Sky and Telescope - July 2016 - 53

Keen-eyed observer Thomas Dobbins wishes that he could see
Venus in the ultraviolet and Jupiter in the infrared.

light, so it will appear bright through a red ﬁlter but dark
through a green or blue ﬁlter.
Here then is an object-by-object guide for using ﬁ lters
when observing the four most rewarding planetary targets in the solar system.

Venus
Enveloped by highly reﬂective clouds that have an apparent surface brightness almost 10 times greater than
the full Moon's, Venus usually appears as featureless
as a frosted light bulb unless you take steps to reduce
its intense glare. Low-contrast markings on a uniform
background are best discerned under moderate levels
of illumination, an attribute of human perception that
psychologists call the Weber-Fechner law. So just reducing the intense brightness of Venus often reveals faint,
diﬀ use markings that would otherwise elude detection.
The simplest way to reduce glare is to use a neutraldensity ﬁlter. Better yet, get a variable polarizing ﬁlter.
An indispensable tool for observing Venus (or the Moon),
this device consists of a pair of identical polarizing ﬁ lters
in a rotating ring mount. Each polarizer acts like a miniature picket fence that permits only light waves vibrating in the direction of the pickets to pass. When their

S&T: SEAN WALKER (4)

The dazzling cloud canopy of Venus usually appears
featureless unless you use a neutral-density or variable
polarizing ﬁlter to reduce its intensity (as at right).

The windswept features in the atmosphere of Venus are
most prominent in the ultraviolet region of the spectrum
(as imaged at left). The eye can't see those wavelengths,
but some cloud details are visible (with reduced contrast)
with the aid of a deep blue or violet ﬁlter (as at right).

Thomas Dobbins

polarization axes are parallel, they transmit the most
light (typically at about 40%). But turn one polarizer so
its axis is perpendicular to the other's, and virtually no
light gets through. You'll be able to adjust the brightness of the image precisely over a broad range simply by
rotating one ﬁlter with respect to the other - much like
operating a dimmer switch.
Even with its dazzling brilliance suﬃciently subdued,
the cloudscape of Venus is extremely bland but rarely
featureless. You can often make out bright polar hazes
known as "cusp caps," occasionally surrounded by dusky
collars. Many reports suggest that these features are best
seen using a green (Wratten 56 or 58) ﬁ lter combined
with a neutral-density or variable polarizing ﬁlter. Seen
in blue and violet light, the caps tend to be lost in the
general brilliance of the planet's limb.
In 1927 Mount Wilson Observatory astronomer Frank
Ross photographed Venus through an ultraviolet (UV)
ﬁlter that transmitted wavelengths of 340 to 400 nanometers. To his surprise, the images captured a host of
rapidly changing features caused by localized concentrations of a UV-absorbing substance high in the planet's
cloud deck. The UV absorber imparts a very subtle
yellow cast to some of the planet's clouds, normally
perceived as pure snow white due to their dazzling brilliance. As Harvard astronomer William H. Pickering
once noted, "The color is strikingly shown if we view
Venus by daylight in the telescope while our terrestrial
clouds are crossing its disk."
The Venusian markings take the general form of diffuse, dusky bands at middle latitudes running roughly
perpendicular to the terminator. You'll often see an equatorial band, sometimes sporting a diverging forked tail
that resembles a horizontally aligned letter Y or the Greek
letter phi (ψ). A C-shaped feature, occasionally present
near the limb, occurs when that diverging fork is distorted
by foreshortening as it rotates onto the disk.
The contrast of the ultraviolet markings on Venus
is greatest at a wavelength of 340 nm, well beyond the
threshold of human vision. Seen at 400 nm, the shortest wavelength perceptible to most of us, their contrast
drops by a factor of ﬁve.
For visual observers, the Wratten 47 violet ﬁlter is
widely reputed to oﬀer the best hope of glimpsing the
UV features. Unfortunately, the eye's response to the
wavelengths transmitted by this dense ﬁlter (only 3% of
the visible spectrum) is feeble because your retina's cone
cells are comparatively insensitive to blue and violet light.
Although Venus is a brilliant subject, with a Wratten 47
ﬁlter you might ﬁnd it disappointingly dim even through
an 8-inch telescope - and perhaps suﬀer eyestrain during prolonged observing sessions. Many observers report
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Sky and Telescope - July 2016

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