Sky & Telescope - November 2020 - 53

approximately equal to the optical path
length of the star diagonal.
Schmidt- and Maksutov-Cassegrains
that focus by moving the primary
mirror can easily accommodate a pair
of star diagonals without recourse to
the Barlow lens, but it's best to use
a 2-inch-format diagonal as the ﬁrst
diagonal of the pair to avoid vignetting.
The ﬁrst star diagonal is inserted in
the focuser and rotated until the output
barrel is parallel to the ground. You
then insert the second diagonal with
the attached Barlow lens into the output barrel of the ﬁrst diagonal. You can
simply swivel the second diagonal to a
comfortable viewing angle, regardless of
the altitude of the object under study.

M A RS: STSCI / N ASA; POL A RIZER: TO M DOBBINS

Dimming the View
Another accessory that increases its
"bang for the buck" when used in an
unconventional way is a variable polarizing ﬁlter. This device consists of a pair
of identical polarizing ﬁlters screwed
together, each serving as a miniature
picket fence that permits only light
waves vibrating in the direction of the
pickets to pass. When the axes of the
polarizers are parallel, the amount of
incident light transmitted is maximized
(typically at about 40%), but when the
axis of one polarizer is oriented perpendicular to the axis of the other virtually
no light is transmitted. You can precisely adjust the brightness of the image
over a broad range simply by rotating
one ﬁlter with respect to the other,
much as with a dimmer switch.
I prefer to separate the polarizing ﬁlters and screw one into the eyepiece and
the other into a short extension tube
that remains stationary. This varies the
image brightness by simply rotating the
eyepiece without interrupting observation. Steer clear of variable polarizing
ﬁlters that cannot be separated in this
fashion, to avoid the annoyance of having to withdraw the eyepiece in order to
rotate one ﬁlter relative to the other.
u Attaching each half of a variable polarizing filter to the eyepiece and the other to an
extension tube allows you to simply rotate your
eyepiece to reduce the glare of a bright planet
or lunar crater.

South

North

West

Even number
of reﬂections

Odd number
of reﬂections

p Newtonian reflectors produce a correct-reading, though upside-down image because they
reflect the light path twice (left). Cassegrains and refractors usually reflect the optical path an odd
number of times, resulting in a view that is right-side-up, but mirror-reversed as seen at right.

One vastly under-appreciated attribute of a variable polarizing ﬁlter is its
ability to render the chromatic aberration of refractors less obtrusive. Not
long after the invention of the telescope, smoked-glass lenses were often
employed to suppress the false color
produced by the era's primitive singleelement objectives. Simply reducing
image brightness makes the defocused
red, blue, and violet light that constitute the purple haze of "secondary
spectrum" of doublet achromat refractors much less conspicuous. Unlike the
"minus violet" ﬁlters that are widely
used for this purpose, a variable polarizing ﬁlter will not impart a yellow cast to
the image, which many observers ﬁnd
objectionable.
Low-contrast markings on a uniform background are best discerned
under moderate levels of illumination,
an attribute of human perception that

psychologists call Weber-Fechner's law.
Dimming excessively bright images
often reveals markings that would otherwise elude detection, notably delicate
lunar rilles or festoons and narrow belts
and rifts on Jupiter.
Variable polarizing ﬁlters are indispensable when observing Venus. Enveloped in a canopy of highly reﬂective
clouds that have an apparent surface
brightness almost 10× greater than
the full Moon, Venus about invariably
appears featureless unless steps are
taken to reduce the intense glare. The
cloudscape of Venus is rarely devoid of
markings if its dazzling brilliance is sufﬁciently subdued. Other targets that are
sufﬁciently bright to beneﬁt from variable polarizing ﬁlters, albeit to a lesser
degree, are the Moon, Mars, and Jupiter.
With my 6-inch f/8 achromat, I'm
consistently able to see more detail on
Jupiter's disc at 140× using a variable
polarizing ﬁ lter transmitting about
25% of incident light than I can at
200× without the ﬁ lter, despite the
smaller image scale that lower magniﬁcation provides. This holds true even
when seeing conditions are excellent
and the higher magniﬁcation should
provide the more detailed view. The
saturation of the planet's subtle pastel
hues is also increased.
¢ Contributing Editor TOM DOBBINS always enjoys observing the planets from
comfortable viewing angles.
sk yandtelescope.org * NOVEMBER 2 02 0

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Sky & Telescope - November 2020

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