Sky & Telescope - May 2020 - 7

Another Kind of Focusing
Mask

Thank You, S&T, from a
New Astronomer

I thoroughly enjoyed Dick Suiter and
Bill Zmek's article "Mesh Focusing
Masks" (S&T: Jan. 2020, p. 30), and
I will deﬁ nitely try making my own
Oleshko mask.
I was surprised that they did not mention the "triangle variation" of the Hartmann mask, which uses three triangles.
When the star is out of focus, three
distinct images appear in the eyepiece.
When focus is achieved, the images
merge. The triangular holes produce diffraction spikes, which become symmetrical when the focus is perfect. The spikes
serve to better ﬁne-tune the focus over
the traditional Hartmann design.
It's easy to make, many websites offer
directions, and it lets in more light than
a two-hole Hartmann mask. It might not
be as accurate as an Oleshko mask, but it
has served me well for years.

I just recently subscribed to S&T, and
as a former professional journalist and
reporter, I am sincerely impressed! The
artwork, diagrams, and photographs are
awesome and informative. The writing
is superior to that of most newspapers
and magazines. The editing is obviously
superb. Someone at S&T knows what
they are doing, and it shows.
Some of the information is a bit
over my head, as I'm only an amateur astronomer with his very ﬁ rst
telescope, but that is expected. Your
experienced astronomers need and
deserve the technical information. I'll
catch up sooner or later.
Larry Oakley's "Greeting the Queen
of Night" (S&T: Jan. 2020, p. 84) was a
neat ﬁ nishing touch! Keep doing that!
I have, on occasion, picked up issues of

Tom Sales
Somerset, New Jersey

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75, 50 & 25 YEARS AGO by Roger W. Sinnott
1945

1970

1995

º May 1945
Forecasting Weather "Quite
recently, Dr. Charles G.
Abbot, retired secretary of
the Smithsonian Institution,
announced that in the future it will
be possible to foretell in January the kind of weather which will
be available for a June wedding.
He believes that in the postwar
world there will be no necessity for
uncertainty as to the weather of
the future. His theory is, of course,
that the weather on the earth is
largely controlled by the variations
occurring from day to day in the
sun's heat. . . .
"It is rather astonishing to realize
that all our weather, cyclonic and
frigid as well as calm and warm,
rainy as well as sunny, is made 93
million miles away, on the sun."
The renowned solar astronomer
was not alone in expecting great
advances in weather forecasting
with the advent of digital computers during World War II.

S&T over the years, and unlike so many
other publications, it keeps getting
better each year!
Talbert McMullin
Gilbert, Arizona

FOR THE RECORD

* The sundial diagram illustrating the
Sun's position at various times and
dates (S&T: Jan. 2020, p. 28) incorrectly showed the hour lines radiating
from the zenith. They actually radiate
from the north celestial pole, as shown at
https://is.gd/shadowsundiagram.

* A 6-minute error in determining Greenwich Time would lead to a navigator's error
of 90 nautical miles, not 6 (S&T: Mar. 2020,
p. 60), when sailing near the equator.
* When computing longitude, subtract
Greenwich Time from local time and multiply by 15, not divide, as stated in "Captain
Cook's Astronomy" (S&T: Mar. 2020, p. 61).

º May 1970
Venus's Day "R. L. Carpenter of the
Jet Propulsion Laboratory remarks:
'The rotation period of Venus has
become particularly interesting not
only because it is retrograde but
also because it is remarkably close
to the so-called synodic resonance
period . . . of 243.16 days. If Venus
should indeed have this period,
then it would rotate backwards
relative to the Earth exactly four
times between each successive
inferior conjunction and thus present the same side to the Earth each
inferior conjunction.' . . .
"He has discussed JPL radar
observations made from 1962 to
1967 at four consecutive inferior
conjunctions . . .
"Carpenter finds 242.982 days
for the sidereal rotation period of
Venus, with an estimated uncertainty of only ±0.04 day. But this
is 0.18 day less than the resonance period, and therefore he
suggests that the rotation of Venus
may not be in synodic resonance
with the earth."

A 1979 study agreed. But a new
wrinkle emerged in 2012: Venus's
day may have lengthened by several
minutes in recent decades.
º May 1995
Io Erupts "In early March an
observing team led by John R.
Spencer (Lowell Observatory) was
monitoring the Jovian moon Io as
part of an educational program for
young students called the Jason
Project. But the participants got
much more than the routine images
they bargained for. The heat-sensitive camera observing Io witnessed
a major volcanic outburst at 18h
Universal Time on March 2nd - the
first such event to be imaged from
Earth as it happened. . . .
"The initial outburst was
probably quite hot, as it shone
brightly at 3.5 microns - a wavelength normally dominated by
reflected sunlight.
"At longitude 95° and latitude
-45°, the eruption site does not
match any of the satellite's known
active volcanic centers."
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 0

https://www.is.gd/shadowsundiagram http://www.skyandtelescope.org

Sky & Telescope - May 2020

Table of Contents for the Digital Edition of Sky & Telescope - May 2020