optics and telescopes
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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 fastestspeed (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.
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 TelescopeObjective 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 telescope1. 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 lens4. Large lenses are heavy gravitational distortion
Law of Reflectionincidence angle=reflection angle
Mirror
Perpendicularto mirror surface
Reflectedlight ray
Incidentlight ray
ir
Angle of reflection r equals angle of incidence i
Reflection : Mirror acts as a lens
Newtonian Telescope
Different designs of Reflecting telescopesPrime 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 arcsecondsλ = wavelength of light, in metersD = diameter of telescope, in meters
(example) Keck telescope, red light. …
€
θ =2.5 ×105λD
Hawaii, Mauna Kea
Light Pollution
Effect of Earth Atmosphere Light = wave Perfect waveform got deformed due to turbulence in atmosphere…
breeze turbulence in atmosphere
Adaptive OpticsUsing a nearby star (e.g., point source), reshape the mirror so that it can become a
perfect (diffraction limited) point source.
Active Opticswind shakes tip/tilt correction
Eliminate the effect of Atmosphere (Adaptive Optics)
Power of Adaptive Optics
Power of Adaptive Optics
Laser-guided Adaptive Optics
In summary…
Important ConceptsRefractive telescope
• disadvantagesReflective telescope
• various designs
Angular resolution
Active OpticsAdaptive Optics
Important TermsRefraction/reflectionFocal lengthlight-gathering powerlight pollution
Aberration (chromatic, spherical)
Chapter/sections covered in this lecture : sections 6-1 through 6-3