AS101 Lecture Notes - Lecture 12: Reflecting Telescope, Refraction, Primary Mirror
ProfessorPatrick Mc Graw
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Astronomy Lecture 11
Why do we bother with telescopes?
- What things did Galileo observe when he pointed his telescope upwards
- Venus rotates just the moon, and that Venus is the brightest planet.
What make a good telescope?
- Size of optical components
- An astronomer with 20/20 vision
Refraction: the bending of light
- Refraction is the behind of a ray of light when it passes through into a new medium
- Light can change direction when interacting with matter
- Prism- white light comes in and it spreads out, light is bending. Short wavelength light bends
more than the long wavelength
- Observation of prisms show refraction depends on the wavelength of light; shorter wavelengths
- Newton’s idea
- A ray of light will reflect from a smooth surface at the same angle that it is incident with
- Special property: for parabolic mirrors, rays of light that are parallel converge at the same point
- A primary or objective lens bends light and brings it to a focus
o The lens must be flawless throughout
- Different wavelengths of light are focussed at different locations :chromatic aberration
- Light bounces from an convex (inwardly curved) primary mirror
- A smaller secondary mirror can be used to redirect the light
Telescopes are light buckets
- If you want to get more light, you need to emit more light
- A larger mirror will produce a brighter image
- Light Gathering power (LGP)- is proportional to the area of the primary mirror or lens
o A two metre mirror has four times the LGP
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