Optics and Telescopes. Lecture 11. Why do we use telescopes?. Human eyes are lenses! Using larger lenses… collect more light magnification. Larger lens can make brighter and magnified images. Change in direction of travel. Refraction. light travels at the fastest
collect more light
light travels at the fastest
speed (e.g., speed of light)
Objective lens (light-gathering) + eyepiece (making image)
Light-gathering power = area of the objective lens
focal length of objective lens
focal length of eyepiece lens
Gemini Telescope (8m)
Large mirrors (nearly defect free : error is less than 8.5 nanometers) are much easier to make.
Hollowed mirror base (honeycomb)
Secondary mirror (or Cassegrain focus hole) does not make a hole in the focused image.
However, support structure creates a diffraction spike from a point source.
Different parts of a spherically concave mirror reflect light to slightly different points image bluring
parabolic mirror (harder to make)
Because of diffraction of light (light waves spread out from a point), there is a limit in angular resolution
Diffraction-limited angular resolution
θ = angular resolution in arcseconds
λ = wavelength of light, in meters
D = diameter of telescope, in meters
(example) Keck telescope, red light.
turbulence in atmosphere
Using a nearby star (e.g., point source), reshape the mirror so that it can become a perfect (diffraction limited) point source.
Aberration (chromatic, spherical)