Performance of Adaptive Optics Systems

Transcription

1 Performance of Adaptive Optics Systems Don Gavel UCSC Center for Adaptive Optics Summer School August, 2008

2 Outline Performance Measures The construction of error budgets AO error contributors AO system simulation Gathering performance data on real AO systems Performance results: Lick AO

3 Performance measures Wavefront error E ~ x, t 2 2 Strehl Ratio S max PSF PSF 0 0, ~ 0

4 Lick 3m Telescope Keck 10m Telescope Strehl ratio

5 Strehl Ratio The Strehl is related to the wavefront variance through Marechal s approximation S PSF PSF exp 0, 0 0, 0 2 ~ ~ 0 1 N P 0 x e 1 2D x 2 d x Valid approximation for small ~ Extended region of validity for AO-corrected wavefronts Exp - 2 ~ 80 dof 300 dof ~ 2 2 (radians)

6 Resolution The Rayleigh criterion: in a diffraction-limited optical system, two point sources are separately distinguishable at a separation ~l/d In AO systems with a Strehl >~0.15, the FWHM of the corrected image is ~l/d F. Rodier introduced the concept of Strehl-resolution = width you have to enclose to get the same energy as in the FHWM of the ideal PSF

7 Contrast Ratio Image contrast Contrast = ratio of halo to core surface brightness Integration time required to detect a faint object in the halo is proportional to (contrast) -2 Keck AO example at l=2m Distance from the primary star, arcseconds

8 Energy in a spectrograph slit D.L. unc The SNR-optimal slit-width transitions to l/d when the Strehl gets > 0.1

9 Additional measures Field performance PSF stability

10 Field Performance of Multi-conjugate AO DM at 0 km DMs at 0,10 km DMs at 0,5,10 km 7 layer model atmosphere with r 0 = 15.6 cm and 0 = 3.1 arcsec Optimize on disk

11 AO system error contributors Fitting error (DM) Control error (sample rate) Measurement error (Hartmann sensor) Isoplanatic error (field angle) Calibration error Laser guide-star specfic errors: cone effect, guide-star elongation To some approximation, we can add these terms in quadrature ~ DM BW SNR iso cal cone

12 DM fitting error d The DM corrects the wavefront up to a spatial frequency of 1/(actuator spacing) 2 2 DM DM P S k F kd d k Example spatial filtering function Kolmogorov turbulence S k r 0 k F( kd) 2 md r DM kd

13 F kd z DM x ( ) DM fitting error Influence Function Spatial Frequency Response x / d kd

14 The fitting error coefficient, m, depends on the type of deformable mirror Segmented mirror d Square segment, m=0.174 d Hexagonal segment, m=0.116 Continuous face sheet DM: m=0.3 Segmented mirrors requre 3 (piston, tip, tilt) actuators per segment Rewriting the fitting error in terms of number of actuators, N a shows its more economical to use a continuous mirror: 2 m DM Na a D N r m Na square hex contin

15 Control bandwidth error The control loop corrects the wavefront up to a temporal frequency of f f 10 2 BW S f F f fc df c s Example temporal filtering function / 3 S f 2 6 r0 v f F f f c f 1 f f c 2 f c 2 f f c 2 BW f g f c 5 3 Greenwood frequency - depends on wind velocity, r0, etc., but simply defined here as the control frequency where the bandwidth term=1 radian 2

16 Wavefront measurement error Reconstructor noise propagator I( x, y ) dxd y 2 1 SNR x d l SNR 2 I( 0, ) d Control loop averaging factor y y Spot-size factor (units: angle on the sky) SNR SNR SNR x y

17 Isoplanatic error If the guide star is not the science object... Light from science object Light from guide star h Turbulent layer Isoplanatic angle: 0 0 r h 2 iso 0 5 3

18 Anisoplanatic error can be controlled by MCAO DM at 0 km DMs at 0,10 km DMs at 0,5,10 km Residual error is the generalized anisoplanatism = (/ m ) 5/3 (Tokovinin&LeLouarn, 2000)

19 Laser guidestar specific errors Cone effect Laser Guidestar at finite altitude Z h d 2 Z h r 0 0 e.g. h=4 km, r 0 =10cm > d 0 =4.5m 2 cone d d 0 5 3

20 Encircled energy The laser guide star has a larger apparent size than a natural star The wavefront measurement error is increased accordingly Lick laser data, from Nov Laser Star StarStar StarStar Laser LGS Spot size (arcsec) DL (d=25cm) 0.4 star (r 0 =11cm) 0.94 LGS 2.16 Radius, arcsec

21 Laser guide star Natural star

22 Optimizing the error budget In the design, select d (subaperture size =~ DM actuator spacing) to trade between DM fitting term and measurement term. This will set the NGS limiting magnitude, or sky coverage. It will also set the optimized wavelength of the AO system: l:r 0 (l)=d. For a laser guide star system, trade measurement error for laser power. Select the optimum d for the predicted LGS brightness. Brighter lasers (and more actuators) get to shorter wavelengths. On-line tuning: Select a frame rate that will best trade off measurement and bandwidth terms Select a natural guide star to trade off brightness (measuement error) for field angle (isoplanatic error)

23 Subaperture size, d Optimizing the error budget Rms wavefront error, nm Simultaneous Solution increasing brightness Guide star magnitude, m v Contoller bandwidth, f c Subaperture size, d

24 Simulating an AO system Heirarchy of modeling Scaling laws Analytic models (usually working in transform space) Monte-carlo wave-optic simulation Tools: Kolmogorov screen generator Wavefront propagation code DM model, WFS model Imaging model

25 Monte-carlo Simulation of an AO system Generate a guide star Near-field propagation Generate a phase screen, add to wavefront s phase wind Continue to propagate Generate another phase screen, add to wavefront s phase... Multiply by the aperture function Apply the DM actuator response model Subtract the DM s phase Run through the WFS model Image residual wavefront Run through the controller model

26 Gathering performance data on a real AO system Telemetry: Wavefront sensor data (slopes, intensities) > controller s rejection curve, bandwidth error term, measurement error term DM actuator commands > simutaneous r 0 Image data: Open loop > r 0 Closed loop > Strehl

27 Error Budget Summary Key Terms in an Astronomical AO Error Budget

28 r0, cm # of occurances # of occurances # of occurances r0 (Fried seeing parameter) Histogram median r0 = 10 cm r0, cm Seasonal Variation of Seeing ( ) Lick seeing statistics Histogram of Wind Speeds Wind Speed, m/sec Greenwood Frequency Histogram Month fg, Hz D. Gavel, E. Gates, C. Max, S. Olivier, B. Bauman, D. Pennington, B. Macintosh, J. Patience, C. Brown, P. Danforth, R. Hurd, S. Severson, J. Lloyd, Recent Science and Engineering Results with the Laser Guidestar Adaptive Optics System at Lick Observatory, Proc SPIE, 4839, pp (2003).

29 Strehl Strehl Strehl Strehl Lick AO System: performance statistics LGS Performance Performance vs Seeing Performance vs Greenwood Frequency BrG Ks Nov01 Ks BrG Oct00 Ks-dim Ks Oct00 H BrG Ks Ks-dim H r0, r0cm fg, Hz Performance vs Guide Star Brightness Brightness, ph/subap/ms BrG Ks H

30 # of occurances # of occurances Lick AO System: performance statistics Strehl Histogram Ks Filter Strehl Histogram BrG Filter Ks-Dim Ks BrG Strehl Strehl

31 Lick AO System: On-line Performance Analysis The spreadsheet errorbudget.xls can help diagnose the sources of Strel loss and aid with on-line AO system parameter adjustments Fill in the seeing and other system parameters in the green boxes and read the Strehl in the blue box Lick Error Budget r m radians@.55 nm Strehl@lambdaObs v-wind 10 m/s counts 100 photo-electrons Fitting tau s read noise 6 electrons Bandwidth fg 9 Hz spot FWHM 2 arcsec SNR mu 1 spot sigma arcsec Calibration d 0.43 m pixel size 2 arcsec Aniso fs 100 Hz crosstalk 0.2 arcsec Strehl fc 10 Hz centroider quad FWHM open loop arcsec at lambdaobs lambda 550 nm SNR note: need to load math package Observed 0.35 lambdaobs 2200 nm theta 0 arcsec Unaccounted calibration 0.85 Strehl (BrG Quad Cell SNR theta0 6 sec Other on-line metrics at the operator interface, based on AO system telemetry data analysis: Seeing r 0 Wind velocity Temporal power spectrum of turbulence Control loop rejection curves wind clearing time scale k -8/3 spectrum noise floor

32 Subaperture intensities Raw Hartmann images Control params Frame rate Control matrix Hartmann slopes Lick AO Telemetry Data Analysis Pipeline Average over illuminated subaps Verify proper background subtraction & photometry Measure Hartmann spot size of internal source Generate phase spectra Determine sensor noise Frame rate Determine guidestar intensity Compute the compensator function Generate controller rejection curve Electronic loop gain Fit effective loop gain Account for sensor noise in phase spectra Determine SNR Derive Hartmann spot size Compute noise averaging factor Determine wavefront measurement error Calculate integrated temporal power rejection SNR BW Actuator voltages Open-loop images Generate tilt spectra Pre-calibrate rms actuator voltage to micron ratio Calculate rms phase correction by DM Account for tilt in phase spectra Determine r0 from rms phase correction Calculate Greenwood frequency Calculate fitting error Compare to Greenwood model Actuator spacing DM

33 Modeling the effect of noise in closed loop HK = I

34 Correcting the closed loop residual phase spectrum for the effects of noise e n e f H cl f f DM Closed-loop transfer function: low-pass Correction transfer function: high-pass Hol f Hcor f 1 Hcl f 1 H f ol 1 1 H ol f e H f H n cor cl e H f n cor f n e e cor cl S S H H 2 2 e cor f cl n S H S H S 2 2 e cl f cl n S H S H S

35 ============================================= Lick 3m error budget /duck5/lickdata/sep00/lgs6data/sep08/cent_07 Saturday 09/09/00 23:03:44 PDT Fitting Error (sigmadm) nm d = cm r0hv = cm Servo Error (sigma_bw) nm fc = Hz fghv = Hz fs = 500 Hz Measurement Error (sigma2phase) nm SNR = control loop averaging factor = spotsizefactor = arcsec TOTAL: nm =============================================

36 ============================================= Lick 3m error budget /duck5/lickdata/may00/lgs6/may21/cent_03 5/22/00, 5:09 UT Fitting Error (sigmadm) nm d = cm r0hv = cm Servo Error (sigma_bw) nm fc = Hz fghv = Hz fs = Hz Measurement Error (sigma2phase) nm SNR = control loop averaging factor = spotsizefactor = arcsec TOTAL: nm =============================================

37 Measuring AO Performance Julian C. Christou and Donald Gavel UCO/Lick Observatory CfAO 2006

38 S h p r r pk pk h p 0 0 where 0 S 1

39 Other Approaches besides Strehl Ratio Image Sharpness (originally described by Muller and Buffington, 1974) S 1 - Size of PSF S 1 h 2 h 2 i i S 3 - Normalised peak value directly related to Strehl Ratio S 3 h pk h i Advantage independent of knowing peak location and value. - Can be applied to extended sources. Disadvantage The numerator is contaminated by an additive noise term n 2. SR h pk h i p Disadvantage sensitive to measurement of peak location and value. Advantage No noise bias pk p i S S 3 3 ( h) ( p)

40 1. Palomar pupil geometry: primary mirror diameter of 4.88m and a central obscuration of 1.8m. No secondary supports modelled. 2. H-band (1.65 microns) with different levels of AO correction. Ideal PSF

41 Sharpness criteria compared with residual wavefront error from the simulations. S 1 has a steeper slope for smaller rms phases. S nm -1 S nm -1 (nm)

42 Relationship between S 1, S 3 and the Strehl Ratio. S 1 and S 3 values generated from noise-free simulations as part of the CfAO Strehl study. Both S 1 and S 3 are normalised to those of the ideal PSF. The effect of constant noise is shown on S 1.

43 Ideal PSF Variation in NGS PSF quality from the Lick AO system (all at 2 microns)

44 Sharpness (normalised S 1 ) compared with Strehl ratio for NGS Lick AO data. Data obtained with different SNR, observing conditions, nights. Dashed line obtained hueristically from the noiseless simulations.. Departure from simulations could be due to either overestimating S 1 (e.g. presence of noise) or underestimating Strehl ratio (not accurately locating the peak). Further analysis on noisy simulations needed. Accuracy of system performance measurements can be obtained from SR and S 1.

45 Science Targets - Basic Astronomy; stellar classification; stellar motion orbits AO Performance - Isoplanatic Issues on-axis vs. off-axis performance - Isoplanatic angle - o Analysis Performance - Measurement of Photometry and Astrometry Lick Observatory Data - NGS - 0.5" Separations 12"

46 Lick NGS Data CrB m Cas Cas 7" 1" 0.5"-7" Del WDS Oph 9" 12" 5"

47 Binary stars permit direct measurement of anisoplanatism by comparing the PSFs. An effective measure of anisoplanatism is the fall off of the Strehl ratio of the off-axis source compared to the on-axis source. SR SR off -axis on-axis exp o where is the binary separation

48 Del (sep = 9.22 arcseconds) ratio = 0.76 ± 0.04 o = 20.1" ± 2.1"

49 70 Oph (sep = 4.79 arcseconds) ratio = 0.84 ± 0.04 o = 14.3" ± 2.5"

50 Summary of Binary Strehl Ratio Measurements Strehl ratio changes vary similarly for both components. Strehl ratio is quite variable for a set of observations ( seconds - minutes) up to changes of 20%. Differential Strehl ratio also varies relative position on the detector? Isoplanatic angle (as determined from differential Strehl ratio) also varies with 15" o 30" with some results implying minutes!

51 Analysis Techniques - Iterative Blind (myopic) deconvolution (Christou-CfAO) - Parametric Blind Deconvolution (PSF Modelling) (Drummond- AFRL) Astrometry and Photometry (on following pages)

52 Summary of Astrometry and Photometry Astrometry between the two techniques shows good agreement ( 0.001") Differential Photometry is in general good agreement ( 0.02 mag) with a few exceptions. - CrB (J = 0.5) - m Cas (J = 0.4; Br = 0.2) - Cas Aa (J = 0.2; K s = 0.2) - Cas Ac (H = 0.15) Christou, J.C., Drummond, J.D., Measurements of Binary Stars, Including Two New Discoveries, with the Lick Observatory Adaptive Optics System, The Astronomical Journal, Volume 131, Issue 6, pp

53 Astronomical AO System Data Analysis Julian Christou (UCSC) Szymon Gladysz (NUI) Gladysz, S., Christou, J., Redfern, M., Characterization of the Lick adaptive optics point spread function, SPIE Proc., 6272, June, 2006

54 High Speed PSF Measurements Data sets obtained at Lick almost monthly between July 2005 and Feb IRCAL fastsub mode ( freeze images) - t exp = 22ms and 57ms - Duty cycle ~ t exp + 30ms field size of arcseconds (64 64 pixels) Target objects: m v ~ 6-8 Typically 10 sets of data each of 1000 frames total frames

55 Long Term PSF Stability Ideal PSF Fiber 1 (Sep-2005) Fiber 2 (Oct-2005) 12-Oct-2005 Reference Change 18-Aug Aug Jul Jul Aug Aug Aug-2004

56 Lick AO Fiber Source 17 Sep Oct Nov % 80% 78% Stable structure in atmospheric-free PSF Strehl Ratios typically 75% -- 82%

57 PSF Structure Fiber Source no better than ~ 80% Strehl ratio. What s the best we can do %? Strong high-order Residual Aberration limiting performance. Relatively stable over minutes hours days months years! No significant change with change of DM references Where is this from? DM flatness Unsensed aberrations in main path Non-common path errors Incorrect SH References Obtain Wavefront map from Phase Retrieval/Diversity measurements. Typically the image is sharpened on the sky Relative peak value metric - other metrics e.g. S 1 First 10 Zernike terms and increasing to 20. Use mirror modes? Important to understand for PSF Reconstruction algorithms. We can deal with the atmosphere but can we deal with the system?

58 Lick NGS Strehl Stability (10000 frames 22-57ms/frame) Christou (UCSC), Gladysz (NUI)

59 Strehl Ratio Distributions Distribution of Strehl ratios (for relative stable performance) all show a similar non-gaussian behaviour. Similar distributions seen in data from Palomar, Keck and AEOS

60 PDF Models x = S x = 100 / (100 - S) x = ln(100 - S)

61 PDF Models Implication is that the instantaneous Strehl ratio has an underlying Gaussian distribution: of r 0! Using Hudgin and Marachel approximations produces a distribution of Strehl ratios similar to that measured, i.e. skewed to a low Strehl ratio tail. Need to obtain simultaneous r 0 and S measurements. Speckle noise dominating.

62 PSF Calibration and Quantitative Analysis The complicated nature of the AO PSF makes quantitative analysis problematic. How well does deconvolution preserve astrometry and photometry? i Cas

63 Separation of the components of CrB Sub-pixel peaks located by Fourier interpolation o Six separate measurements of a binary star on different days on different positions on the IRCAL detector. o Separation depends upon location on detector o Precision for each location ~ 2 mas (= 0.03 pixels = 1.5% l/d) o Separation dispersion ~ 50 mas AMOS

64 Julian C. Christou, Austin Roorda, and David R. Williams, Deconvolution of adaptive optics retinal images, J. Opt. Soc. Am. A 21, (2004)

65

66

67

68

69

70

71

72 Deconvolution of final images, using data from the wavefront sensing Image g x hx x f xdx nx PSF Object Noise = + Fourier Transform: G f Hf Ff Nf Then (in the Fourier domain): H * G multi frame H Then solve for object F 2 F H * N 0 multi frame

73

74 F f F f f

75 Summary conclusion AO Performance Measurement AO performance-hitters (intro to error budget) AO modeling and simulation AO performance metrics Sharpness and anisoplanatism measures from the AO corrected science image Spectral analysis of telemetry from the AO system (wavefront sensor and deformable mirror signals) Astronomical AO data analysis Vision science AO data analysis CfAO Summer School, 2008