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66 AUSTRALIAN SKY & TELESCOPE JULY 2016

Characteristicsoffiltersforplanetaryobserving

Wratten Colour Visible-light Dominant

number transmission wavelength (nm)

25 Red 14% 617

23A Light Red 25% 606

85 Salmon 68% 595

21 Orange 46% 589

15 Deep Yellow 66% 583

12 Yellow 74% 576

8 Light Yellow 83% 572

11 Yellow-Green 40% 550

56 Light Green 53% 552

58 Green 24% 533

82A Very Light Blue 73% 476

80A Light Blue 29% 479

38A Deep Blue 17% 478

47 Violet 3% 463

30 Magenta 27% 420&602

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

and80A)filtersrestrictobservationtotheuppermost

layers of the Martian atmosphere. These choices

accentuatethebrightnessandsharpentheboundaries

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 25 cm, 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 pleasing views of Mars are

provided by the Wratten 30 magenta filter, which

transmits both red and blue light at the opposite ends

of the visible spectrum while blocking yellow-green

wavelengths, simultaneously enhancing the contrast of

surface and atmospheric features. Baader Planetarium’s

Contrast Booster and Orion’s Mars Observation Eyepiece

Filter have dielectric coatings with similar spectral

characteristics.

Jupiter and Saturn

Although both the warm hue and the intensity of

the Great Red Spot vary markedly over the years, the

visibility of Jupiter’s most famous feature invariably

improves when you use a light blue (Wratten 80A

or 82A) filter. Blue filters also selectively darken the

belts’ ruddy hues, increasing their contrast with the

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.

The filters discussed in this article offer a variety of enhancements. Those with

low transmission values work best (or only) with larger-aperture telescopes.

S&T: SEAN WALKER

Planetary observing