigh
this shortcoming.
Because near-IR light penetrates the upper-atmosphere
hazes on all the gas giants, giving us a clearer view to
the layers below, astronomers use these images to studyfeatures partially obscured in visible wavelengths. This
eff ect is particularly visible on Jupiter. The planet’s main
cloud deck contains almost all the active structures of its
upper atmosphere: high- and low-pressure systems, erup-
tive plumes, and large swirling storms.
Saturn is a similar case, though imaging the Ringed
Planet through near-IR is even more revealing than it
is for Jupiter. The planet’s colder temperatures produce
thicker hazes above its cloud belts, so standard color
fi lters yield low-contrast images compared to near-IR pic-
tures. High-resolution near-IR images often record small
storms barely visible in standard visible wavelengths.
Perhaps the most interesting features we can record
with a near-IR fi lter are the atmospheric belts and occa-
sional bright spots on Uranus. These tiny details are
beyond even the Hubble Space Telescope’s reach in visible
wavelengths. Uranus has in recent years displayed a faint
equatorial belt as well as a brighter belt near the boundary
of its northern polar region. On Neptune, amateurs have
detected a bright near-IR spot in its southern hemisphere
as recently as 2013.
So what about the rocky planets? Mars displays very-
high-contrast albedo features in near-IR light, though
these same details are better resolved in red light. During
the onset of large dust storms, the haze-penetrating char-
acteristic of a near-IR fi lter is useful for increasing the
visibility of the boundaries of these dust clouds in relation
to the known albedo features.
Venus presents faint, grayish atmospheric band-
ing localized at a level too deep to be detected visually
(roughly 10 km lower in the atmosphere). Such banding is
interesting to follow day to day, as these cloud movements
are studied less than in other wavelengths.
Imaging Mercury through near-IR fi lters is most ben-
efi cial due to the planet’s close proximity to the Sun, mak-
ing it impractical to image at night (S&T: Oct. 2009, p.
70). Near-IR fi lters slightly improve the contrast between
the planet and the bright daylight sky when Mercury isCCAPTURINGAPTURING COLORCOLOR,,
Although a wealth of detail can be captured in visual wavelengths (left), shooting Jupiter through a methane fi lter (center) reveals
that certain storms, particularly the large Great Red Spot, reside high above the surrounding atmosphere. Images recorded through a
near-infrared fi lter (right) show details near the limb and poles of the planet that are otherwise hidden by high-altitude haze.CHRISTOPHE PELLIER (3)