Sky & Telescope - May 2024 - 53

OLD PHOTOGRAPH OF SOLAR GRANULATION: ATLAS DE PHOTOGRAPHIES SOLAIRES
(1903); GRANULATION CLOSEUP: NSO / AURA / NSF; SIMULATION: HEINRICH SIEDENTOPF
/ JAHRSCHRIFT DER ASTRONOMIE VOL 76 (1941)
Henri Bénard discovered that polygonal
patterns spontaneously form when
a thin layer of liquid is heated from
below. Three decades later, the German
astronomer Heinrich Siedentopf simulated
the appearance of solar granulation
by heating a shallow pan of molten
paraffin on a hot plate. He added
exceedingly fine particles of aluminum
to render rising and falling motions in
the viscous fluid visible.
By the mid-20th century, astronoreport
enjoying their best views of solar
granulation only an hour after sunrise.
Seeing tends to improve late in the
afternoon and early in the evening as
solar heating subsides.
Whenever possible, avoid looking
mers used spectrographs to record the
Doppler shifts produced by vertical
motions in the solar photosphere. This
feat removed any lingering doubts that
granules are ascending cells of buoyant
hot plasma that cool to form darker
lanes of denser, descending plasma.
Small differences in temperature create
a surprisingly large range of brightness,
because luminosity varies by the fourth
power of temperature, a relationship
called the Stefan-Boltzmann law.
A typical granule has a diameter
of about 1,500 km - about the size
of the state of Texas - and a lifespan
of roughly 5 to 10 minutes. At any
given moment, a network of about 4
million granules covers the seething
photosphere.
Observing solar granulation presents
special challenges because the Sun
is not only a source of intense light,
but extreme heat as well. To achieve
diffraction-limited performance, large,
state-of-the-art solar telescopes employ
liquid-cooled optical elements and
elaborate ventilation systems or vacuum
light paths, often supplemented with
adaptive optics. To maximize their
potential resolution, they are located on
the summits of tall coastal peaks, where
gentle laminar winds produce excellent
seeing conditions.
For amateur observers of the Sun, the
safe, full-aperture solar filters based on
reflective metallic coatings deposited on
substrates of glass or thin Mylar films
prevent 99.99% or more of the Sun's
light and heat from entering a telescope.
The problems posed by heat-induced
turbulence within the instrument that
professional solar astronomers grapple
with at great expense are eliminated. It
pA remarkably detailed 1877 photograph of
solar granulation by Jules Janssen
is the telescope's immediate surroundings
that remain problematic.
Sub-arcsecond daytime seeing is
very rare at most observing sites. At
night the best seeing is found near the
zenith, where the line of sight through
the atmosphere is shortest. During the
day, however, the best seeing usually
occurs early in the morning when the
Sun is between 15° and 30° above the
horizon. As the Sun gains altitude,
it warms the ground, and convective
turbulence begins to play havoc with
a telescope's ability to resolve fine
detail. Many experienced observers
over rocky terrain devoid of vegetation.
Rooftops and large masses of concrete
or asphalt also degrade the view. A wide
expanse of lawn or a grass-covered field
makes a far better observing site. Views
over bodies of water are often excellent.
Although often impractical, observing
from a high balcony or other elevated
perch can dramatically improve viewing
by positioning the scope well above the
worst ground-level convection.
Even under mediocre seeing conditions,
solar granulation is visible in a
4-inch telescope at a magnification of
about 100×. On rare occasions when
the atmosphere is momentarily very
calm, a telescope with an aperture
of 6 to 8 inches at a magnification of
250× will provide breathtaking views
of the simmering solar cauldron, where
features form and dissipate in a matter
of minutes.
¢Contributing Editor TOM DOBBINS
writes his observing columns after deep
dives into the historical record.
pThe patterns of convective Bénard cells created in Heinrich Siedentopf's laboratory more than
80 years ago (right) are uncannily similar to solar granulation seen at left recorded with the Daniel
K. Inouye Solar Telescope at Haleakala¯ Observatory on Maui.
sk yand tele scope .o r g * MAY 2024 53

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