Sky & Telescope - August 2020 - 6

FROM OUR READERS

Herman Heyn's Focal Point (S&T: Mar.
2020, p. 84) reminded me of the 2017
total solar eclipse, when my son, Phil,
and I traveled to Murphy, North Carolina and set up my 100-mm binoculars
in the front yard of a hotel.
Many others had stayed there for
the same reason, so Phil and I were
among quite a crowd of observers. My
binoculars with objective lens ﬁ lters
attracted quite a stream of viewers,
and at one point a gentleman asked
whether he could get a picture of the
partially eclipsed Sun by holding his
cell-phone camera up to the eyepiece.
Phil, who had some experience with
digiscoping, helped him. I was busy
taking photos with my DSLR camera
when I suddenly heard whoops of

levels are continuously monitored and are
well below the limit set by the Occupational
Safety and Health Administration (OSHA)
during regular operations.

Howard L. Ritter, Jr.
Perrysburg, Ohio

Locating Oneself Without GPS

I'd like to thank Sean Walker for the
excellent article (S&T: Mar. 2020, p. 30)
describing his recommended workﬂow
for extracting a high-quality HDR image
from photos taken at different exposures during totality of a solar eclipse. I
had just such a set from the 2017 solar
eclipse, and previous processing attempts
never quite lived up to my expectations.
Thanks to S&T, I now have an eclipse
image of which I am very proud.
Andrew Thomas
Seattle, Washington

S Caught by cell through binoculars, a jet flies
across the Sun during the 2017 solar eclipse.

S This image of 2017's solar eclipse was processed using Sean Walker's technique.

Recycling with John Dobson

2020, p. 24) did not include discussion
on the health hazards of using mercury
in liquid-mirror telescopes. Mercury is
a highly toxic substance, and a 4-meter
mirror provides a very large evaporating
surface. The likelihood of gaseous mercury expanding beyond telescope dome
is another problem. I would like to know
how these hazards are being controlled.

I enjoyed Jerry Oltion's article about resurrecting an original Dobsonian (S&T:
Apr. 2020, p. 36) and thought I'd add
an anecdote. My ﬁrst day job was as an
electronic technician at the Lawrence
Hall of Science in Berkeley. Computers in those days used huge reel-to-reel
tape decks to store data. They came in
big aluminum cans 2 feet wide and 2
inches deep. Dobson and his buddies
used to come by to collect the empties
to use as altitude bearings. His 22-inch
that he used for public observing nights
at LHS had two of them.
Dave Lusby
Sandia Park, New Mexico

The Dangers of Mercury
As one who has taught chemical safety
in the workplace, I was surprised that
Govert Schilling's article (S&T: Apr.
6

delight and amazement: The cell phone
had captured a jet in transit across the
Sun! The three of us all went home
happier than we'd expected.

AUGUST 2 02 0 * SK Y & TELESCOPE

Bill Wood
Melbourne, Australia

"

Govert Schilling replies: According

to Paul Hickson (University of British
Columbia), mercury evaporation is greatly
reduced by a thin layer of mercury oxide
that forms on the rotating mirror surface.
Moreover, the Mylar cover traps mercury
vapor that forms when the mirror is started
and stopped. A charcoal ﬁlter then sucks
out the vapor and removes it from the air.
Of course, notes Hickson, mercury vapor

I was quite interested in Ted Rafferty's
article (S&T: Mar. 2020, p. 58) about
Captain James Cook. Rafferty mentions
that the charts of Newfoundland that
Cook prepared were used for more than
100 years. Indeed, they were. I used them
as a commercial ﬁsherman in the late
1970s. We did have a LORAN-A navigation system, but it was a pain to use and
did not work well close to the shore.
Cook included drawings of the view of
the headlands around the approaches of
the bays, which made it easier for sailors
to ﬁnd their bearings on what is (to this
day) a rough and undeveloped shoreline.
On the charts themselves, I could see
the soundings made in neat rows from
when Cook's men painstakingly rowed
back and forth while casting the lead
line. Sadly, they missed more than a few
rocks. Newfoundlanders call these sunkers. I have personal experience with one
of these, but that's another story.
Sailing in the wake of Cook taught
me that it isn't technology that gets us
there but the use of our minds.
Ted White
Victoria, British Columbia

I found Ted Rafferty's article fascinating. Although I am somewhat familiar
with Harrison's solution of the longitude problem using chronometers, I
was intrigued by the descriptions of the
lunar and Jupiter-moon methods used
to determine longitude. In a somewhat
related problem, how did the explorers
of Antarctica navigate to the South Pole
in 1911-12, and how did they determine
their position with just the Sun?
Colin Unsworth
Newtown, Pennsylvania

"

Ted Rafferty replies: The navigation
Robert Scott and Roald Amundsen
used to reach the South Pole in 1911-12
depended on the determination of latitude.
Scott employed a theodolite for determining
latitude, Amundsen a sextant. Amundsen

SUN A ND PL A NE: HOWA RD RIT TER / PHIL
RIT TER; ECLIPSE: A NDRE W THO M AS

Eclipse Fun
Sky & Telescope - August 2020

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