Sky and Telescope - November 2018 - 53

LE AH TISCIONE / S&T

very minute planetary detail can only
". . . a 10- or 12-inch instrument of high quality is capabe discerned "by the high powers which
ble of revealing at least 75% of what can be seen on the
may be used with large instruments."
However, he argued that, "What the
Moon or brighter planets through even the largest Earthminor telescope lacks in point of light it
based instruments."
gains in definition. When the seeing is
good in a large aperture it is superlative
not marked, but with good seeing it is
of meters overhead. Small wonder that
in a small one. When unusually high
most striking."
Thomas Romney Robinson (1792-1882),
powers may be employed in the former,
Today fans are integral components
who used the 72-inch "Leviathan of
far higher ones proportionately may be
of many commercial telescopes. But
Parsontown" reflector for many years,
used with the latter."
reported in 1871 that "any differences of even with the aid of fans, it is difficult
Harvard College Observatory
for a mirror of more than 50 mm (2
temperature between the speculum and
astronomer William Henry Pickering
inches) thickness to closely "follow"
the air in the tube is capable of injuring
(1858-1938) founded an observatory in
falling evening temperatures. That
or even destroying definition, though
the hills of Jamaica where the modest
thickness is typical of the thin mirrors
the speculum be absolutely perfect.
diurnal temperature variation routinely
of modern 16- to 24-inch reflectors that
Hence there are few hours in the year
made the humid atmosphere more
when the 6-foot can display its full pow- must be supported by elaborate flotation
tranquil than at sites at high elevacells to keep their optical figure from
ers." One of Robinson's colleagues comtions in the Andes Mountains. Despite
distorting under their own weight.
plained that during a two-year interval
the often superb conditions, Pickering
So, for lunar and planetary observers,
he enjoyed only three hours of excellent
alleged that 11- to 15-inch instruments
how much does telescope size matter?
definition through the great reflector.
showed finer detail than larger ones on
Instruments with apertures smaller than
The use of fans to provide a laminar
nine nights out of ten. Even on the very
8 inches are certainly capable of providcurrent of air that sweeps away the
best nights, "Nothing was to be gained
ing very satisfying views, but they are
warm boundary layer and accelerates
by using more than 20 inches."
less than optimal in terms of resolution
cooling was pioneered in the 1920s by
Many large telescopes are even more
and lack the image brightness required
Pickering. He reported that "with poor
handicapped by their own thermal
to reveal the muted pastel hues of many
seeing due mainly to currents in the
properties than they are by atmospheric
planetary markings. Based on five
upper air the resulting improvement is
turbulence. At most locations the
decades of observing through
temperature on a clear evening
a vast array of telescopes,
falls at a rate of 2°C to 3°C
Incoming
starlight
I'd venture to say that under
per hour. In still air a glass
excellent conditions, a 10- or
primary mirror 30 mm (1.2
Undistorted light wave
12-inch instrument of high
inches) thick cools at a rate
quality is capable of revealing
of about 3°C per hour, but a
Turbulent atmosphere
at least 75% of what can be
76-mm (3-inch) thick mirror
seen on the Moon or brighter
cools at a rate of less than 1°C
Distorted
planets through even the largper hour. Assuming that its
light wave
est Earth-based instruments.
thickness-to-diameter ratio is
The larger apertures required to
held constant, the mass of a
Small total
Large total
distortion
see the remaining 25% involve
mirror (corresponding to its
distortion
rapidly diminishing marginal
heat capacity) increases with
Large
Small
returns. Many of the finest
the cube of its diameter, but its
aperture
aperture
images of the Moon and plansurface area (corresponding to
ets have been captured using
its ability to transfer heat to its
Large angular
Small angular
12- to 16-inch instruments.
surroundings) only increases
displacement
displacement
It's no coincidence that this
by the square of its diameter.
is the same size "sweet spot"
Large mirrors are very
p The effects of atmospheric seeing on small and large telescopes
determined
by so many visual
inefficient at shedding heat.
are shown here. A light wave arriving from the target (a planet or star)
observers generations ago.
Convection produces a thin,
is distorted by the turbulent atmosphere. When the distorted wavefront enters a telescope, its average "tilt" determines the target's
turbulent boundary layer of
position, while the total range of distortion influences the blurriness
warmer air just above the
¢ Contributing Editor THOMAS
of the view. A small-aperture telescope sees a large displacement,
surface of the optic that can
A. DOBBINS currently observes
but not much distortion, so the target appears relatively sharp but
blur the image every bit as
the planets using a 10-inch f/8
dances around, while a large telescope displays a blurry view while
much as turbulence thousands
remaining relatively still.
Newtonian reflector.
sk yandtelescope.com * N OV E M B E R 2 018

53


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Sky and Telescope - November 2018

Table of Contents for the Digital Edition of Sky and Telescope - November 2018

Contents
Sky and Telescope - November 2018 - Cover1
Sky and Telescope - November 2018 - Cover2
Sky and Telescope - November 2018 - 1
Sky and Telescope - November 2018 - Contents
Sky and Telescope - November 2018 - 3
Sky and Telescope - November 2018 - 4
Sky and Telescope - November 2018 - 5
Sky and Telescope - November 2018 - 6
Sky and Telescope - November 2018 - 7
Sky and Telescope - November 2018 - 8
Sky and Telescope - November 2018 - 9
Sky and Telescope - November 2018 - 10
Sky and Telescope - November 2018 - 11
Sky and Telescope - November 2018 - 12
Sky and Telescope - November 2018 - 13
Sky and Telescope - November 2018 - 14
Sky and Telescope - November 2018 - 15
Sky and Telescope - November 2018 - 16
Sky and Telescope - November 2018 - 17
Sky and Telescope - November 2018 - 18
Sky and Telescope - November 2018 - 19
Sky and Telescope - November 2018 - 20
Sky and Telescope - November 2018 - 21
Sky and Telescope - November 2018 - 22
Sky and Telescope - November 2018 - 23
Sky and Telescope - November 2018 - 24
Sky and Telescope - November 2018 - 25
Sky and Telescope - November 2018 - 26
Sky and Telescope - November 2018 - 27
Sky and Telescope - November 2018 - 28
Sky and Telescope - November 2018 - 29
Sky and Telescope - November 2018 - 30
Sky and Telescope - November 2018 - 31
Sky and Telescope - November 2018 - 32
Sky and Telescope - November 2018 - 33
Sky and Telescope - November 2018 - 34
Sky and Telescope - November 2018 - 35
Sky and Telescope - November 2018 - 36
Sky and Telescope - November 2018 - 37
Sky and Telescope - November 2018 - 38
Sky and Telescope - November 2018 - 39
Sky and Telescope - November 2018 - 40
Sky and Telescope - November 2018 - 41
Sky and Telescope - November 2018 - 42
Sky and Telescope - November 2018 - 43
Sky and Telescope - November 2018 - 44
Sky and Telescope - November 2018 - 45
Sky and Telescope - November 2018 - 46
Sky and Telescope - November 2018 - 47
Sky and Telescope - November 2018 - 48
Sky and Telescope - November 2018 - 49
Sky and Telescope - November 2018 - 50
Sky and Telescope - November 2018 - 51
Sky and Telescope - November 2018 - 52
Sky and Telescope - November 2018 - 53
Sky and Telescope - November 2018 - 54
Sky and Telescope - November 2018 - 55
Sky and Telescope - November 2018 - 56
Sky and Telescope - November 2018 - 57
Sky and Telescope - November 2018 - 58
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Sky and Telescope - November 2018 - 60
Sky and Telescope - November 2018 - 61
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Sky and Telescope - November 2018 - Cover3
Sky and Telescope - November 2018 - Cover4
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