Lesson 18: Collecting Light with Telescopes
In this lesson,
we will talk about the key properties that
determine the effectiveness of a telescope and learn the basics of telescope
design. We will then discuss new telescope technologies and see what are the
advantages of placing telescopes in space.
Telescopes are
essentially giant eyes that collect more light than our own eyes. By combining
this light-collecting capacity with cameras and other instruments such as
spectrographs that disperse the light into spectra, we can record and analyze
light in detail.
Light-collecting area is the first key property of a telescope that
determines how much light the telescope can collect at one time.
For example,
a 10-meter telescope has a light-collecting area of 10 meters in diameter.
The second key property of a telescope is angular resolution. Angular
resolution is the smallest angle over which we can tell that two dots (or two
stars) are distinct.
The figure above shows the same two stars;
the image on the left was taken using a telescope with low angular resolution
while the image on the right has a higher angular resolution.
Telescopes collect far more light and
allow us to
see far more detail than we could with the naked eye. The human eye has an
angular resolution of about 1 arc minute or 1/60th of a degree. Our eyes cannot
see stars separately if the distance between them is less than 1 arc minute.
In contrast, the Hubble telescope has an amazing angular resolution of 0.
05 arcsecond for visible light,
meaning that you could read this document from a distance of over 1 kilometer!
The angular resolution determines the amount of detail we can see with a
telescope.
Telescopes come in
2 basic designs - refracting and reflecting. Refracting telescopes operate
similar to the eye using transparent glass lenses to collect and focus light.
Galileo's earliest telescopes were refracting telescopes.
Instead of a lens, which can get very thick and
heavy,
Reflecting telescopes use a precisely curved mirror to gather light. The
primary mirror reflects the gathered light to a secondary mirror that lies in
front of it. The secondary mirror then reflects the light to a focus at a place
where the eye or instruments can observe it. This can be a hole in the primary
mirror or along the side of the telescope.
Telescopes today are specialized and
can observe different wavelengths of light beyond the visible spectrum,
allowing us to learn far more than we could learn from visible light alone.
Astronomers have developed interferometry, a technology where multiple smaller
telescopes can work together to obtain the angular resolution of a much larger
single telescope. An example of this is the Karl Janksy Very Large Array in New
Mexico that consists of 27 radio telescopes that can be moved along train
tracks. Working together, the telescopes achieve an angular resolution that
would otherwise require a single radio telescope with a 40 kilometer diameter.
Modern day astronomers also use telescopes to study sources of information
other than light - neutrinos, cosmic rays and gravitational waves. Neutrinos
are subatomic particles produced by nuclear reactions in stars. Astronomers
have used neutrino telescopes underwater and in deep mines to learn about the
Sun and stellar explosions.
High energy subatomic particles from
space, called cosmic rays, are
also being studied by astronomers. So far, little is known about their origin.
Finally,
gravitational waves as predicted by Einstein's general theory of relativity are
now being directly observed with the recent development of gravitational wave
telescopes.
Telescopes are placed in
space so that
they can avoid the distorting effects of the Earth's atmosphere such as light
pollution and turbulence from air movement.
Examples of telescopes in space include the Chandra X-ray Observatory,
the Hubble Space Telescope and the new James Webb Space Telescope for 2018.
Much of the electromagnetic spectrum can be observed only from space and not
from the ground. Only radio waves, visible light, the longest wavelengths of
ultraviolet light, and small parts of the infrared spectrum can be observed
from the ground. Telescopes in space allow us to observe the rest of the light
that does not penetrate Earth's atmosphere.
Twinkling of stars occurs because of air turbulence. Stars tend to twinkle more
on windy nights and when they are close to the horizon. Above the atmosphere in
space, they do not twinkle at all.
However, building large telescopes on the ground is much cheaper than launching
and maintaining a telescope in space. Now, telescopes with a new technology
called "adaptive optics" are able to overcome much of the blurring caused by
our atmosphere.
All of these new developments are helping astronomers to
learn more about the universe than ever before.