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http://www.skyandtelescope.com.au 65

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, diffuse markings that would

otherwise elude detection.

Thesimplestwaytoreduceglareistousea

neutral-densityfilter.Betteryet,getavariable

polarising filter. An indispensable tool for observing

Venus (or the Moon), this device comprises a pair of

identical polarising filters in a rotating ring mount.

Eachpolariseractslikeaminiaturepicketfencethat

permitsonlylightwavesvibratinginthedirection

ofthepicketstopass.Whentheirpolarisationaxes

are parallel, they transmit the most light (typically at

about 40%). But turn one polariser so that its axis is

perpendicular to the other’s, and virtually no light will

getthrough.You’llbeabletoadjustthebrightness

oftheimagepreciselyoverabroadrangesimplyby

rotatingonefilterwithrespecttotheother—much

likeoperatingadimmerswitch.

Even with its dazzling brilliance sufficiently

subdued, the cloudscape of Venus is extremely

blandbutrarelyfeatureless.Youcanoftenmakeout

brightpolarhazesknownas‘cuspcaps,’occasionally

surrounded by dusky collars. Many reports suggest

that these features are best seen using a green

(Wratten 56 or 58) filter combined with a neutral-

densityorvariablepolarisingfilter.Seeninblueand

violetlight,thecapstendtobelostinthegeneral

brillianceoftheplanet’slimb.

In 1927, Mount Wilson Observatory astronomer

FrankRossphotographedVenusthroughanultraviolet

(UV)filterthattransmittedwavelengthsof340to400

nanometres. To his surprise, the images captured a

hostofrapidlychangingfeaturescausedbylocalised

concentrations of a UV-absorbing substance high in

theplanet’sclouddeck.TheUVabsorberimpartsa

very subtle yellow cast to some of the planet’s clouds,

normallyperceivedaspuresnowwhiteduetotheir

dazzling brilliance. As Harvard astronomer William

H. Pickering once noted, “The colour 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

YortheGreekletterphi(ψ). A C-shaped feature,

occasionallypresentnearthelimb,occurswhenthat

diverging fork is distorted by foreshortening as it rotates

onto the disk.

ThecontrastoftheultravioletmarkingsonVenus

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

contrastdropsbyafactoroffive.

Forvisualobservers,theWratten47violetfilteris

widelyreputedtoofferthebesthopeofglimpsingthe

UV features. Unfortunately, the eye’s response to the

wavelengths transmitted by this dense filter (only 3%

ofthevisiblespectrum)isfeeblebecauseyourretina’s

cone cells are comparatively insensitive to blue and

violet light. Although Venus is a brilliant subject, with

aWratten47filteryoumightfinditdisappointingly

dimeventhrougha20-cmtelescope—andperhaps

suffer eyestrain during prolonged observing sessions.

Many observers report better results using the Wratten

38Abluefilter.

At the opposite end of the spectrum, you’ll find red

(Wratten 23A and 25) and orange (Wratten 21) filters

veryusefulwhenobservingVenus—andMercuryas

well—highintheskyduringdaylighthoursbecause

these greatly reduce the apparent brightness of the

brilliantbluebackgroundsky.

Mars

Colour 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 usually appears virtually opaque at wavelengths

shorterthan450nm(inthedeep-blueregionofthe

Below left: The

dazzling cloud

canopy of Venus

usually appears

featureless

unless you use a

neutral-density or

variable polarising

filter to reduce its

intensity (right-

hand image).

Below right:

The windswept

features in the

atmosphere of

Venus are most

prominent in the

ultraviolet region

of the spectrum

(left-hand

image). 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

S&T: (right).

SEAN WALKER (4)

S&T:

SEAN WALKER / SOURCE: TELESCOPES.NET