Astronomical Observing Techniques 2017 Lecture 13: Twinkle, twinkle star... No more!

Transcription

1 Astronomical Observing Techniques 2017 Lecture 13: Twinkle, twinkle star... No more! Christoph U. Keller

2 Overview 1. The Power of Adaptive Optics 2. Atmospheric Turbulence 3. Basic Principle 4. Key Components 5. Error Terms 6. Laser Guide Stars 7. Adaptive Optics Operations Modes Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 2

3 The Power of AdapHve OpHcs oldweb.lbto.org/ao/aopressrelease.htm

4 Io with and without Keck AO Io image taken with Keck adaptive optics; K-band, 2.2micron Io image based on visible light taken with Galileo spacecraft orbiter. Io image taken with Keck adaptive optics; L-band, 3.5micron Io image taken without Keck adaptive optics. cfao.ucolick.org/pgallery/io.php Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 4

5 NGC 7469 cfao.ucolick.org/ao/why.php Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 5

6 Seeing: r 0, τ 0, θ 0 (from Lecture 2) Fried parameter r 0 : average turbulent scale over which RMS opdcal phase distordon is 1 rad r 0 increases as λ 6/5 Seeing Δθ at good sites at 0.5μm: cm atmospheric coherence (or Greenwood delay) Dme: maximum Dme delay for RMS wavefront error to be < 1 rad ( ν is mean propagadon velocity) IsoplanaDc angle θ 0 : angle over which RMS wavefront error is smaller than 1 rad 6/5 2 r0 ( λ) = λ Cn ( z) dz 0 λ Δθ = r 0 3/5 1/5 ~ λ τ 0 = r 0 v θ 0 = 0.314cosζ r 0 h keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 6

7 ResoluHon & SensiHvity Improvement λ 1. Angular resoludon: θ = θ = r0 2. Point source sensidvity: S / N ~ D λ D gain = D r 2 0 gain in t 1 int ~ 4 D PHARO LGS Ks image 500s integ., 40" FOV, 150 mas FWHM WIRO H image Kobulnicky et al. 2005, AJ 129, keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 7

8 AdapHve OpHcs Principle Maximum scale of tolerated wavefront deformadon is r 0 à subdivide telescope into apertures with diameter r 0 Measure wavefront deformadon Correct wavefront deformadon by bending back patches of size r 0 Number of subapertures is (D/r 0 ) 2 at observing wavelength à requires hundreds to thousands of actuators for large telescopes r 0 D keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 8

9 AdapHve OpHcs Scheme (SCAO) Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 9

10 Wavefront DescripHon: Zernike Polynomials Expansion into a series of orthogonal terms: keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 10

11 Zernike Amplitudes for Kolmogorov Turbulence Units: Radians of phase / (D / r 0 ) 5/6 Tip-tilt is single biggest contributor Focus, astigmatism, coma also big High-order terms go on and on. Reference: Noll keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 11

12 Wavefront Sensors Shack Hartmann Most common principle is the Shack Hartmann wavefront sensor measuring sub-aperture Dlts: f Pupil plane Image plane keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 12

13 ExPo Wavefront Sensor at WHT Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 13

14 Wavefront Sensing Sunspot wavefront sensor images at 955Hz Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 14

15 Curvature, Pyramid Wavefront Sensors Curvature Wavefront Sensor Pyramid Wavefront Sensor Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 15

16 Deformable Mirrors (DM) Fit mirror surface to wavefront r 0 sets number of degrees of freedom segmented mirrors rarely used anymore mostly condnuous face-sheet mirrors keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 16

17 Membrane Deformable Mirror micromachined deformable mirror (OKOtech/Flexible OpDcs) with 37 actuators 600-nm thick, 15-mm diameter silicon nitride membrane electrostadc actuators Individual Actuators Mirror Surface Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 17

18 AdapHve Secondary Mirrors DM part of telescope à adapdve secondary mirrors no addidonal opdcs à lower emission, higher throughput more difficult to build, control, and handle DM for MMT Upgrade Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 18

19 { Shack-Hartmann Wavefront Analysis centroid (center of gravity) calculadon on each subaperture: Δx = i max i=i min i max j max j= j min j max I(i, j) i I(i, j), Δy = i max i=i min i max j max j= j min j max I(i, j) j I(i, j) i=i min j= j min i=i min j= j min Zero Position { Δy Δx keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 19

20 Influence Matrix slope of mirror surface and Shack-Hartmann star posidons are propordonal to actuator posidon linear reladonship between actuator a and star posidon c: cn = N k=1 a k b nk (For a single spot x-offset or y-offset) combine equadons for each spot posidon n into matrix equadon: C = BA C = star posidons A = actuator posidons B = influence matrix describing influence of specific actuator posidon on star posidons keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 20

21 Measuring the Influence Matrix measure centroid posidons in subapertures for different seqngs of actuator k for actuator k and subaperture n, slope of best fit line is element (n, k) of influence matrix B Spot Offset (pixels) Actuator 1, Trial 1, Spot 17 (horizontal) r = E+00 2.E+04 4.E+04 6.E+04 8.E+04 Squared Voltage keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 21

22 Determining the Control Vector Influence matrix B is known, C from wavefront sensor Find control vector A to correct for error in wavefront Matrix inversion of B? A = B 1 C Overdetermined system More centroid measurements than actuators No exact soludon A exists for given set of centroids No exact B -1 exists (B is rectangular) Singular Value DecomposiDon: approximate B -1 in best possible way (minimizes wavefront error) keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 22

23 Typical AO Error Terms Fiqng errors from insufficient approximadon of wavefront by deformable mirror due to finite number of actuators (d: actuator spacing) Temporal errors from Dme delay between measurement and correcdon, mostly due to exposure and readout Dme Measurement errors from wavefront sensor CalibraDon errors from non-common aberradons between wavefront sensing opdcs and science opdcs Angular anisoplanadsm from sampling different lines of sight through the atmosphere, mostly limits field of view σ fit σ d r 0 2 temp 2 σ aniso t τ 0 2 σ measure ~ S / 2 σ calibration θ θ 0 5/3 N ~??? 5/3 5/3 keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 23

24 Angular AnisoplanaHsm Angular anisoplanadsm severely limits wide-field imaging sky coverage (finding a guide star within the isoplanadc angle) keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 24

25 Typical CorrecHon and Residuals cfao.ucolick.org/pgallery/gc.php Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 25

26 Sky Coverage To sense the wavefront one needs a bright reference/guide star within the isoplanadc angle Galactic Equator ( l < 5 ) Intermediate Latitudes (-35 < l < -25 ) Galactic Pole (l < -80 ) All sky average (-90 < l < 90 ) Cummulative Sky Coverage (mi < mag) Sky coverage given by star counts integrated over given bands in galactic latitude from USNO-B1.0 catalog. Star counts are cummulative, i.e., guide star sky coverage is for all stars brighter than given magnitude Guide Star Magnitude (I) Cumulative sky coverage, i.e., the chance of finding stars brighter than given magnitude, for a random target as a function of I-band magnitude using the USNO-B1.0 catalogue. keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 26

27 Laser Guide Stars SoluDon to the sky coverage problem: create your own guide star Sodium LGS excite atoms in sodium layer at aldtude of ~ 95 km. Rayleigh beacon LGS scauering from air molecules sends light back into telescope, h ~ 10 km Since the beam travels twice (up and down) through the atmosphere, Dp-Dlt cannot be corrected à LGS-AO sdll needs a natural guide star, but this one can be much fainter (~18mag) as it is only needed for Dp-Dlt sensing keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 27

28 Layer of neutral sodium atoms in mesosphere (height ~ 95 km, thickness ~10km) from smallest meteorites Resonant scauering occurs when incident laser is tuned to D2 line of Na at 589 nm. Sodium Beacons keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 28

29 WHT Rayleigh Guide Star Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 29

30 Rayleigh Beacons Due to interacdons of the electromagnedc wave from the laser beam with molecules in the atmosphere. Advantages: cheaper and easier to build higher power independent of Na layer Disadvantages: larger focus anisoplanadsm laser pulses à Dming Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 30

31 Focus AnisoplanaHsm The LGS is at finite distance H above the telescope and does not sample all turbulence and not the same column of turbulent atmosphere ( cone effect ): The contribudon to the wavefront error contribudon from focus anisoplanadsm is: 2 σ FA D = d 0 5/3 d 0 ~ λ 6/5 where depends only on wavelength and turbulence profile at the telescope site. H à very large telescopes need muldple LGSs due to this cone effect. keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 31

32 Ground Layer AO GLAO Useful if ground layer (= ground + dome + mirror seeing) is the dominant component Uses several WFS and guide stars within a large FOV (several arcmin) WFS signals are averaged à control one DM ReducDon of FWHM ~ factor of two (only!) GLAO is thus a "seeing enhancement" technique Advantage: wider fields and shorter wavelengths keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 32

33 Laser Tomography AO LTAO Uses muldpe laser beacons each laser has its WFS combined informadon is used to opdmize the correcdon by one DM onaxis. reduces the cone effect system performance similar to natural guide star AO but at much higher sky coverage. Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 33

34 MulH-Conjugate AO MCAO to overcome anisoplanadsm, the basic limitadon of single guide star AO MCAO uses muldple NGS or LGS MCAO controls several DMs each DM is conjugated to a different atmospheric layer at a different aldtude at least one DM is conjugated to the ground layer best approach to larger corrected FOV keller@strw.leidenuniv.nl Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 34

35 MCAO: Performance Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 35

36 MulH-Object AO MOAO MOAO provides correcdon not over the endre FOV of several arcmin but only in local areas within several arcmin à muldobject spectroscopy. needs (several) guide stars close to each science target. picks up the WFS light via small "arms inserted in the FOV. each science target has its DM systems work in open loop (!) Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 36

37 Extreme AO XAO XAO is similar to SCAO high Strehl on-axis and small corrected FOV however, Strehl values in excess of 90% requires many thousands of DM actuators requires minimal opdcal and alignment errors main applicadon: search for exoplanets, SPHERE on VLT Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 37

38 Student-Built ExPo AdapHve OpHcs Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 38

39 ExPo AdapHve OpHcs at WHT Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 39

40 MSc Astronomy & InstrumentaHon lectures: Astronomical Telescopes and Instruments DetecDon of Light High Contrast Imaging Astronomical Systems Design Project Management opdon to take courses at TU Del (not required) opdon for major research thesis in industry Astronomical Observing Techniques 2017, Lecture 13: Adaptive Optics 40