IWU Astronomy Expedition to Observe Solar Eclipse of 21 August 2017
by Thushara Perera
Early in the morning on the day of the Great American Eclipse, I and a
team of current and former IWU students set off for Carbondale, IL to
observe the total solar eclipse. The transportation was through a
local company—Rasmussen Travels—owned by a former IWU
physics student and my expenses were covered by the Mellon Center for
Curricular and Faculty Development at IWU. We took with us a
telescope and solar filters that belong to the IWU observatory and my
personal DSLR camera. Below is a photo of our team and our
instruments, at the SIU-Carbondale campus. We also brought with us a
video recorder to document the experience.
People from left to right: Alex (Krystyna's
brother), Krystyna Lopez (Physics Major, class of 2017), Ben Liao
(Physics/Pre-Engineering Student, class of 2019), myself (Thushara
Perera, Physics Faculty), Jordan (Alex Rasmussen's wife), Alex
Rasmussen (former IWU Physics student and owner of tour company).
The telescope on the left is a 6-inch Cassegrain Reflector with a
makeshift counterweight and the camera on the right is a Cannon DSLR
with a 400-mm telephoto lens. Both are equipped with solar filters.
We got to the SIU campus and set up our instruments soon after the
eclipse began (11:50 AM). The following photo was taken with a phone
placed at the eyepiece of the telescope (using some exposure control).
At this point about about 75% of the sun's diameter and about 50% of
the sun's surface area has been eclipsed. i.e. the
eclipse magnitude is about 0.75 and the obscuration is
50%. As a result of telescope optics, this image is a mirror
reflection of what it would actually look like on the sky (as viewed
through eclipse glasses, for example).
Although it was a sunny day in Carbondale, there were some clouds in
the sky and, as a result, we almost did not get to witness the sun
(and moon) at totality. However, due to sheer luck, the clouds parted
for a minute or so during totality and we were able to witness the
full glory of a total solar eclipse! The drama leading up to this
event and other parts of our trip are documented in the short
(8.5-minute) movie below.
One perception-related inaccuracy in the video is due to the fact that
the video recorder adapts very well (too well) to low-light
situations. As a result the background illumination during totality
appears brighter than it was; it really was quite dark. Even so,
our surroundings weren't truly as dark as during nighttime mainly due
to a sunset-like glow on the horizon in all directions. This
effect is due to the finite size of the moon's complete shadow
(the umbra), which had an approximate radius of 35 miles in
Carbondale. What we see as the "360-degree sunset" is the
illumination of the sky (at distances in excess of 35 miles) in
regions not experiencing the total eclipse.
Below are some of the photos we took during totality.
These images are in chronological order. You can click on an image to
make it larger. The two bookend images, which have higher contrast,
were taken with a phone camera at the eyepiece of the telescope. They
show the sun's corona (its outer
atmosphere) and coronal "streamers" that point outward from the
sun. These are magnetically guided flows of ions, which source the
solar wind. In the lower contrast frames obtained from the video
recording (the middle images), the corona blends in with the thin
cloud cover to give a unique visual effect. In most of these images,
you see small crimson-colored "beads" of light at a few points on the
moon's circumference (click on an image to see them clearly). These
are called Bailey's Beads and are chance views of the
chromosphere, the solar atmospheric
layer that lies within the corona, afforded by valleys on the lunar
surface. The corona and chromosphere are usually not visible to us
because they are dimmer than the sky itself during daytime.
The following photos were taken just as the sun was re-emerging
from totality.
The fist image is usually called the "diamond ring," which shows just
a single spot of the sun's
photosphere (though lunar valleys), as the total eclipse is just
ending (or beginning). The photosphere is the innermost layer of the
sun's atmosphere, which is what we usually recognize as the sun during
daytime, and lies within the chromosphere.
With the excitement of totality behind us we were able to take some
good-quality photos of the partially eclipsed sun using the camera.
You can blow up these images by clicking on them. The part of the sun
visible in these images is the photosphere, which is
responsible for the glow we recognize as the sun in the daytime sky,
on a regular day. During this eclipse, the presence of a line
of sunspots, right through the
middle, made it easy to focus telescopes/cameras on the sun. As you
can see in this time sequence, more and more sunspots are revealed as
the sun comes out of the moon's shadow. Two other notable aspects of
the photosphere are its sharp outer edge, as if the sun is a solid
sphere rather than a gaseous object. In reality the sun is a
ball of gas, but the gas that comprises the photosphere has a high
opacity, with an optical depth of only 400
km. The edges appear sharp because 400 km is very small compared to
the solar diameter (about 1.4 million km). Another notable feature
is limb darkening, which refers to the darker shade of the sun
near the edges, as if the sun is a solid ball illuminated from the
front. In reality, this effect is due to a steep temperature gradient
within the 400-km thickness of the photosphere, from 5800 Kelvin at
the bottom to about 4400 Kelvin at the top. The photo parameters,
for future reference, were, sensitivity: ISO 400, lens focal length:
400 mm, filter: ND 1/100000 equivalent, f-stop: 8, shutter speed:
1/2500 seconds.
A fun effect to observe during partial eclipse phases is shown below.
The tiny holes in the hat result in a splash of circular patches on
the ground on a regular day. However, during an eclipse, the patches
take on the shape of the eclipsed sun!
The box of crackers acts as a flat screen here. Each tiny hole in the
hat works as a pin-hole projector. A small pin hole in front of a very
bright source acts as an arbitrary focus for light rays from the
source. i.e. the light rays cross paths at that point and, as a result, the
projected image is a mirror reflection of the actual scene.
After an exciting day of observing, we went for lunch/dinner at a
restaurant in Carbondale and witnessed the following recreation of the
day's events!
