16.684 Space Systems Product Development Chart: 1 February 13, 2001 MIT Space Systems Laboratory Introduction to Optics part II Overview Lecture Space Systems Engineering presented by: Prof. David Miller prepared by: Olivier de Weck Revised and augmented by: Soon-Jo Chung 16.684 Space Systems Product Development Chart: 2 February 13, 2001 MIT Space Systems Laboratory Interferometer Types (NASA, AirForce) Space Technology 3-2005 Air Force UltraLITESIM-2006 Michelson Interferometer Precision Astrometry Michelson Interferometer Fizeau Interferometer Earth Observing Telescope TPF - 2011 NGST - 2007 Michelson Interferometer A Common Secondary Mirror (MMT, Fizeau) Primary Mirror = 8 m diameter 16.684 Space Systems Product Development Chart: 3 February 13, 2001 MIT Space Systems Laboratory Interferometer Types (Ground) Michelson Interferometer (Visible) Keck Interferometer-2006 Michelson Interferometer (Infrared) Twin 10 m Keck Telescopes and four 1.8 m outriggers Baseline 85m Palomar Testbed Interferometer Michelson Interferometer (Infrared) Testbed for Keck and SIM Mark III Interferometer Keck Observatory: Multiple Mirror Telescope (MMT) Fizeau Interferometer (Visible, Infrared) 36 hexagonal segments => 10 m overall aperture 16.684 Space Systems Product Development Chart: 4 February 13, 2001 MIT Space Systems Laboratory Michelson Interferometer Independent Light Collectors feed light to a common beam combiner. Get interfered fringes => Inverse Fourier Transform (CLEAN,MEM) Suitable for Astronomical Objects: Unchanged over a long period of time Beamsplitter Collector Collector Michelson Interferometer S t e l l a r w a v e f r o n t Detector Detector Optical delay line Object Imaging with Michelson Interferometer FT "FT -1 " Baseline orientations: + y x yvvv uu u x u-v (Fourier plane) Reconstructed image 16.684 Space Systems Product Development Chart: 5 February 13, 2001 MIT Space Systems Laboratory Fizeau Interferometer Gives a direct image of a target from a large combined primary mirror, and a wide field of view (Imaging applications in space and MMT) Suitable for Wide Angle Astrometry And for rapidly changing targets (Terrestrial, Earth Objects) 1 2 3 Telescope Telescopes Fizeau Interferometer Fizeau Interferometer DetectorDetector Sparse aperture Telescope "Beam combiner" 16.684 Space Systems Product Development Chart: 6 February 13, 2001 MIT Space Systems Laboratory Comparison (Fizeau, Michelson) Fi zea u Int erfer omet er Miche lson Inte rfe romete r Pro duce a dir ec t im age o f its targ et ( Full Instan t u- v c over age pr ov ided) Takes a sub se t o f u- v po ints obtaine d a peri od o f tim e. Wide angle(field) of view im aging app lications Ast rome try, Nulling Inter ferom etry Rapidly Cha nging t arge ts (Terre strial, Ea rth Objects) Targ et u nc han ged (Ast ron omi cal Objects) Takes t he c omb ine d sci en ce ligh t from all the ap erture s an d focuses it into C CD Me as ure s po ints in Fourier tran sform of ima ges => Inver se FFT ne eded U- V re so lution dep end s on b oth the separa tion and the size of aper ture s Angula r re so lution de pend s solely on the separa tion of aper ture s Optima l Con figura tion: Golay (m inim um aper ture size ) The angul ar res olution im pro ves as t he sep ara tion incre as es 16.684 Space Systems Product Development Chart: 7 February 13, 2001 MIT Space Systems Laboratory CCD Beam Combiner Common Secondary vs Sub Telescope Phased Sub- Telescope Arrays Common Primary Sub Telescope Fizeau Precise Off Axis Configuration Off Axis Optical Aberration Need Combiner + Phase sensing and compensater mechanism (complex) Less Central Obstruction(Off Axis) On Axis Suffers Central Obstruction Hard to change the Configuration Can employ Off-the-shelf telescopes Common Secondary Mirror Array AirForce is studying two options for UltraLITE(Golay 6) 16.684 Space Systems Product Development Chart: 8 February 13, 2001 MIT Space Systems Laboratory Optical Arrays -0.3 -0.2 -0.1 0 0.1 0.2 0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 Optical Array Configuration X - Distance [m] Y - D is tance [m] Instead of using a single aperture use several and combine their light to form a single image Aperture positions (uv) are critical - look at combined PSF / Transmissivity of the Optical Array Transmissivity Function: Derivation see separate handout (Mennesson) 16.684 Space Systems Product Development Chart: 9 February 13, 2001 MIT Space Systems Laboratory CDIO: Breaking the Paradigm -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 Optical Array Configuration X - Distance [m] Y - D i s t a n c e [m ] Golay-3 0.6 m telescope -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 Optical Array Configuration X - Distance [m] Y - D i s t a n c e [m ] Physical Aperture Layout Monolithic 0.6 m telescope Compare Architectures with Quantitative Metrics PSF 16.684 Space Systems Product Development Chart: 10 February 13, 2001 MIT Space Systems Laboratory Effective Radius of Optical Array How to find the effective radius(Reff) of the array? ? =the radius of the array thought as that of a monolithic aperture. ? UV coverage plot x,y is any point within aperture ? : the maximum radius of uv plot without any holes ? ? Fill Factor: the array’s total collecting area over the area of a filled aperture with the same uv coverage(the same Reff) u=± x 2 ? x 1 λ v=± y 2 ? y 1 λ R uv R eff R eff = 0.5R uv 16.684 Space Systems Product Development Chart: 11 February 13, 2001 MIT Space Systems Laboratory Golay Configurations ?Optimum Golayis D eff -dependent ? Labor moves Golay benefits to larger D eff ? Golay’s sacrifice Encircled Energy EE=83.5% EE=26.4% EE=9.3% EE=3.6% EE=2.2% 16.684 Space Systems Product Development Chart: 12 February 13, 2001 MIT Space Systems Laboratory Technological Trends ? Lightweight (Low-Area-Density) Optics - 15kg/m 2 ? Deployable Optics ? Adaptive and Active Optics ? Membrane Mirrors and Inflatables ? Ultra-Large Arrays (CCD Mosaics) ? Distributed Optical Arrays ? Space Based Astronomy ? White light interferometry 16.684 Space Systems Product Development Chart: 13 February 13, 2001 MIT Space Systems Laboratory Optical Performance Criteria ? Sensitivity(Effective Collecting Area) ? Point Spread Function(PSF): Frequency used merit function(Irradiance distribution),Can measure Phase difference, can derive Resolution,EE, MTF(OTF) ? Encircled Energy: particularly relevant merit function of the optic performance of an optical system whose purpose is to collect light and direct it thru the entrance slit of a spectrometer ? Modulation Transfer Function(MTF): For many imaging applications involving extended objects containing fine structure, the MTF is a more appropriate performance criterion than PSF. Practical Cutoff Frequency(Fr) -> Cutoff Frequency(Fc) 16.684 Space Systems Product Development Chart: 14 February 13, 2001 MIT Space Systems Laboratory Optical Sensitivity 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 N=8 N=6 N=2 Phase SubTelescope Common Secondary E f f e c t i v e C o l l ec t i n g A r e a REFLECTANCE R ? Sensitivity of Phased Telescope Array (Effective Collecting Area) R: the reflectance N: the number of reflections ? N geoeff RAA = 2 types may be viable concept for quasi-monochromatic applications; however, phased telescope arrays will suffer substantial sensitivity losses for broadband spectral applications A Region: UV B Region: Visible (Color) C Region: Quasi-Monochromatic A B C 16.684 Space Systems Product Development Chart: 15 February 13, 2001 MIT Space Systems Laboratory PSF,EE (Golay Array –3) Preliminary Calculation(0.3m GR) => D=2.0m,approx needed !! For Reff=1m, r=0.5m and Array Radius(L)= 1.m (Golay 3,monochrome) r=0.3685m and Array Radius(L)= 1.1m (Golay 6,monochrome) Using Analysis Tool: Evaluate Configuration Using ?PSF(Point Spread Function) ?Encircled Energy ?MTF(Modulation Transfer Function) ?FF(Fill Factor) 16.684 Space Systems Product Development Chart: 16 February 13, 2001 MIT Space Systems Laboratory Modulation Transfer Function ? An image of an extended object is far more complex thant point source(e.g) astrometry. PSF is not enough! -> MTF of each configuration is necessary ? Both Resolution and Contrast (Modulation) Transfer IMPORTANT 0 5 10 15 20 25 30 35 40 45 50 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 TextEnd -10 -8 -6 -4 -2 0 2 4 6 8 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 25 30 35 40 45 50 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Intensity Of Object Image Object Optical Array PSF of Optical Array Intensity Of Image Modulation= I max ? I min I max + I min FFT(I object) OTF=FFT(PSF) FFT(I image) * Real Image InvFFT MTF=abs(OTF) 16.684 Space Systems Product Development Chart: 17 February 13, 2001 MIT Space Systems Laboratory Optical Control and Beam Combining Optics Control Actuators Piezoelectric translators (PZTs) Fast steering mirrors (Tilt and Tip) Optical delay lines (or inchworm positioners) Alignment mirrors Sensors Laser interferometers Quad cells Charge Coupled Device (CCDs) cameras Avalanche Photo Diodes (APDs) 16.684 Space Systems Product Development Chart: 18 February 13, 2001 MIT Space Systems Laboratory Fine Optical Pointing and Phasing: Single Aperture ? Light path – Light enters spacecraft by reflecting off primary and then secondary, after which it is collimated (not converging or diverging except for diffraction effects) – Reflects off two-axis (tip and tilt) Fast Steering Mirror (FSM) ? FSM controls out the low level line-of- sight (LOS) jitter in the deadband of the attitude control – Lens focuses light onto camera ? LOS jitter control using FSM – Feedforward attitude sensors to command FSM – If bright point source in FOV, measure motion on camera and command FSM to minimize its motion Primary Mirror Secondary Mirror Fast Steering Mirror Light Rays CameraLens 16.684 Space Systems Product Development Chart: 19 February 13, 2001 MIT Space Systems Laboratory CCD Fig.10, Cassegrain Type beam combiner (above) and refractive lense combiner Beam Combining Goal: 1. maintain the optical phase difference of each beam to a fraction of a wavelength 2. align images from telescopes to within a fraction of a resolution element over the whole field of view (Additionally) Field of curvatures on the order of wavelength, finer than required for conventional telescopes Beam Combiner Phase(Piston Error) Contributor: 1. Lateral Pupil Geometry Error (Pupil Mapping Error) =>the elimination of this error : golden rule of separated telescope ( significant in wide field of view telescope) 2. Piston Error: part of piston error is induced by pupil geometry e 3. Tilt Error : Measured separately from piston error. X-Tilt Error, Y-Tilt Error 16.684 Space Systems Product Development Chart: 20 February 13, 2001 MIT Space Systems Laboratory Lateral Pupil Geometry Error Physical Meaning: If the subapertures of the entrance pupil have a D, and separated by S, with magnification M, the exit pupil must have dimensions for D/M and separation S/M a a )sin(aMP ε= p Correct Pupil Geometry Incorrect Pupil Geometry Difficult to measure lateral pupil geometry => abstract optical quantity Use the relationship and Kalman filter to estimate ε Pg ψ )sin( M P g ψ ε= ε :lateral pupil geometry error 16.684 Space Systems Product Development Chart: 21 February 13, 2001 MIT Space Systems Laboratory Optical Tolerance p γ r Tilt induced piston error )sin(γrP t = ])sin([)sin( )( p M r pppm gtp ++= ++= ψ εγ FOV=4km,h=500km,piston error55nm =>1μm Lateral pupil Geometry Error <1/10 of Airy Disk Diameter => 33 μrad Alignment Error (Image Rotation) 1/10 of λ, r=0.5m => 110 nradTilt 1/10 of wavelength = 55 nmPiston Total Piston Error P :the optical path difference between beams ToleranceErrors DFOV HalfAngle )44.2)(1.0( λ <?Φ 16.684 Space Systems Product Development Chart: 22 February 13, 2001 MIT Space Systems Laboratory Beam Combining Layout OPDA(Optical Path Difference Adjuster) :is driven by ?Piezoelectric translator(PZT) = Fine Control Translation Range (-12 μm),Resolution(5nm), SlewRate(4.5 μm/s) Angular Tilt Range(700 μrad),Resolution (200 nrad) ?Burleigh inchworm positioners = Coarse Control Translation Range(5mm) with a resolution of 0.1 μm CCD Retro Mirror Sensor System (Interferometer) Scraper(Scan) Mirror OPDA 16.684 Space Systems Product Development Chart: 23 February 13, 2001 MIT Space Systems Laboratory Sensor System Optics(Example from AFRL) Polarizing Beam Splitter Filter Beam Splitter Direction to Scraper Mirror Line Scan Camera Line Scan Camera Beam Expander Argon Laser Motor Field Scan Mirror (ψ) Piston Sensing : Lateral Effective Photo detector(LEP) : Split Polarizer Tilt Sensing : Microscope Lens 16.684 Space Systems Product Development Chart: 24 February 13, 2001 MIT Space Systems Laboratory Fine Optical Pointing and Phasing: Multiple Aperture ? Now need to stabilize both absolute and relative LOS jitter (Differential Wavefront Tilt) ? Also need to interfere same wavefront of light at combiner – Use Optical Delay Lines (ODLs) ? ODLs add and subtract optical pathlength from the companion telescope to the combiner. ? Used to control small positional mis-alignment and S/C attitude error ? Typically a multi-stage device with voice coils and piezoelectrics Primary Mirror Secondary Mirror Fast Steering Mirror Light Rays Camera Lens Secondary Mirror Light Rays Fast Steering Mirror Optical Delay Line Fold Mirrors Beam Combiner 16.684 Space Systems Product Development Chart: 25 February 13, 2001 MIT Space Systems Laboratory References [1] Larson, W.J., Wertz, J.R., “Space Mission Analysis and Design”, Second Edition, 9.5 Designing Visual and IR Payloads, pp.. 249-274, Microcosm, Inc,1992 [2] Born Max, Wolf Emil, “Principles of Optics”, Electromagnetic Theory of propagation interference and diffraction of light, Sixth Edition, Cambridge University Press, 1998 [3] Hecht E. “Optics”, Addison-Wesley, 1987 [4] Günter Diethmar Roth, “Compendium of Practical Astronomy”, Volume 1, Instrumentation and Reduction Techniques, Springer Verlag, Berlin, New York, ISBN 3-540-56273-7, 1994 16.684 Space Systems Product Development Chart: 26 February 13, 2001 MIT Space Systems Laboratory Optical System Design Process From SMAD Chapter 9 1. Determine Instrument Requirements 2. Choose preliminary aperture 3. Determine target radiance 4. Select detector candidates 5. “Optical Link Budget”, SNR considerations 6. Determine Focal Plane architecture and scanning schemes 7. Select F# and telescope/optical train design 8. Complete preliminary design and check MTF 9. Estimate weight, power and ACS requirements 10. Iterate and document - code optics software module