Sky and Telescope - June 2016 - 6

Letters

Wow, Gary Seronik's "Telescope Making
Then and Now" (S&T: Jan. 2016, p. 72)
brings back memories! I've been a Sky &
Telescope subscriber since 1959, and back
then we created a lot of our own equipment. I got started by constructing a
4¼-inch Newtonian, including grinding
my own mirror!
My dad built, with factory parts, an
8-inch Newtonian. (Considering he was
the president of a manufacturing company, he was quite mechanically skilled.)
He bought the ½ 0 -wave primary and
secondary mirrors from Cave Optical,
along with the tube, spider, and mirror
mounts. The German equatorial mount
was from Magnusson.
I learned and really enjoyed the many
hours of helping him lay out a to-scale
diagram of the optical path and, later,
properly drilling the holes for the mounts
and ﬁnder scope. I spent several years
hauling that telescope outside to three
concrete pads I'd dug. Then in 1961 he
built a 10-by-10-foot observatory with a
bed, desk, and roll-oﬀ roof. I even had a
darkroom in the basement for developing
my pictures of the Moon.
A few years later we built a 16-inch f/16
Cassegrain. He, along with his toolmaker
at work, built a skeleton tube out of angle
iron (bent into circles) and All Thread
rods. Then we made a fork mounting
out of channel iron with a 2-inch equatorial shaft inserted into a part scavenged
from an injection-molding machine.
He managed to obtain the mirrors from
John Hindle (son of John H. Hindle) in
England. All this was mounted to a 12-by12-inch concrete column that extended 8
feet high and 4 feet into the ground!

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June 2016 sky & telescope

Ash Dome was just 15 miles down
the road, and we paid a visit. But rather
than buy a ready-made dome, my dad
bought a silo, which Ash Dome also sold.
We erected it, cut out a 30-inch slot, and
made a pair of Palomar-type shutters.
That scope never had the use it deserved
- by then high-school studies and girls
took up much of my time! Sadly, there
was no room for it at my home when I got
married. I still have the optics - but just
for nostalgia.
Paul Rybak
Topton, North Carolina

Did Cassini See
the Great Red Spot?
Having written Sky & Telescope's May
1968 cover story, "The Discovery of
Jupiter's Red Spot," I was somewhat
taken aback by a caption in Amy Simon's
article (S&T: Mar. 2016, p. 18) that reads,
"Historians generally (but not universally) believe [that 17th-century] observers saw something other than the Great
Red Spot."
I don't know who these "historians"
are, but I made a strong case that the feature seen by Cassini and by his nephew
Giacomo Maraldi during 1665 to 1713
was, indeed, the Great Red Spot.
Perhaps the most signiﬁcant diﬀerence
between the 17th-century spot and the
modern one has been its size. Cassini's
spot was apparently 9½° to 13½° across,
roughly a third of the GRS's typical width
during the 20th century. But, as the main
point of Simon's article emphasizes, it
has now shrunk to the size of Cassini's
feature.
Everything else regarding the ancient
and modern spots, such as latitude and
cycles of fading and reappearance, is
similar. It's been claimed that Cassini's
spot had a very diﬀerent rotation period,
but those early measurements were not
robust; during 1693 the spot might well
have had a period similar to that of the
modern Great Red Spot.
Clark R. Chapman
Southwest Research Institute
Boulder, Colorado

VATICAN MUSEUM

Fond Telescope Memories

Donato Creti's 1711 painting shows a detailed
Jupiter and three of its moons looming large
in the sky. The oval spot above center (which
would be toward south in that era's refracting
telescopes) bears some remarkable similarities
to the Great Red Spot.

Amy Simon replies: The size diﬀerence is
indeed a key factor, but other descriptions
I've read have been a bit vague about some
of the spot's other aspects as well. However, the exceptionally long gap in credible
reports, almost 200 years, is probably the
real proof. The GRS was distinct enough
in the late 1800s that earlier observers, had
they seen it, surely would have reported it.
Cassini, Hooke, and likely other contemporaries saw something, but there's no clear
evidence it is the same storm we see today.

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016