AST201H1 Chapter Notes - Chapter 6: Spectral Resolution, Adaptive Optics, Sky Radio

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29 Jan 2013
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CHAPTER 6: TELESCOPES
6.1 Eyes and Cameras: Everyday Light Sensors
How does your eye form an image?
Basic components:
1) Lens: bends light and creates an image on retina
- refraction occurs because light moves slower in denser matter results in a change
in direction
- Parallel rays converge at a focus - light that enters the lens farther from the center
are bent more and rays that pass directly though are not bent at all
- X parallel rays do not converge but still follow rules from above. The rays bend to
form an image place where the image appears in focus is called the focal plane of
the lens
2) Pupil: does not play direct role in image formation
- Controls how much light enters the eye by adjusting the size of its opening
- Dilates in low light, constricts in bright light
3) Retina: contains light-sensitive (cones and rods)
How do we record images?
Similar to camera except camera is permanent
- Detector: any device that records light, including photographic films and various
types of digital-imaging chips
- Camera acts as the lens and detector acts as retina
Recording with a camera has 2 important advantages:
1) Recorded image is more reliable and detailed than a drawing
2) Can control the exposure time which means better detail because more photons reach
the detector
Today, most images are recorded digitally with electronic detectors called
charge-coupled devices CCDs a silicon chip engineered to be extraordinarily sensitive to
photons
- It is divided into picture elements (pixels)
- When light hits a pixel, it causes a bit of electric charge to accumulate - more
photons = more electric charge
CCDs have 3 major advantages over photographic film:
1) More sensitive to light
2) Wider dynamic range than photographic film they can record both dim and bright
light at the same time
3) Digital image can be manipulated through image processing to bring out details that
would have been otherwise missed
6.2 Telescopes: Giant Eyes
What are the two most important properties of a telescope?:
1) Light-collecting area: tells us how much total light the telescope can collect at one time.
- characterize the size of telescope as the diameter of its light collecting area
- note: a small increase in diameter can mean a big increase in the light-collecting area
(ex. 10m tele is 5x the diameter of a 2m tele, but the area is 25 times as great (52 = 25)
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CHAPTER 6: TELESCOPES
2) Angular Resolution: the smallest angle over which we can tell that two dots/stars are
distinct
- Large tele. can have amazing angular resolution; but they must be situated on ground
which has other complications
- Ultimate limit to a tele’s resolving power is the interference caused by electromagnetic
waves of light
Note: that’s why even high quality tele in space cannot have perfect angular resolution
- Diffraction limit: the angular resolution that a telescope could achieve if it were limited
only by the interferences of light
Depends on the diameter of the primary mirror + the wavelength of the light being
observed
Larger tele = smaller diffraction limit (better angular resolution
For longer wavelength of light, diffraction limit is larger therefore, radio tele must
be far larger than visible light tele to achieve the same angular resolution
What are the two basic designs of telescopes?
1) Refracting telescope
- Uses transparent glass lenses to collect and focus light, similar to eye
- Diagram:
2) Reflecting telescope
- Uses precisely curved primary mirror to gather light; It reflects the gathered light to a
secondary mirror in front of it; It then reflects the light to a focus
- Diagram:
- The fact that the secondary mirror blocks some light from the primary mirror is not a
problem because it only blocks a small fraction of incoming light
- Used more often because:
a) Refracting: Since light passes through the lens of a refracting telescope, the lenses
must be made from HQ glass shaped precisely.
Reflecting: While, reflecting telescopes only require a precisely shaped surface. (the
quality of the mirror is not a factor)
b) Refracting: Large glasses lenses are heavy; therefore, hard to stabilize at the top.
Reflecting: The primary mirror of reflecting telescopes is mounted at the bottom
and does not have the stabilizing problem.
c) Refracting: lenses cause chromatic aberration (lens brings different colours of light
into focus at slightly different places can be fixed by using combo lenses)
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