Sky & Telescope - June 2023 - 7

system due to planetary perturbations at
its current return. "
If you would like to do your own calculations,
here is a link to the comet's orbital
elements posted on Gideon van Buitenen's
comets page: https://is.gd/VanBuitenen.
Comparing Detectors
Congratulations on your fantastic
article " Stepping Up to CMOS " by
Ron Brecher (S&T: Mar. 2023, p. 58). I
thoroughly enjoyed the very clear sideby-side
comparison of both types of
imaging chips.
CMOS detectors have indeed come
a long way, including that their issues
with amp glow are now practically
resolved, as the article explained.
One question keeps puzzling me,
though. How does binning compare
between CMOS and CCDs? Due to the
different read-out systems, I suspect
CMOS does lose some advantage
because binning averages are made
after the data are read-out, not on the
chip beforehand as with a CCD. On
the other hand, CMOS has better read
noise to compensate.
Tom Alderweireldt
Schilde, Belgium
Sean Walker replies: CMOS binning
occurs after the sensor is read out
and thus simply doubles the read noise. But
it is very low in the first place. There's not as
big a gain compared to CCDs, but it's still
useful for over-sampled systems. Here is an
article one manufacturer published on the
matter: https://is.gd/Binning.
"
Blast From the Past
As I perused my February issue, I did a
double-take when I saw the cover from
50 years ago showing Nicolaus Copernicus,
in Roger Sinnott's 75, 50 & 25 Years
Ago (S&T: Feb. 2023, p. 7). The February
1973 issue was the first issue I received
as a new subscriber when I was 13 years
old. Back then, I was observing with a
2.4-inch Unitron refractor. Now my primary
telescope is a 17.5-inch Dobsonian.
I still have that original issue - and the
2.4-inch Unitron! Thank you for your
wonderful magazine, which has been
part of my exploration of the cosmos for
the past half century.
David Hasenauer
Monrovia, California
FOR THE RECORD
* In " What Is Opposition? " (S&T: Mar.
2023, p. 72), the interior planets achieve
eastern elongation as they catch up with us
in our orbit, and western elongation occurs
after each has passed us by.
* In " Comet Prospects for 2023 "
(S&T: Feb. 2023, p. 48), 1.9 astronomical
units is equal to 280 million kilometers and
180 million miles.
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
1948
1973
º June 1948
Compound Reflectors " Another
combination of conic curves has
lately received notice among
amateurs. It consists of an ellipsoidal
primary mirror and a spherical
secondary. The favorable qualities
of this combination, such as its
relative simplicity of manufacture,
were recognized and advocated
some years ago by [Horace E.] Dall,
who made several telescopes of
that design. A few years later, Alan
R. Kirkham published formulas for
computing the aberration of the
spherical secondary . . .
" Albert G. Ingalls [of Scientific
1998
American] has proposed the name
'Dall-Kirkham' for this telescope, as
a merited and more dignified one
than the misnomer 'modified Cass.' "
Allyn J. Thompson, author of Sky
Publishing's Making Your Own Telescope,
had just begun a series of
articles on Cassegrain-type reflectors.
Unfortunately, Thompson's
sequel has never been released.
º June 1973
Old Eclipse Site " A few years ago,
on a short spring vacation, my family
and I set out to find Separation,
[Wyoming,] to see what remained
of a day 90 years past. In the hope
of finding the stone telescope piers,
we carried with our camping equipment
the Naval Observatory volume
on the eclipse, . . . which contains
Sampson's and Newcomb's
description of their site. . . .
" An 1882 map showed us
roughly where the town and the
original railbed had been . . . For a
day or so we searched along this
desolate road, looking in particular
for the semicircular sand dune,
which had given protection to
Sampson and Newcomb. [We] were
just about to give up when my wife
Marjorie found a small and unusual
stone [that was] soft, like old mortar.
Perhaps the piers lay buried
beneath us . . . "
The excitement of reading about
that 1878 eclipse, and John A.
Eddy's successful hunt for the expedition
site, has never left me.
º June 1998
Asteroid Impact? " On March
11th, Brian Marsden (Central
Bureau for Astronomical Telegrams)
announced what seemed like the
scariest [potentially hazardous
object] yet - a 19th-magnitude
asteroid designated 1997 XF11. It
had been discovered last December
. . . He found that it would pass
a scant 40,000 kilometers above
Earth's surface on October 26,
2028. This was deep within the
orbit's estimated margin of error of
about 180,000 km!
" [The] issue became moot within
one day of the [announcement
when prediscovery images greatly
reduced the chance of an impact.] "
As Stuart Goldman explained,
this whiplash news was a publicrelations
nightmare for astronomy.
It sparked big changes in how
alarming discoveries were handled.
Nowadays, for example, NASA's
Sentry risk page at https://cneos.
jpl.nasa.gov/sentry/ gives up-todate
details on all known hazardous
minor planets.
sk yand tele scope .o r g * JUNE 2023
7

https://www.is.gd/VanBuitenen https://www.is.gd/Binning https://cneos.jpl.nasa.gov/sentry/ https://cneos.jpl.nasa.gov/sentry/ https://skyandtelescope.org/

Sky & Telescope - June 2023

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