Imaging Extremes

8/14/2019 Imaging Extremes

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Imaging Extremes


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Look at the images in the next few slides.

They are all real images captured on camera.

The first picture is an electron micrograph of abutterflys wings and the last one shows galaxies in

their formative stages (Hubble ultra deep probe photo).


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Awe inspiring?

So what is an image after all?


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Image

- a visual reproduction of object information

Imaging

- the process of reproducing object information

What is transferred from object space to image space?

-object information carried by e.m.radiationIn which wavelength range?

- depends on : spectral comp. of the light source/

spectral information content of object/spectral sensitivity of detector/

desired resolution.


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Is perfect imaging possible?

Ideal imaging

Real life Imaging


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Airy pattern


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Rayleigh Criterion

Lmin

= 1.22 f / d

Sparrow Criterion

Lmin

= 0.47f / d

Resolution increases with

decrease wavelength

decrease in focal length increase imaging aperture

For the human eye:

for =550nm

f =20mm

d=2mm (2-8mm)

Lmin=6.7 microns ~ 0.1 mm

min = Lmin/f = 1min of arc


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~25cm ~20mm

The limitation (!) of the human eye

The human eye as an instrument cannot magnify. It always minifies!

B t h i t lli t d i i iti i l !


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But humans are intelligent and inquisitive animals!

The limit of angular resolution of the human eye has to be overcome.

The mystery of our existence has to be unravelled

We had to gather information from two extreme regions and probe

-closer and closer in the microscopic world

- farther and farther into the stars

In both these regimes the 0.1 micron linear resolution and 1min of

arc angular resolution proved utterly hopeless.

Optical instruments like microscopes and telescopes were developed


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Microscopes are used to view smaller and smaller objects.

Its purpose is to magnify the object so that the image produced in

the retina is at least 0.1 mm.

f has to be small ( typical f.l. : 16 1.6 mm)

d cannot be made large due to design problems

How small can be ?

The lowest in the visible wavelength is ~ 0.3

UV microscopy is possible.

X-ray microcopy not possible because of material limitations.

What else?

Is it ssibl t tili th t f ti l s?


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Is it possible to utilize the wave nature of particles?

De-Broglie proposed that moving particles exhibit wave-like

properties

The wavelength is given by = h / mass x velocity

What particle?Of course electrons.

Accelerate electrons through a potential difference to make

them travel faster. The faster they travel the smaller will beits wavelength!

How small?

Say V=10KV, eV=1/2 mv2, = 0.0000123 microns = 0.123Ao

Which is 10-4 magnitude smaller than visible light

Resolution will be 104 magnitude higher!

Lmin=0.00024nm = 2.4Ao (actually equal to 2.2A using relativistic mechanics)


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Electromagletic lens

Scancoils

Electron gun

detector

To vacuum pump

Focussing

lens

Electron beam

Ordinary electron

microscopes have a

resolution of ~1nm

-which can be pushed to

0.1nm = .0001micron


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The Other Extreme capturing starlight

Stars are low intensity point sourcesTo capture more light - aperture d must be large.

Large aperture calls for reflecting type objectives.

Large aperture also increases resolution.

Reflector of Mt.Palomar Observatory telescope is 200 in dia.

A telesope is an afocal system and has angular magnification.


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Ever since the dutch spectacle maker Hans Lippershey

(c1570-c1619) assembled the first telescope and GalileoGalilei became the first of the human species to observe the

craters of the moon (1609), there was no looking back.


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//

eye

FoFo

objective eyepiece eye

Angular Magnification

M = / /


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Schmidt plate

Reflecting Telescopes

Paraboloid primary mirror

Schematic of a Schmidt-Cassegrainian Telescope

Secondary mirror


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An almost perfect device to unravel the mystifying universe!

But man proposes nature disposes.

All observatories are on mountain tops why?

Atmospheric Turbulence distorts the image

A distant star is a point object for all practical purposes

The wave reaching the earth surface should have a planewavefront

Atmospheric turbulence distorts the wavefront

It also causes the image to be vacillating

We have no control over the atmosphere!

In 1730 Newton writes The only remedy to eliminate the tremors

of the atmosphere is a most serene and quiet air such as may

be found on tops above the highest mountains above thegrosser clouds.


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Causes of atmospheric tremor:

Temperature changes less than 1 deg.C cause eddies in wind velocity

The change in velocity causes minute changes in atmospheric density

Minute variations of refractive index (~ 10-6 ) results

These minute variations accumulate

The refractive index profile of the atmosphere continually changes

Effects:

Twinkling : Random interference between light waves from the same star

passing through slightly different atmospheric paths

Quiver : Wandering of the image

Spreading : The spreading of an well defined image to a light patch


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As far as technology is concerned, the human species havenever resigned to fate !

There were two options available to defeat atmospheric turbulance:

May be a telescope can be placed beyond the atmosphere

May be the distorted wavefront can be corrected before imaging

The first option gave us the Hubble TelescopeThe second option grew into a front line area of optics Adaptive

Optics


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The Hubble Telescope

1914 Dr.Lyman Spitzer proposed the outrageous idea

1960 The US space program developed and excelled and Dr.Spitzer lobbied

in the US Congress and NASA

1977 Congress approved funds for a telescope in space and decided to name it

after Edwin Hubble whose observations confirmed that the Universe is

expanding an indirect confirmation of the Big Bang Theory.

1990 Hubble Telescope goes into orbit

It took8 years to build, has more than 400,000 parts and 26,000 miles of

electrical wiring, has five scientific instruments including a spectroscope.

Aperture of primary mirror 94.5 Secondary mirror 12

Orbit 612 km Orbital Speed 28,000km/hr

Orbital period 97 min

Cost : 2.2 billion dollars


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What is ADAPTIVE OPTICS ?

the capability of adjusting to different

conditions or environment

A term referring to optical components that

modify the propagation of light in some way or

other

Adaptive

Adaptive Optics : The technology by which a distortedoptical signal is corrected automatically, often by gathering

information about the environment through which it passes

obviously through optics that can adapt itself to changing

environmental needs.

Optics


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We are gifted by nature with such a technology

the animal eye-brain system is a perfect example of adaptive control

Optic Nerve

Oculomotor/

Trochlear Nerve

Closed loop

architecture

-Dilation and contraction of Iris (intensity control)

-Tracking by eye movement ( image tilt control)

-Squint (convergence angle control / phase)

Adaptive optics pre-supposes that light from stars that reach the earth are


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Adaptive optics pre supposes that light from stars that reach the earth are

parallel rays, i.e., the wavefront is a plane.

Is a starlight a parallel light beam??


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Sun is our nearest star at 8 light minutes away.

Distance : ~8 light minutes / 150x106 kms

Diameter of Earth = 12,756 km

100m

etres

Largest the Telescope has diameter (~ 9 metres dia) d is

~ 0.27 nanometres for the sun.

The next closest star is 4 light years away!!

Is a starlight a parallel light beam??

d

What does Adaptive optics aim to do?


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Schmidt type phase plate

What does Adaptive optics aim to do?


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Corrector Sampler

Imager

Wavefront sensor

The building blocks of a typical AO system

Waveform sensor : The Shack Hartmann Sensor waveform tilt meas.


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Waveform sensor : The Shack Hartmann Sensor waveform tilt meas.

Basic Principle:

x= 0

y= 0

X/

Y/

x= x/ / f

y= y/ / f


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V2

V1

V3

V4

X-tilt = [(V1+V4)-(V2+V3)]/V

Y-tilt = [(V1+V2)-(V3+V4)]/V


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Reproduced froma publication by

Ronald Shack and

Ben C.Platt


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CorrectorSampler

Imager

Wavefront sensor

The building blocks of a typical AO system

In other words, mirrors can be deformed in a predefined


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, pmanner so as to reflect a corrected wavefront !!


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Correctors

Deformable mirrors Segmented Mirrors

Bimorph MirrorsTip/Tilt mirrors


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Tip/Tilt mirror

Deformable mirror


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Bimorph structure

(flux bucket structure)

The polarisation of the piezo-

electric plate is chosen such that

when voltage is applied to an

electrode, one of the plates

expands, and the other contracts.

This differential expansioncauses the bimorph to bend,

much in the same way as a bi-

metallic strip will bend when

heated.


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Segmented mirror of

primary (10 m, 91)

at SALT

Segmented mirror used in AO

loop

Bl k di i it d
http://www.ing.iac.es/PR/AR1997/int04.jpghttp://www.news.wisc.edu/newsphotos/images/SALT_Observatory_mirror03.JPG


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Corrector

Imager

SHS

Block diagram revisited


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Effect of adaptive optics on a star image at the Keck

telescope. This is cheating a bit because the star is its own

guide star!


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Imaging Extremes

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