Sky and Telescope - July 2016 - 54

OBSERVING

Exploring the Solar System
Martian surface
features stand out
far more boldly in red
or orange light than
in green light. Only
clouds high in the
planet's atmosphere
are evident when
viewed in blue light.
S&T: SEAN WALKER

better results using the Wratten 38A blue ﬁlter.
At the opposite end of the spectrum, you'll ﬁnd red
(Wratten 23A and 25) and orange (Wratten 21) ﬁlters
very useful when observing Venus - and Mercury as
well - high in the sky during daylight hours because
these greatly reduce the apparent brightness of the brilliant blue background sky.

Mars
Color ﬁlters will enable you to study the Red Planet's
meteorology. Although the Martian atmosphere is very
tenuous, it scatters an appreciable amount of light and

Characteristics of Filters for Planetary Observing
Wratten
number

Color

Visible-light
transmission

Dominant
wavelength (nm)

Red

14%

617

23A

Light Red

25%

606

Salmon

68%

595

Orange

46%

589

Deep Yellow

66%

583

Yellow

74%

576

Light Yellow

83%

572

Yellow-Green

40%

550

Light Green

53%

552

Green

24%

533

82A

Very Light Blue

73%

476

80A

Light Blue

29%

479

38A

Deep Blue

17%

478

Violet

463

Magenta

27%

420 & 602

The ﬁlters discussed in this article oﬀer a variety of enhancements.
Those with low transmission values work best (or only) with largeraperture telescopes.

July 2016 sky & telescope

usually appears virtually opaque at wavelengths shorter
than 450 nm (in the deep-blue region of the visible spectrum). In 1871 Lord Rayleigh demonstrated that the scattering of light by gas molecules is inversely proportional
to the fourth power of its wavelength. Consequently, violet
light at a wavelength of 400 nm is scattered 9.4 times
more than red light at 700 nm.
You can use these scattering properties to your
advantage. Violet (Wratten 47) and blue (Wratten 38A
and 80A) ﬁ lters restrict observation to the uppermost layers of the Martian atmosphere. These choices accentuate the brightness and sharpen the boundaries of high
cirrus clouds, hazes over the planet's poles and along
the morning and evening limbs, and the orographic
(mountain-generated) clouds that form over the Tharsis
and Elysium shield volcanoes.
A rare and poorly understood phenomenon known as
the "blue clearing" occurs when the surface features of
Mars become visible in blue and violet light for periods
of several days or even weeks, usually around the date
of opposition. Use Wratten 38A blue and Wratten 47
violet ﬁlters to detect and monitor these clearings, which
might be limited to only one hemisphere and ﬂuctuate
rapidly in intensity.
Features that appear white when unﬁltered - and
brighter through a green (Wratten 56 or 58) ﬁlter than
in blue or violet light - correspond to patches of surface
frost or low-lying fogs. Dust clouds appear brighter and
have sharper boundaries through yellow (Wratten 12 and
15) ﬁlters than they do when viewed in green or blue light.
Use Wratten 85 salmon and Wratten 21 orange
ﬁlters to dramatically increase the contrast between the
planet's ochre "deserts" and its duskier "maria." The
distinction between these surface features is greatest
through red ﬁlters: either Wratten 23A or, for apertures
greater than 10 inches, the denser Wratten 25. Red light
also penetrates the "hood" of haze and cirrus that's usually present over the Martian polar caps, sharpening its
boundaries and revealing its true extent.
Some of the most aesthetically pleasing views of Mars
are provided by the Wratten 30 magenta ﬁ lter, which
transmits both red and blue light at the opposite ends
of the visible spectrum while blocking yellow-green

Sky and Telescope - July 2016

Table of Contents for the Digital Edition of Sky and Telescope - July 2016