AS101 Chapter Notes - Chapter 4.2-4.5: Infrared Window, Compton Gamma Ray Observatory, James Webb Space Telescope

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28 Jan 2013
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Chapter 4.2-4.5
Astronomers build optical telescopes to gather light and focus it into sharp images
- Requires careful optical and mechanical designs
- Leads astronomers to build very large telescopes
Two kinds of Telescopes
Astronomical telescopes focus light into an image in one of two ways
1) A lens bends (refracts) the light as it passes through the glass and brings it to a focus to form an
image
2) A mirror (a curved piece of glass with a reflective surface) forms an image by bouncing light
Thus results in two different types of telescopes..
1) Refracting Telescopes use a lens to gather and focus the light
2) Reflecting Telescopes use a mirror
Main lens in a refracting telescope is called the primary lens, and the main mirror in a reflecting
telescope is called the primary mirror
Both kinds of telescopes form a small, inverted image that is difficult to observe directly, so a lens called
the eyepiece is used to magnify the image and make it convenient to view
Focal length is the distance from a lens of mirror to the image it forms of a distant light source such as a
star
Surfaces of lenses and mirrors must be shaped and polished to have no irregularities larger than a
wavelength of light
Creating the optics for a large telescope can take moths, of even years; involve huge, precision
machinery; and employ several expert optical engineers and scientists.
Refracting telescopes have serious disadvantages
They suffer from an optical distortion that limits their usefulness
When light is refracted through glass, shorter wavelengths bend more than longer wavelengths
You see a colour blur around every image
Called chromatic aberration and it can be only particularly corrected
The glass in primary lenses must be pure and flawless because the light passes all the way through it
The weight of the lens can be supported only around its outer edge
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Light reflects from the front surface of a reflecting telescope’s primary mirror but does not pass through
it, so reflecting telescopes have no chromatic aberration
Mirrors are less expensive to make than similarly sized lenses and the weight of telescope mirrors is
easily supported
Every large astronomical telescope built since 1900 has been a reflecting telescope
Optical telescopes gather visible light, but astronomers also build radio telescopes to gather radio
radiation
Radio Waves from celestial objects, like visible light waves, penetrate Earth’s atmosphere and reach the
ground
The dish reflector of a typical radio telescope focuses the radio waves so their intensity can be measured
Because radio wave lengths are so long, the disk reflector does not have to be as perfectly smooth as
the mirror of a reflecting optical telescope
Figure 4.4
In most radio telescopes, a dish reflector concentrates the radio signal on the antenna
The signal is then amplified and recorded
For all but the shortest radio waves, wire mesh is an adequate reflector
The Powers of a Telescope
Astronomers struggle to build large telescopes because a telescope can help human eyes in three
important ways (these are called the three powers of a telescope)
- Two most important of these three powers depend on the diameter of the telescope
Most celestial objects of interest to astronomers are faint, so you need a telescope that can gather
large amounts of light to produce a bright image.
Light-gathering power refers to the ability of a telescope to collect light
Catching light in a telescope is like catching rain in a bucket (the bigger the bucket, the more rain it
catches)
The light-gathering power is proportional to the area of the primary mirror, that is, proportional to the
square of the primary’s diameter
A telescope with a diameter of 2 meters has four times (4x) the light-gathering power of a 1-meter
telescope
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One reason radio astronomers build big radio dishes is to collect enough radio photons, which have low
energies, and concentrate them for measurement
The resolving power refers to the ability of the telescope to reveal fine detail
One consequence of the wavelike nature of light is that there is an inevitable small blurring called a
diffraction fringe around every point of light in the image, and you cannot see any detail smaller than
the fringe
Astronomers can’t eliminate diffraction fringes, but the fringes are smaller in larger telescopes, meaning
they have better resolving power and can reveal finer detail
- ie. A 2-meter telescope has diffraction fringes ½ as large, and thus 2x better resolving power,
than a 1-meter telescope
The size of the diffraction fringes also depends on wavelength, and at the long wavelengths of radio
waves, the fringes are large and the resolving power is poor, which is another reason radio telescopes
need to be larger than optical telescopes
One way to improve resolving power is to connect two or more telescopes in an interferometer, which
has a resolving power equal to that of a telescope as large as the maximum separation between the
individual telescopes
The first interferometers were built by radio astronomers connecting radio dishes kilometers apart
Modern technology has allowed astronomers to connect optical telescopes to form interferometers
with very high resolution
Aside from diffraction fringes, two other factors (optical quality and atmospheric conditions) limit
resolving power
A telescope must contain high-quality optics to achieve its full potential resolving power
Even a large telescope shows little detail if its optical surfaces have imperfections
When you look through a telescope, you look through miles of turbulence in Earth’s atmosphere, which
makes images dance and blur, a condition that astronomers call seeing
A related phenomenon is the twinkling of a star
The twinkles are caused by turbulence in Earth’s atmosphere, and a star near the horizon, where you
look through more air, will twinkle more than a star overhead
Even with good seeing, the detail visible through a large telescope is limited, not just by its diffraction
fringes, but by the steadiness of the air through which the observer must look
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