Sky & Telescope - December 2019 - 6

FROM OUR READERS

Early this year a fellow member of the
Smoky Mountain Astronomy Society
(SMAS) and I rode to the top of Chilhowee Mountain in eastern Tennessee
to take pictures of the elusive Omega
Centauri. The summit overlooks Great
Smoky Mountains National Park and
has a very good view of the southern sky.
Unfortunately, clouds lingered there
for the whole night, and we had to pick
another target. The Hercules cluster,
M13, was rising in the east and in
perfect position for photography. Three
hours and 20 minutes later, I had accumulated a hundred 2-minute exposures,
and Jammey Church processed them. We
created a beautiful picture with brilliant
colors, but the story doesn't end there.
We proudly showed the results to
other SMAS members, with one taking
an acute interest. Michael McCullough

Duane Dunlap * Knoxville, Tennessee

Tidal Triggers

Over the Limit

In the News Note "Apollo-era Data
Reveal the Moon's Tectonic Activity"
(S&T: Aug. 2019, p. 8), Javier Barbuzano
notes that six of the 28 detected moonquakes occurred during lunar apogee,
when the Moon is farthest from Earth.
He explains, "The connection with apogee is important, because that's when
Earth's tidal pull per lunar surface area
is largest."
I thought that the farther the Moon
is from us, the weaker Earth's tidal pull
(gravitational stress) should be - or am
I mistaken?

Don Ferguson states that both his
3.5-inch and 7-inch Questar telescopes
exceed the resolving power predicted
by Dawes' equation (S&T: June 2019, p.
36). For example, while the Dawes limit
is 1.3Ǝ for a 3.5-inch scope, Ferguson's
achieves 1Ǝ. He gets similar results with
the 7-inch. Part of this is likely due
to the high quality of modern optical
components, compared to what Dawes
was working with in the 1860s, but I've
always suspected that Dawes was being
a bit conservative, too. (Interestingly,
Questar claims a 1Ǝ resolution for the
3.5-inch in its spec sheet.)
This leads me to wonder whether a
3.5-inch refractor would get even better
results. The 89-mm Questar scope is a
Maksutov with a central obstruction of
27.9 mm, giving it an effective aperture
of just 84.5 mm. Would a good apochromatic refractor of this aperture achieve
the same results as Ferguson's Questar?
Either way, I'm jealous of him for owning two of these marvelous telescopes!

Michel Bonnement
Courseulles sur Mer, France

"

Kelly Beatty replies: Researcher

Thomas Watters and his coauthors
point out that the quakes result from a
combination of global contraction, Earthinduced diurnal tides, and the Moon's orbital
recession. The fault scarps they found are
due to compression of the lunar crust, and
(counterintuitively) the diurnal and recession
stresses are most compressive near the tidal
axis when the Moon is near apogee.

6

was more interested in the objects captured in the background. After studying
the uncropped original (https://is.gd/
Dunlap_M13), he discovered more than
20 galaxies surrounding the cluster.
Researching the subject further, we
came across Bob King's "M13 with a Side
of Galaxies, Please" on the S&T website
(https://is.gd/M13_galaxies), which
helped greatly, but the story doesn't end
there either.
McCullough turned to the SIMBAD
database to identify additional galaxies.
Then he used redshift values to determine their corresponding distances in
light-years and an online estimator to
get the light-travel time, assuming a ﬂat
universe. One of the galaxies captured
in our image is more than 2 billion
light-years away!
This experience shows the value of
being in an astronomy club and reading
Sky & Telescope.

D EC E M B E R 2 019 * SK Y & TELESCOPE

Tom Sales
Somerset, New Jersey

p Messier 13, the majestic globular cluster
in Hercules, shares the field with many fainter
deep-sky companions - such as the 12thmagnitude galaxy NGC 6207 at upper left.
North is up.

I have owned ﬁve 3½-inch Questars
since 1966, and on the night of August
7, 1966, I elongated Gamma2 (γ 2)
Andromedae at 160× and 200× with
one of those scopes when the seeing
was virtually perfect. According to the
13th edition of Norton's Star Atlas, γ 2
Andromedae's 5.4- and 6.6-magnitude
components have an orbital period of
55 years and were at their widest separation in 1971.
Based on this and my visual observations of other doubles, I concluded the
telescope's resolution was around 3.5″/a
(with a the aperture in inches) - the
same as Ferguson found many years
later, rather than the Dawes value of
4.56″/a.
The ability of the Questar's optics to
resolve below the Dawes limit is readily
explained. As pointed out by Henry E.
Paul in his 1965 classic Telescopes for
Skygazing, the presence of a central
obstruction decreases the size of the
Airy disk of a diffracted point source.
Current Questar literature states this
obstruction is 32.5% of the aperture's
diameter. This results in a decrease in

DUANE DUNL AP & JA MME Y CHURCH

Membership
Has Its Privileges
https://www.is.gd/Dunlap_M13 https://www.is.gd/Dunlap_M13 https://www.is.gd/M13_galaxies
Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019
