Sky & Telescope - March 2024 - 6

BALL SCOPE: PETER B. VAN DER WAL
FROM OUR READERS
The Bottle of Labeyrie
I'd like to react to Jerry Oltion's " The
Darkness Before the Dawn " (S&T: Nov.
2023, p. 72) on the construction of a
ball scope. He noted that Pierre Lemay
had done it 10 years earlier. Both Jerry
and Pierre were probably not aware that
the famous French optical engineer and
astronomer Antoine Labeyrie built two
ball scopes from concrete for stellar
interferometry at the CERGA (Center
for the Research of Geodynamics and
Astronomy) Observatory on the Plateau
de Calern near the town of Grasse in
Provence, France. Now the observaBringing
the Sun Indoors
I enjoyed Jerry Oltion's article " Build a
Solar Projector " (S&T: Dec. 2023, p. 72).
This spring, I built a variation on that
theme: a solar projector with a variablefocal-length
lens system that projects
a focusable image of the Sun into my
house. The impetus was an old Gaertner
Scientific coelostat that I had inherited,
thinking " wouldn't it be fun to have a
live solar image in the house. "
The projector lens system consists
of two simple lenses, one positive and
the other negative. With the appropriate
choice of focal
lengths, the effective
Bernd Enders built
a solar projector that
projects an image of
the Sun into a frame
on his living-room wall
from 40 feet away.
6 MARCH 2024 * SKY & TELESCOPE
pThis massive, concrete ball scope is one
of a pair that sits on the Plateau de Calern in
southeastern France.
tory is part of Côte d'Azur University in
Nice. Labeyrie is known for introducing
speckle interferometry in 1970. In 1977,
I spent two weeks on the Plateau de
Calern for my research and met Labeyrie.
I found the above picture of what my
French colleagues called the " bottle of
Labeyrie " in my photo archives.
Peter B. van der Wal
Stephanion Observatory
Stefani, Greece
focal length of the system is a function
of the lens spacing, so the system can
project a solar image anywhere from a
few feet all the way to infinity.
The trick turned out to be finding
a suitable combination of commercial
lenses. (I wasn't prepared to make my
own lenses!) After numerous online
purchases of inexpensive lenses (with
focal lengths often very different from
the advertised focal lengths) and some
experimentation, I came up with a functioning
solar projector with an image
distance of about 40 feet, approximately
f/430, projecting a 4-inch image onto a
screen framed by a gold frame hanging
on my living-room wall!
Bernd Enders
San Rafael, California
Three Cheers for M33
I thoroughly enjoyed Howard Banich's
article " Messier 33: An Observer's
Guide " (S&T: Nov. 2023, p. 58) and
the sketches of the Triangulum Galaxy
and its detailed nuances. We have a
tendency, I think, to consider galaxies
as single entities. Banich's description
of M33's components was a delight.
It prompted me to look at my
attempts at imaging the galaxy from
2020. Living with a suburban lightpolluted
sky means I don't bother
trying to observe or image galaxies
much. But occasionally I have a
chance at darker skies, and then I do.
So it made me happy that I could find
some of the celestial bodies Banich
describes in my photo taken from a
Bortle 4/5 site, even with a smallaperture
setup. The next time I get
a chance at darker skies, I'll attempt
longer exposures on M33 and visual
observations as well.
Steve Altic
Columbus, Ohio
Howard Banich replies: I'm delighted
you enjoyed my article and
sketches of M33, and your point about
thinking of galaxies as single entities is
excellent. I certainly do that at times, but
when I remember to use my informed
imagination to consider what makes up a
galaxy that I can barely see, or one that's
edge-on from our perspective, I often
think of M33. Imagine what M31 must
look like seen face-on!
Anyway, good luck with imaging and
observing when you get under a dark sky!
"
Sky & Telescope - March 2024

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