Optics and Telescopes Lecture 11

Why do we use telescopes? Human eyes are lenses! Human eyes are lenses! Using larger 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 speed (e.g., speed of light) in vacuum.

Refraction of light by lens

Parallel light rays from distant objects If a lens is located very far from the light source, only a few of the light rays are entering the lens. If a lens is located very far from the light source, only a few of the light rays are entering the lens. These rays are essentially parallel. These rays are essentially parallel.

Extended object  Extended image A lens creates an extended image of an extended object.  each point on an extended object passes through a lens and produces an image of that point.  collection of point images = image of an extended object.

Refractive Telescope Objective lens (light-gathering) + eyepiece (making image) Light-gathering power = area of the objective lens magnification = focal length of objective lens focal length of eyepiece lens

Refractive Telescope

Disadvantages of refractive telescope 1.Hard to make defect free lenses (especially larger one) 2.Glass is opaque to certain wavelengths (UV is 100% blocked!) 3.Very difficult to make larger lens 4.Large lenses are heavy  gravitational distortion

Law of Reflection incidence angle = reflection angle Mirror Perpendicular to mirror surface Reflected light ray Incident light ray i r Angle of reflection r equals angle of incidence i

Reflection : Mirror acts as a lens

Newtonian Telescope

Different designs of Reflecting telescopes Prime focus is good but inconvenient. Prime focus is good but inconvenient.

All Modern telescopes are Reflecting telescopes Gemini Telescope (8m) (1)Primary mirror (2)secondary mirror (3)Cassegrain focus Large mirrors (nearly defect free : error is less than 8.5 nanometers) are much easier to make. Hollowed mirror base (honeycomb)

Secondary mirror making a hole in the image? 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 A solution -parabolic mirror (harder to make) -correcting lens Spherical Aberration

Angular resolution 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. …

Hawaii, Mauna Kea

Light Pollution

Effect of Earth Atmosphere Light = wave Light = wave Perfect waveform got deformed due to turbulence in atmosphere… Perfect waveform got deformed due to turbulence in atmosphere… breezeturbulence in atmosphere

Adaptive Optics Using a nearby star (e.g., point source), reshape the mirror so that it can become a perfect (diffraction limited) point source. Active Optics wind shakes  tip/tilt correction

Eliminate the effect of Atmosphere (Adaptive Optics)

Power of Adaptive Optics

Laser-guided Adaptive Optics

In summary… Important Concepts Refractive telescope disadvantages Reflective telescope various designs Angular resolution Active Optics Adaptive Optics Important Terms Refraction/reflection Focal length light-gathering power light pollution Aberration (chromatic, spherical) Chapter/sections covered in this lecture : sections 6-1 through 6-3