SkyandTelescope.com July 2014 75component of the Venusian atmosphere is carbon dioxide,
which is translucent at the same wavelength, allowing the
thermal emission from the surface to escape into space.
Thermal emissions from the Venusian surface are
very weak in comparison to the blinding glare of sunlight
refl ected by its daytime clouds. For this reason, they can
only be observed on the night side of the planet, when
Venus is visible from Earth as a razor-thin crescent
shortly before and after inferior conjunction. To image
this thermal signal you’ll need a 1-micron IR fi lter and
you’ll have to take exposures several seconds long, delib-
erately overexposing the dayside crescent (S&T: Oct. 2010,
p. 72). Some darker regions can be seen on the glowing
night side: these are the mountaintops and other high-
lands, which appear darker due to the lower temperatures
found at their higher altitudes.Ultraviolet Light
The one other wavelength region useful for studying our
solar system’s planets resides at the opposite end of the
spectrum. These are UV wavelengths, from 300 to 400
nm. Invisible to our eyes, UV light is greatly aff ected by
our atmosphere; short wavelengths are blurred by our tur-
bulent atmosphere and scattered by water vapor and thin
clouds. Even our telescope optics block some UV light —
glass components from Barlow lenses or corrector plates
in Schmidt-Cassegrain telescopes absorb some UV, and
the sensitivity of CCD detectors is very low in this region
of the spectrum. The UV light accessible to amateurs
corresponds to the bands between 300 to 400 nm, also
known as UVC, the only one of the three UV bands that
manages to penetrate Earth’s ozone layer.
Imaging through a UV fi lter enhances details in the
upper cloud layers of the terrestrial planets. Venus par-
ticularly benefi ts from the use of a UV fi lter (S&T: Oct.,2007, p. 96). Its upper atmosphere contains a still-uniden-
tifi ed compound that strongly absorbs light at the UV
wavelength centered at 365 nm. The distribution of this
compound isn’t uniform in the Venusian atmosphere, so
UV-fi ltered images reveal the presence of large vortices
above the planet’s polar regions, often appearing as bright
cusps, while atmospheric waves near the equator form
features that resemble a Y or Psi (Ψ) shape. Following
these features for a number of consecutive days enabled
French amateur Charles Boyer in 1957 to determine the
rotation rate of the Venusian atmosphere to be around
four terrestrial days (S&T: June 1999, p. 56).
Images of the other planets through UV fi lters show
many of the same features recorded through blue and
violet visual fi lters, except Jupiter. Reddish storms in the
Jovian atmosphere appear particularly dark in UV wave-
lengths, and transient coloration events in the belts and
storms are easier to detect than at longer wavelengths.
In this recent golden age of planetary imaging, the
quality data amateurs routinely produce has led to some
exciting discoveries. And with the limited resources dedi-
cated to solar system studies at NASA, ESO, and other
professional observatories, amateur observations beyond
the visible spectrum are more valuable than at any time
in the past. So consider broadening your spectral range to
participate in this growing fi eld of pro-am collaboration.
You could provide valuable data that unlocks new discov-
eries in our knowledge of the solar system. ✦Christophe Pellier is a long-time planetary imager and
observer. Follow his imaging adventures at http://www.planetary-
astronomy-and-imaging.com.Right: While imaging Venus with a 1,000-nm fi lter shortly before
the planet’s inferior conjunction in 2004, the author was the fi rst
to record thermal emissions from the planet’s surface. The faint
darker markings in the bottom-right of the globe are cooler moun-
taintops. Left: This image was taken prior to the overexposed ver-
sion at right, demonstrating the visual appearance of the planet
on the same evening.Left: Venus presents swirls and banding in ultraviolet light,
whereas it has a mostly bland appearance in other wavelengths.
Right: UV images of Mars reveal the planet’s water vapor clouds.CHRISTOPHE PELLIER (2)
VENUS: SEAN WALKER; MARS: CHRISTOPHE PELLIER