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Lecture 4
Part 1: Finish Geometrical Optics Part 2: Physical Optics
Claire Max UC Santa Cruz
January 21, 2016
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Aberrations
• In optical systems
• Description in terms of Zernike polynomials
• Aberrations due to atmospheric turbulence
• Based on slides by Brian Bauman, LLNL and UCSC, and Gary Chanan, UCI
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Optical aberrations: first order and third order Taylor expansions
• sin θ terms in Snell’s law can be expanded in power series
n sin θ= n’ sin θ’
n ( θ - θ3/3! + θ5/5! + …) = n’ ( θ’ - θ’3/3! + θ’5/5! + …)
• Paraxial ray approximation: keep only θ terms (first order optics; rays propagate nearly along optical axis)
– Piston, tilt, defocus
• Third order aberrations: result from adding θ3 terms – Spherical aberration, coma, astigmatism, .....
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Different ways to illustrate optical aberrations
Side view of a fan of rays (No aberrations) “Spot diagram”: Image at
different focus positions
Shows “spots” where rays would strike hypothetical detector
1 2 3 4 5
1 2 3 4 5
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Spherical aberration
Through-focus spot diagram for spherical aberration
Rays from a spherically aberrated wavefront focus
at different planes
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Hubble Space Telescope suffered from Spherical Aberration
• In a Cassegrain telescope, the hyperboloid of the primary mirror must match the specs of the secondary mirror. For HST they didn’t match.
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HST Point Spread Function (image of a point source)
Before COSTAR fix After COSTAR fix
Core is same width, but contains only 15% of energy
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Point spread functions before and after spherical aberration was corrected
Central peak of uncorrected image (left) contains only 15% of central peak energy in corrected image (right)
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Spherical aberration as “the mother of all other aberrations”
• Coma and astigmatism can be thought of as the aberrations from a de-centered bundle of spherically aberrated rays
• Ray bundle on axis shows spherical aberration only
• Ray bundle slightly de-centered shows coma
• Ray bundle more de-centered shows astigmatism
• All generated from subsets of a larger centered bundle of spherically aberrated rays
– (diagrams follow)
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Spherical aberration as the mother of coma
Big bundle of spherically aberrated rays
De-centered subset of rays produces coma
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Coma
• “Comet”-shaped spot
• Chief ray is at apex of coma pattern
• Centroid is shifted from chief ray!
• Centroid shifts with change in focus!
Wavefront
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Coma
Through-focus spot diagram for coma
Rays from a comatic wavefront
Note that centroid shifts:
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Spherical aberration as the mother of astigmatism
Big bundle of spherically aberrated rays
More-decentered subset of rays produces astigmatism
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Astigmatism
Through-focus spot diagram for astigmatism
Side view of rays
Top view of rays
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Different view of astigmatism
Credit: Melles-Griot
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Wavefront for astigmatism
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Where does astigmatism come from?
From Ian McLean, UCLA
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Concept Question
• How do you suppose eyeglasses correct for astigmatism?
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Off-axis object is equivalent to having a de-centered ray bundle
Ray bundle from an off-axis object. How to view this as a de-centered ray bundle?
For any field angle there will be an optical axis, which is to the surface of the optic and // to the incoming ray bundle. The bundle is de-centered wrt this axis.
Spherical surface
New optical axis
⊥
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Aberrations
• In optical systems
• Description in terms of Zernike polynomials
• Aberrations due to atmospheric turbulence
• Based on slides by Brian Bauman, LLNL and UCSC, and Gary Chanan, UCI
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Zernike Polynomials
• Convenient basis set for expressing wavefront aberrations over a circular pupil
• Zernike polynomials are orthogonal to each other
• A few different ways to normalize – always check definitions!
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Piston
Tip-tilt
From G. Chanan
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Astigmatism(3rd order)
Defocus
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Trefoil
Coma
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Spherical
“Ashtray”
Astigmatism(5th order)
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Aberrations
• In optical systems
• Description in terms of Zernike polynomials
• Aberrations due to atmospheric turbulence
• Based on slides by Brian Bauman, LLNL and UCSC, and Gary Chanan, UCI
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Units: Radians of phase / (D / r0)5/6
Reference: Noll
Tip-tilt is single biggest contributor
Focus, astigmatism, coma also big
High-order terms go on and on….
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Seidel polynomials vs. Zernike polynomials
• Seidel polynomials also describe aberrations
• At first glance, Seidel and Zernike aberrations look very similar
• Zernike aberrations are an orthogonal set of functions used to decompose a given wavefront at a given field point into its components
– Zernike modes add to the Seidel aberrations the correct amount of low-order modes to minimize rms wavefront error
• Seidel aberrations are used in optical design to predict the aberrations in a design and how they will vary over the system’s field of view
• The Seidel aberrations have an analytic field-dependence that is proportional to some power of field angle
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References for Zernike Polynomials
• Pivotal Paper: Noll, R. J. 1976, “Zernike polynomials and atmospheric turbulence”, JOSA 66, page 207
• Books: – e.g. Hardy, Adaptive Optics, pages 95-96
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Let’s get back to design of AO systems Why on earth does it look like this ??
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Considerations in the optical design of AO systems: pupil relays
Pupil Pupil Pupil
Deformable mirror and Shack-Hartmann lenslet array should be “optically conjugate to the telescope pupil.” What does this mean?
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Define some terms
• “Optically conjugate” = “image of....”
• “Aperture stop” = the aperture that limits the bundle of rays accepted by the optical system
• “Pupil” = image of aperture stop
optical axis object space image space
symbol for aperture stop
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So now we can translate:
• “The deformable mirror should be optically conjugate to the telescope pupil”
means
• The surface of the deformable mirror is an image of the telescope pupil
where
• The pupil is an image of the aperture stop – In practice, the pupil is usually the primary mirror of the
telescope
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Considerations in the optical design of AO systems: “pupil relays”
Pupil Pupil Pupil
‘PRIMARY MIRROR"
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Typical optical design of AO system
telescope primary mirror
Science camera
Pair of matched off-axis parabola mirrors
Wavefront sensor (plus
optics) Beamsplitter
Deformable mirror
collimated
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More about off-axis parabolas
• Circular cut-out of a parabola, off optical axis
• Frequently used in matched pairs (each cancels out the off-axis aberrations of the other) to first collimate light and then refocus it
SORL
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Concept Question: what elementary optical calculations would you have to do, to lay out this AO system? (Assume you know telescope parameters, DM size)
telescope primary mirror
Science camera
Pair of matched off-axis parabola mirrors
Wavefront sensor (plus
optics) Beamsplitter
Deformable mirror
collimated
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Review of important points
• Both lenses and mirrors can focus and collimate light
• Equations for system focal lengths, magnifications are quite similar for lenses and for mirrors
• Telescopes are combinations of two or more optical elements
– Main function: to gather lots of light
• Aberrations occur both due to your local instrument’s optics and to the atmosphere
– Can describe both with Zernike polynomials
• Location of pupils is important to AO system design
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Part 2: Fourier (or Physical) Optics
Wave description: diffraction, interference
Diffraction of light by a circular aperture
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Levels of models in optics
Geometric optics - rays, reflection, refraction
Physical optics (Fourier optics) - diffraction, scalar waves
Electromagnetics - vector waves, polarization
Quantum optics - photons, interaction with matter, lasers
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Maxwell’s Equations: Light as an electromagnetic wave (Vectors!)
∇⋅E = 4πρ
∇⋅E = 0
∇×E = − 1c∂B∂t
∇× B = 1c∂E∂t
+ 4πc
J
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Light as an EM wave
• Light is an electromagnetic wave phenomenon, E and B are perpendicular
• We detect its presence because the EM field interacts with matter (pigments in our eye, electrons in a CCD, …)
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Physical Optics is based upon the scalar Helmholtz Equation (no polarization)
• In free space
• Traveling waves
• Plane waves
∇2E⊥ =1c2
∂2
∂t 2 E⊥
E⊥ x,t( ) = E⊥ 0,t ± x c( )
E⊥ x,t( ) = E k( )ei ω t−k⋅x( )
k2 E = ω c( )2 Ek =ω / c
k Helmholtz Eqn., Fourier domain
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Dispersion and phase velocity
• In free space
– Dispersion relation k (ω) is linear function of ω – Phase velocity or propagation speed = ω/ k = c = const.
• In a medium – Plane waves have a phase velocity, and hence a wavelength, that
depends on frequency
– The “slow down” factor relative to c is the index of refraction, n (ω)
k =ω c where k ≡ 2π λ and ω ≡ 2πν
k ω( ) =ω vphase
vphase = c n ω( )
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Optical path – Fermat’s principle
• Huygens’ wavelets
• Optical distance to radiator:
Δx = v Δt = c Δt nc Δt = n Δx
Optical Path Difference = OPD = ndx∫• Wavefronts are iso-OPD surfaces
• Light ray paths are paths of least* time (least* OPD)
*in a local minimum sense
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What is Diffraction?
In diffraction, apertures of an optical system limit the spatial extent of the wavefront
Aperture
Light that has passed thru aperture, seen on screen downstream
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Diffraction Theory
We know this
Wavefront U
What is U here?
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Diffraction as one consequence of Huygens’ Wavelets: Part 1
Every point on a wave front acts as a source of tiny wavelets that move forward.
Huygens’ wavelets for an infinite plane wave
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Diffraction as one consequence of Huygens’ Wavelets: Part 2
Every point on a wave front acts as a source of tiny wavelets that move forward.
Huygens’ wavelets when part of a plane wave is blocked
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Diffraction as one consequence of Huygens’ Wavelets: Part 3
Every point on a wave front acts as a source of tiny wavelets that move forward.
Huygens’ wavelets for a slit
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The size of the slit (relative to a wavelenth) matters
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Rayleigh range
• Distance where diffraction overcomes paraxial beam propagation
L
D λ / D LλD
= D ⇒ L = D2
λ)
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Fresnel vs. Fraunhofer diffraction
• Very far from a point source, wavefronts almost plane waves.
• Fraunhofer approximation valid when source, aperture, and detector are all very far apart (or when lenses are used to convert spherical waves into plane waves)
• Fresnel regime is the near-field regime: the wave fronts are curved, and their mathematical description is more involved.
S
P
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Regions of validity for diffraction calculations
The farther you are from the slit, the easier it is to calculate the diffraction pattern
Near field Fresnel Fraunhofer (Far Field)
N = D2
Lλ≥1 N = D2
Lλ<<1N = D2
Lλ>>1
D
L
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Fraunhofer diffraction equation
F is Fourier Transform
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Fraunhofer diffraction, continued
• In the “far field” (Fraunhofer limit) the diffracted field U2 can be computed from the incident field U1 by a phase factor times the Fourier transform of U1
• “Image plane is Fourier transform of pupil plane”
F is Fourier Transform
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Image plane is Fourier transform of pupil plane
• Leads to principle of a “spatial filter” • Say you have a beam with too many intensity fluctuations
on small spatial scales – Small spatial scales = high spatial frequencies
• If you focus the beam through a small pinhole, the high spatial frequencies will be focused at larger distances from the axis, and will be blocked by the pinhole
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Details of diffraction from circular aperture
1) Amplitude
2) Intensity
First zero at r = 1.22 λ/ D
FWHM λ/ D
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Heuristic derivation of the diffraction limit
Courtesy of Don Gavel
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Rayleigh resolution
limit: Θ = 1.22 λ/D
2 unresolved point sources
Resolved
Credit: Austin Roorda
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Diffraction pattern from hexagonal Keck telescope
Ghez: Keck laser guide star AO!
Stars at Galactic Center!
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Conclusions: In this lecture, you have learned …
• Light behavior is modeled well as a wave phenomena (Huygens, Maxwell)
• Description of diffraction depends on how far you are from the source (Fresnel, Fraunhofer)
• Geometric and diffractive phenomena seen in the lab (Rayleigh range, diffraction limit, depth of focus…)
• Image formation with wave optics