November 18, 2019 0

Module 6 / Lecture 2 : Instrumentation for Astronomy

Module 6 / Lecture 2 : Instrumentation for Astronomy


Hello! In this lecture we’ll discuss how Earth’s
atmosphere affects astronomical observations. Most of our astronomical observatories are
on the ground. And although the atmosphere is great for breathing and all of that, it
does pose a few problems for astronomers. The sky is bright during the day because of
atmospheric scattering of the Sun’s light. The much dimmer light of most celestial objects
gets drowned out so visible light astronomers can only work at night. And they can only work if it’s clear. The
sky is often filled with those pesky clouds. But even if it’s at night and spectacularly
clear, there are additional effects that stymie the astronomer- light pollution, atmospheric
turbulence, and the fact that most forms of light can’t even reach the ground. Just as our atmosphere scatters sunlight in
the daytime, it also scatters city lights at night. We call this light pollution. Human-produced light pollution hinders our
view of the stars and negatively affects astronomical research. But it’s also an environmental problem. Light pollution disrupts ecosystems, it affects
human circadian rhythms, and it wastes energy. It’s estimated that approximately $2.2 billion
dollars per year is lost due to poor lighting in the United States alone. And the problem
of light pollution is only getting worse. If you’re interested in the issue of light
pollution and how to fix it, the International Dark-Sky Association is a nonprofit that works
to counter light pollution. Information on the IDA can be found online at darksky.org. Atmospheric turbulence is another problem
for the astronomer. To understand turbulent, imagine a penny at
the bottom of a very still pool. It’s easy to see. Now imagine someone does a cannonball
into the pool. The water gets wavy and the image of the penny jumps around. Being on Earth, under the atmosphere is sort
of like being at the bottom of a pool. To do astronomy we have to look through layers
of air. The more still the air is, the easier it is to see the stars. The uneven heating and cooling of the atmosphere
creates moving bundles, or pockets, of air. These air pockets act like little lenses.
When the parallel light rays from a star hit the bundles, the rays bend in unpredictable
ways. Therefore when a telescope on the ground looks
up at the night sky through the atmosphere, we get a twinkling and blurry image. Atmospheric turbulence tends to limit the
angular resolution of ground-based telescopes to no better than about half an arcsecond,
even if a telescope can theoretically achieve a smaller angular resolution. One technique to counter turbulence is called
adaptive optics. Sophisticated, deformable mirrors controlled
by computers can correct in real-time for the distortion caused by the turbulence of
the Earth’s atmosphere. A computer calculates the necessary changes
by monitoring distortions in the image of a bright star near the object of interest.
If there is no bright star nearby, a laser is used to create an artificial star to monitor
for distortions. When you’re choosing the location of the next
great Earth-based observatory, you want to minimize the effects of bad weather, light
pollution, and turbulence as much as possible. The best sites are dark, dry, calm, and high
above the densest parts of the atmosphere. There are some sites around the world that
meet these criteria. For example, the 4300-meter summit of Mauna Kea in Hawaii. Of course, the best solution to our atmospheric
troubles is to put our telescopes into space, where there is no daylight, no clouds, no
light pollution, and no turbulence. This is why the Hubble Space Telescope, despite its
modest mirror size of only 2.4 meters has been so enormously successful. But there’s another good reason to put telescopes
outside of Earth’s atmosphere. Earth’s atmosphere poses a problem that no
ground-based telescope can overcome. Our atmosphere blocks most forms of light. Astronomical phenomena
emit light at all energies of the electromagnetic spectrum, so of course we want to look at
everything. Only we can’t. At least not from the ground. This figure shows the approximate depths to
which different forms of light penetrate our atmosphere. Only radio waves, visible light,
and the very longest wavelengths of ultraviolet light and small parts of the infrared spectrum
can be observed from the ground. Therefore, the most important reason for putting
telescopes into space is to allow us to observe the rest of the electromagnetic spectrum. The Hubble Space Telescope is probably the
most famous space observatory, but there are many others that operate in parts of the electromagnetic
spectrum that do not reach the ground. NASA plans to launch an even more powerful
infrared observatory. The James Webb Space Telescope will be the successor to the Hubble
Space Telescope. The primary mirror is much larger and the potential science that can
be done with the James Webb is very exciting. Before I leave you, I wanted to share this
artist’s work. Thierry Cohen takes images from densely populated cities like New York,
Rio de Janeiro and Shanghai and removes the artificial urban lights. He puts the sky in as it would look if there
were no light pollution. The images are lovely and haunting and best observed on your computer
from a dark room. I hope you enjoy. I will talk to you again soon.

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