Sky and Telescope - July 2015 - 8

Letters

To the Ends of the Earth

Seeing Red
Here's a minor issue regarding the
"Phenomena of Jupiter's Moons" printed
in most issues - at least that's what I
remember it being called (I'm composing
this comment in the dark while simultaneously viewing the magazine with a
red headlamp on). This section's title and
dates are printed in red, and when viewed
with the headlamp they become invisible!
I don't typically consider dark-adapting
before taking on Jupiter, but I do often
check this tabulation during the night
before Jupiter comes up. Seen with just
the red light from my headlamp, the listing can be quite baﬄing.
I wear out every monthly issue from
use because you have such a great magazine, but would you consider ﬁ xing those
"invisible" labels?
Riley Friedrich
West Springﬁeld, Massachusetts
Editor's note: Great feedback! Jupiter in
July is sinking into the sunset, but when we
resume the table we'll adjust the color to be
more readable under red light.
8

July 2015 sky & telescope

ROBERT SCHWARZ

Here at the geographic South Pole, we
have the biggest site for astrophysics
in Antarctica. We've got the South Pole
Telescope, IceCube Neutrino Observatory, BICEP3 (which followed BICEP1 and
BICEP2), and the SPUD/Keck Array.
What we don't have is a subscription
to Sky & Telescope - with recent budget
cuts, all our magazine subscriptions got
canceled. Therefore, last year I wrote to
S&T and asked for some old issues. (Since
we only get mail from November through
February, when the station is accessible
from the outside world, even old magazines are new to us.)
As the photo shows (taken when the
ambient temperature was -65°F), they've
arrived. Thanks so much!
Robert Schwarz
South Pole Station, Antarctica
Astronomers at Amundsen-Scott South Pole Station can now catch up on celestial happenings
with back issues of Sky & Telescope.

Musings on Gravitational Lensing
I enjoyed Govert Schilling's article
"Hubble Goes the Distance" (S&T: Jan.
2015, p. 20). It did raise a few questions in
my mind, though. I can understand how a
large cluster of galaxies such as Abell 2744
can act as a gravitational lens, albeit a very
crude, irregular lens (as I might grind!).
Such a lens would have many individual
focal points that, I assume, result in the
arcs, lines, and "blobs" highlighted on
page 23. So how could such an imperfect
lens magnify or brighten a distant galaxy
behind it? The light of a distant galaxy
would be focused in many places because
of the diﬀerent ray paths. Also, it seems to
me that the distant, lensed galaxy might
appear dimmer than it actually is because
its light is spread out over a larger area (for
example, as pictured on page 26). Finally,
is it possible to deconstruct the ray paths
somehow to obtain a clearer picture of the
distant galaxy?
Philip Petersen
League City, Texas

Govert Schilling responds: The distant background source/galaxy is indeed "magniﬁed"
over a larger (sometimes smaller) area on the
sky as seen from Earth due to an intervening
gravitational lens. However, in the process
the source's "surface brightness" is conserved
- the number of photons seen per square
arcsecond of sky does not change. So if the
source is magniﬁed by some factor M, then
its ﬂux becomes M times greater (that is, we
see more total light coming from it) but not
M times brighter per unit of angular area.
Any point of the source has exactly the same
surface brightness before and after lensing.
Regarding your second question about
deconstructing the ray paths, indeed observers have been doing this for several decades,
leading to increased resolution of the lensed
source. One complication is that, in general,
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Sky and Telescope - July 2015

Table of Contents for the Digital Edition of Sky and Telescope - July 2015
