A (6)

64 AUSTRALIAN SKY & TELESCOPE JULY 2016

V

eteran planet observers have long maintained

that the eye can be trained to see details that

elude the novice. But sometimes even the best-

trained eye can use a bit of help at the eyepiece.

Formorethanahalfcentury,colourfiltershave

been indispensable tools for visual observers of the

planets. Most of the filters available today are discs

of tinted glass mounted in cells that thread into the

barrels of standard 1¼- or 2-inch eyepieces. They

employ the numerical designations of the Kodak

Wrattenseriesofgelatinfilters,originallydeveloped

for photographic applications more than a century ago.

In recent years, interference filters — produced

bythevacuumdepositionofmultiplethinlayers

ofdielectricfilmsonglass—haveappearedon

the market. These filters offer higher overall light

throughput and more complex spectral properties

than the tried-and-true Wratten series. But the latter

are available in far greater variety and still provide an

inexpensivewaytoimproveyourviewsoftheplanets.

When you observe a planet telescopically at a

modest altitude above the horizon, its disk exhibits

Getbetterviewsoftheplanets

blueandredfringesalongitstopandbottomlimbs.

This phenomenon, known asatmospheric prismatic

dispersion,resultsbecausethehigherrefractiveindexof

air at shorter wavelengths causes the blue component

of the image to be lifted to a greater extent than the

redcomponent.Atanaltitudeof30°,theseparation

between blue and red light is about 1 arcsecond, the

resolution limit of a 114-mm telescope. Even at an

altitude of 45°, the visible spectrum smears out over 0.6

arcsecond, the resolution limit of a 20-cm telescope.

By reducing the intensity of blue light, yellow

(Wratten 12 and 15) and yellow-green (Wratten 11)

filters sharpen images of low-lying planets and enable

you to see finer details. These filters are also invaluable

for eliminating the defocused blue and violet light that

produces the objectionable purple fringing formed by

refractors with achromatic lenses.

The principal benefit of colour filters is enhancing

contrast.Withrareexceptions,planetarymarkingshave

delicate,pastelhues—notthevivid,saturatedcolours

thatappearinthevastmajorityofspacecraftand

webcamimagessooftenseenthesedays.Colourfilters

candramaticallyimprovethecontrastandvisibility

of features that differ even slightly in hue. A reddish

feature, for example, reflects red light but absorbs green

andbluelight,soitwillappearbrightthroughared

filterbutdarkthroughagreenorbluefilter.

Here, then, is an object-by-object guide for using

filters when observing the four most rewarding

planetary targets in the Solar System.

Venus

Envelopedbyhighlyreflectivecloudsthathavean

apparent surface brightness almost 10 times greater

These simple observing accessories can bring out

moredetailswhenyouobservebright planets.

Coloured filters are useful
aids when viewing the planets
telescopically. They’re sold both
individually and in sets.

A variable
polarising filter
lets you ‘dial in’
the optimum
brightness for
viewing the
brilliant disk of
Venus or the
Moon. ORION TELESCOPES & BINOCULARS

THOMAS DOBBINS

Planetary observing