Sky & Telescope - December 2024 - 7

telescopes and considerably better than
any planned for the immediate future.
Alas, van Speybroeck is no longer with
us, and when his mirror is gone, it's
not coming back. Sub-arcsecond X-ray
imaging could be lost for at least 10
years. New mirrors are being designed
but will be tricky and time-consuming
to build, and often resolution must be
sacrificed for larger area.
Hopefully the NASA budget can be
increased or rearranged to keep Chandra
operational as new observations
from the James Webb Space Telescope
and other missions come in. The August
S&T News Note concerning this issue
was much appreciated.
Fred Seward
Concord, Massachusetts
The MAGIC Telescopes
I was just reading " Magic in the Air "
by Javier Barbuzano (S&T: Aug. 2024,
p. 12) and, as fascinating as the
gamma-ray search is, one detail keeps
nagging me that goes uncommented:
They are building lightweight, 23-meter,
optical-frequency telescopes with carbon
fiber and actuators! Is this really
the case, and can they be aimed at other
deep-sky targets as well?
Joshua Stern
Los Angeles, California
"
Camille M. Carlisle replies: Cherenkov
telescopes like those of the
Cherenkov Telescope Array Observatory
cannot be used like optical telescopes: They
focus at a distance of 10-15 km above the
primary mirror, and their detectors are only
sensitive to green-bluish light and designed
to catch flashes that last for tens of nanoseconds
at most - in other words, they're
built to detect brief flashes of blue light
in Earth's atmosphere, not to resolve the
images of celestial objects like nebulae and
galaxies. The angular resolution of any image
would be rather poor when compared
with dedicated optical instruments such as
the 10-meter Gran Telescopio Canarias.
Some astronomers have nevertheless explored
the possibility of using the Cherenkov
telescopes for catching optical transients
or for imaging bright sources. But these
are only ideas at this stage, and even if astronomers
modified the receivers to be able
to detect a wide range of wavelengths, the
images would be blurred and defocused.
FOR THE RECORD
* Mars reaches opposition on January
16th, not January 12th as shown in the
graph on page 72 of the September 2024
issue. Also, the graph does not include
magnitude information, contrary to what is
stated in the caption.
* On page 59 in the October 2024 issue,
Herbig-Haro objects vary on time scales of
a few years, not days.
SUBMISSIONS: Write to Sky & Telescope, 1374 Massachusetts Ave., 4th Floor, Cambridge, MA 02138, USA, or email:
letters@skyandtelescope.org. Please limit your comments to 250 words; letters may be edited for brevity and clarity.
75, 50 & 25 YEARS AGO by Roger W. Sinnott
1949
1974
º December 1949
Radio Astronomy " Mr. [Grote]
Reber brought with him a large
stack of tracings [of] radiation from
the sky at a wave length of slightly
less than two meters. . . . There
was clearly [a surge] at the exact
time the Milky Way passed over his
antenna. An annoying factor was the
presence of numerous violent and
sharp disturbances which Mr. Reber
explained to us were the result of
various electrical appliances, such
as a dentist's drill a block or two
away, a trolley line on a street in his
vicinity, or the faulty ignition of a
passing car.
" As we continued our discussion
1999
[and] after G. P. Kuiper had visited
him at his house in Wheaton, Ill.,
and inspected the remarkable radio
telescope which he had built at his
own expense . . . we began to see
emerging before our eyes a completely
new branch of astronomy. "
So recalled Yerkes astronomer
Otto Struve.
º December 1974
Jupiter's Family " The 20th-magnitude
moving object discovered
near Jupiter in September [is now]
confirmed to be a satellite of that
planet. New photographic positions
of Jupiter XIII were secured on the
evenings of October 16th and 17th
by Charles Kowal with the 48-inch
Palomar Schmidt telescope . . . "
Finding this moon (later named
Leda) opened the floodgates.
Today, Jupiter has nearly 100 known
moons, and Saturn half again as
many as that.
º December 1999
Dwindling Deuterium " One of
the most eagerly sought numbers
in cosmology is the density of
baryons, the particles that make up
atomic nuclei. By comparing the
baryon density to censuses of visible
galaxies, astronomers can tell
whether ordinary matter is mostly
luminous (that is, residing in stars)
or dark (lurking unseen in black
holes, gas clouds, or planets and
brown dwarfs). New spectroscopic
measurements support the notion
that there are more baryons than
meet the eye - implying that baryons
constitute a significant fraction
of the universe's dark matter.
" The most sensitive baryon
indicator is deuterium, [which] was
created in the Big Bang [but largely]
destroyed as the primordial fireball
expanded and cooled. The nuclear
reactions that consumed deuterium
would have run faster if baryons had
been more abundant. Thus plentiful
deuterium is a sign of low cosmic
baryon density, and vice versa. But
reading the cosmic deuterium meter
is far from easy [because] 'there's
always the worry that what appears
to be the deuterium line is hydrogen
at a different redshift,' says Todd M.
Tripp (Princeton University). "
While studies to learn the cosmic
abundance of deuterium go on, a
step forward came in 2005 when
Alan Rogers and colleagues at
MIT's Haystack Observatory first
detected it at radio wavelengths.
sk yand tele scope .o r g * DECEMBER 2024
7

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Sky & Telescope - December 2024

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