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