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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Thanks