Kerri Cahoy (MIT), Peter Lawson (JPL), John Taranto (Thorlabs), Michael Feinberg (Boston Micromachines),
Anne Marinan (MIT), Ma@ Webber (MIT)
Sagan Workshop Thursday Hands-‐on Session: Building a Coronagraph
Acknowledgements
• Thank you to John Taranto and Thorlabs for providing addiIonal opIcal hardware for today’s labs!!! J
• Thank you to Michael Feinberg and Boston Micromachines for the “Sensorless” demonstraIon
• Thank you to Gene Serabyn, Dimitri Mawet, for Vector Vortex occulIng mask
• Thank you to Jeremy Kasdin, Elizabeth Young, and Alexis CarloU for Shaped Pupil mask
• Thank you to John Trauger for M2 mask
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Goals
• Learn how a coronagraph works… with hardware – Understand pupil plane, image plane – Apodizers, occulIng masks, Lyot stops – PoinIng sensiIvity, detectors
• Learn about high actuator count deformable mirrors – Needed for wavefront control and speckle management
• Learn about wavefront control methods, how these mirrors are used (sensored and sensorless)
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Overview
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• Break into 4 groups: A, B, C, D
• Walk to Keith Spalding Building (#6) on corner of California & Wilson
• 1) Coronagraph in KS-‐410E
• 2) Shack Hartmann in KS-‐410
• 3) Sensorless Demo in KS-‐415
• 4) MATLAB in KS-‐300
Laser Safety
• TLS001-‐635 is a class 3R laser • The 635 nm laser is 2.5 mW and split into 4 setups, so it is less powerful than solo… but! – Do NOT stare into laser! – Use a sheet of paper or detector to trace laser path, avoid looking directly at laser light
– Avoid wearing jewelry near hands that could unpredictably reflect laser light elsewhere (watch, rings, etc.)
– Make sure laser terminates in some type of beam stop (detector, object, beam dump)
– Reduce power if aligning opIcs, use as li@le power as possible
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Coronagraph Lab
• We’ve set it up – take a look, and then take the posts out and put it back together: 1. Use a piece of paper to see how the beam behaves through the system. 2. Fully open the iris closest to the camera to see how the Lyot stop blocks
diffracIon spikes; save images with the camera of “before” and “aoer”. 3. Remove all posts except laser and camera (do not move the “feet”). 4. Start by puUng in the 25 mm lens and observing its collimaIng funcIon. 5. Put in the 3 remaining lenses one at a Ime. Observe how they focus/relay
light. IdenIfy pupil and image planes. Capture the resulIng PSF. 6. Put in irises. Capture resulIng PSF, compare with (5). 7. Put in apodizer (if any, do separately) and occulIng mask. Adjust poinIng.
Capture resulIng PSF, compare with (6).
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Amplitude coronagraph(Spot/Linear Occulter/
Vector Vortex)
Iris/ Phase mask Lyot Stop
(Iris -‐ optional)
Pinhole Laser Source
25 mm lens 200 mm lens 100 mm lens 100 mm lens
Detector
Shack Hartmann Demo
• Thorlabs “AO Kit” demonstraIon using a Shack Hartmann wavefront sensor and Boston Micromachines MEMS deformable mirror
• Learn about the closed-‐loop control, introduce disturbances and observe the response
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http://www.ctio.noao.edu/~atokovin/tutorial/part3/wfs.html
“Wavefront Sensorless” Correction
• Use Boston Micromachines MEMS deformable mirror and algorithm that opImizes based on detected image to improve focus of the beam
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http://www.bostonmicromachines.com/wsaod-l.htm
MATLAB Shack Hartmann Simulation
• Simulate a Shack Hartmann wavefront sensor – Use Zernike modes – Generate spot offsets in x and y
– Understand modal decomposiIon
– See least squares approach to wavefront reconstrucIon
– See the effect of system parameters like number of lenslets, etc. on performance
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Wavefront Reconstruction: Modal vs. Zonal
• Zonal: write the wavefront gradients in terms of finite-‐differences, and numerically integrate the data to recover the wavefront.
• Modal: the wavefront is described in terms of funcIons that have analyIc derivaIves. The measured slope data is then fit to the derivaIve of these funcIons, allowing a direct determinaIon of the wavefront from the fit coefficients.
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References
• M2 mask:h@ps://exep.jpl.nasa.gov/files/exep/HCIT-‐Milestone2Signed-‐2008-‐08-‐08.pdf
• Vector Vortex mask: h@p://arxiv.org/pdf/0912.2287v1.pdf
• Shaped pupil: h@p://arxiv.org/pdf/0912.2287v1.pdf
• Band-‐limited Lyot: h@p://arxiv.org/pdf/astro-‐ph/0203455v1.pdf
• Shack Hartmann: h@p://betagaugereplacement.org/documents/wavefront/Shack-‐Hartmann-‐2.pdf
• Example Wavefront Sensorless Algorithm: h@p://www.opIcsinfobase.org/josaa/abstract.cfm?uri=josaa-‐15-‐10-‐2745
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MATLAB Simulation Activity
• Connect via VNC Viewer • Run MATLAB • Open sumZernike.m • sumZernike(n, m, x_in, y_in)
>> x=linspace(-‐10,10,100); >> y=x; >> out = sumZernike(2,2,x,y); >> imagesc(out)
• Try your own n, m
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Zernike Modes
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MATLAB Simulation Activity 1/2
• Open Spo{ieldGeneraIonV06.m • Read through it together on the projector • Run it with the default n, m vectors
– Note image output (does it make sense to you?) and resulIng mode coefficients: >> Spo{ieldGeneraIonV06 Coefficients for aberrated wavefront terms Max RMS error (um) divided by Max(Znm) (um) for all contribuIng (n,m) Zernike terms ans = 1.0e-‐03 * 0.6667 0.6667 0.0004 0.0002 0.0009
• Change it to create a distorted wavefront of your choice, such as: – Run it with single Znm aberraIons (1,1) or (1,-‐1) – Run it with single Znm aberraIons (2,0) or (2,2) or (2,-‐2) – Run it with single Znm aberraIon (3,1) or (3,-‐1) – Modify it to create a superposiIon of modes of your choice – Note output, the resulIng mode coefficients
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Solving for modes using only Δx, Δy
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http://scien.stanford.edu/pages/labsite/2003/psych221/projects/03/pmaeda/index.html
Modal wavefront reconstruction
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MATLAB Simulation Activity 2/2
• Open parIalZernike.m – This is used to help calculate dZ/dx and dZ/dy
• Open WavefrontAnalysisV05.m – Run it (do not clear all or close all) on the output from Spo{ieldGeneraIonV06.m
• Do numbers match?
• How might you expect this to change if you added some detector or centroiding noise into the output of Spo{ieldGeneraIonV06?
• How would results change if you reduced the number or pitch of the lenslets?
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