Lecture 9: Speckle Interferometry. Full-Aperture Interferometry. Labeyrie Technique. Knox-Thompson Technique. Bispectrum Technique

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1 Lecture 9: Speckle Interferometry Outline 1 Full-Aperture Interferometry 2 Labeyrie Technique 3 Knox-Thompson Technique 4 Bispectrum Technique 5 Differential Speckle Imaging 6 Phase-Diverse Speckle Imaging Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 1

2 Full-Aperture Interferometry Diffraction Limit theoretical angular resolution of telescope proportional to λ/d λ wavelength D diameter of telescope Diameter Wavelength Diffraction Limit 10 cm 500 nm cm 500 nm cm 5000 nm cm 500 nm cm 500 nm Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 2

3 Index of Refraction of Air index at sea level: wavelength dependence of index of refraction about /λ 2 air is largely achromatic 1 K temperature difference changes n by temperature of atmosphere is not uniform variation of 0.01 K along path of 10 km: 10 4 m 10 8 = 10 4 m = 100 waves at 1µm refractive index of water vapour is less than that of air moist air has smaller refractive index Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 3

4 Seeing light from astronomical source travels in straight line through space Nonuniform refractive index fluctuation - masses of warm or cold air refract light differently Different parts of wavefront interfere with each other in image plane on the ground, it looks like the astronomical object is at several places in the sky at the same time temperature fluctuations change about a hundred times per second blurred image when telescope could provide much better angular resolution Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 4

5 Seeing resolution limited by Earth s atmosphere to 0. 5 independent of D atmosphere is turbulent medium with small-scale temperature fluctuations Speckle-Interferometry: suitable observational method and post-facto reconstruction teqchnique to eliminate the angular smearing due to seeing Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 5

6 Interference telescope with focal length f working at wavelength λ combines light that passed through different parts of atmosphere describe seeing as many small telescopes that are differently affected by the atmosphere quality of seeing described by Fried s Parameter r 0, diameter of telescopes that would be diffraction-limited under current seeing conditions typical values in the visible r 0 : 5 30 cm Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 6

7 Speckles image of point source is cloud of small dots: speckles speckle diameter λ/d speckle cloud diameter λ/r 0 speckle life time in visible about 10 ms, longer at larger wavelengths Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 7

8 Seeing for Single and Binary Stars Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 8

9 Seeing and Solar Granulation Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 9

10 Mathematical Description of Seeing image of a point source: Point Spread Function (PSF) image formation due to time-varying PSF i (t) = o s (t) i observed image o true object, constant in time s point spread function convolution Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 10

11 Fourier Domain after Fourier transformation: I (t) = O S (t) I observed image O true object, constant in time S optical transfer function describing seeing and instrument isoplanatic patch: constant PSF over an area of 3 10 extended objects as sum of point sources convolved with PSF Fourier frequency f object in Fourier space has amplitude, phase: O(f ) = O(f ) e Ψ(f ) Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 11

12 Labeyrie Technique Basic Idea developed in 1970 by Antoine Labeyrie exposure time shorter than time constant of seeing ( 20 ms in the visible) clever average of many images that is free from atmospheric influences assumption: PSF constant during exposure time image of binary star consists of two identical, overlapping speckle clounds Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 12

13 Average Autocorrelation Autocorrelation can detect identical, but shifted speckle clouds Labeyrie technique: average autocorrelation contains diffraction-limited information autocorrelation in image space corresponds to power-spectrum in Fourier Space I 2 = O 2 S 2 speckle transfer function S 2 from calibration point source Labeyrie technique allows reconstruction of Fourier amplitudes average auto-correlation is not image Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 13

14 Autocorrelation Image Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 14

15 Knox-Thompson Technique Reconstructing the Fourier Phases Knox and Thompson (1973) phases crucial for extended objects (star clusters, galaxies, Sun) phases Ψ (O) with Knox and Thompson approach: I( f )I ( f δf ) = O( f )O ( f δf ) S( f )S ( f δf ) ( ( ) ( )) ( ( )) ( ( )) Ψ O f O f δf = Ψ O f + Ψ O f δf Ψ f phase of S( f )S ( f δf ) is zero integrate phase differences in two directions to recover object phases iterative approach minimizes sum of squares of phase differences Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 15

16 Average and Best Frame of Image Series Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 16

17 Average and Knox-Thompson Reconstruction Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 17

18 Comparison of Techniques Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 18

19 Bispectrum Technique Weigelt 1977 product of three Fourier components I( f 1 )I( f 2 )I ( f 1 + f 2 ) phase of average atmospheric factor is again zero ( ) ( ) ( phase of bispectrum: Ψ f1 + Ψ f2 Ψ f1 + ) f 2 many more correlations between Fourier components bispectrum is 4-dimensional large computational effort today most-often used speckle technique Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 19

20 Dutch Open Telescope Speckle Movie DOT Blue Continuum Movie of AR10425 Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 20

21 Differential Speckle Imaging The Idea narrow-band filter images do not have enough signal to do direct speckle imaging measure PSF in broad-band channel (b) to deconvolve simultaneous exposures in narrow-band channel (n) I n = O n S, I b = O b S approximation of O n : O n = In = S In I b O b. avoid division by 0 via: O n (In /I b ) I b 2 = Ib 2 O In I b = b Ib 2 O b Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 21

22 Application to Solar Zeeman Polarimetry BUT, in reality I n = O n S + N n, I b = O b S + N b account for random photon noise with appropriate optimum filter account for anisoplanatism by overlapping segmentation with segment sizes on the order of the isoplanatic patch high spatial resolution at good spectral resolution can be achieved Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Data Analysis, Lecture 9: Speckle Imaging 22

23 Phase-Diverse Speckle Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 23

24 Raw Data Reconstruction Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 24

25 Adaptive Optics Real-Time Wavefront Correction Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 25

26 Evolution of Small-Scale Fields in the Quiet Sun Christoph U. Keller, Utrecht University, Astronomical Data Analysis, Lecture 9: Speckle Imaging 26