Wavefront Sensing in Astronomy

Transcription

1 Wavefront Sensing in Astronomy by INAF Arcetri Observatory (Florence - Italy) ragazzoni@arcetri.astro.it

2 Why WaveFront Sensing in Astronomy? Because most of visible and Near IR Astronomy is still made on the ground Because the atmosphere is blurring images Because Adaptive Optics become a mature technique from nearly two decades Pupil size: 4..8m in diameter Temporal scale: 1..10mSec.

3 Which are the most four popular WaveFront Sensors? - Shack-Hartmann - Hartmann- Shack - I do not remember the name but there is a lenslet array in the pupil plane and a detector in the focal plane - Oh, have no idea how it works but I know there is a funny grid of dots in some screen at a certain point. Yes also in Astronomy there has been (and partially still there is) a SHS (Shack-Hartmann Syndrome)

4 Remember We usually struggle with a very limited amount of photons (tens ) We need something working fast, in real time Turbulence of the atmosphere is (rather) well characterized

5 Big telescopes (8..10m class) in top-sites (Hawaii, Canary, Arizona desert, Chilean desert) Today an 8m class telescope cost around MEuros and night operations costs of the order of ~1Euro/sec.

6 An instrument for an 8m-class Telescope is designed, built, and Commissioned from a group of Engineers/astronomers It costs in the range 3..10MEuros and requires a group of people for ~2..4 years

7 Instruments are Large (2..5m), Weighty ( Kg) but several components can be found by specialized companies

8 It is generally a complex opto-mechanic system and the issues are always the same: Flexures Software Funds

9 Usually DMs are BIG Also because of Lagrange invariance (A-Omega or etendue ) There are interesting exceptions

10 Open vs. closed loop..!

11 Center of our Galaxy (la via lactea, or milky way) Our Sun

12 WafeFront Sensors Zoo (I) Shack-Hartmann (focal plane WFS) Curvature (pupil plane WFS) Pyramid (pupil plane WFS) Smartt (pupil plane WFS) Dumbb (pupil plane WFS) Modulating grating (pupil plane WFS) Z-invariant (pupil plane WFS) A lot (really) more

13 WaveFront Sensors Zoo (II) Focal Plane vs. Pupil Plane Can provide: The phase directly (Smartt, but for small perturbations) The first or second derivatives along xy The first or second derivatives along radial or tangential directions The Zernike polynomials directly (Yess!!! See next talk!!!)

14 Which are the current borderlines in WFS in Astronomy? extreme Adaptive Optics (XAO) for very high contrast imaging (a planet around nearby star) Very large number of sensed modes for Extremely Large Telescopes (ELTs), up to one million sensed modes Tomographic, wide field, wavefront sensing for Multi Conjugated Adaptive Optics

15 Which are the current borderlines in WFS in Astronomy? extreme Adaptive Optics (XAO) for very high contrast imaging (a planet around nearby star) I think this is not of direct interest Very large number of sensed modes for Extremely Large Telescopes (ELTs), up to one million sensed modes Techniques to make calibration faster?? Tomographic, wide field, wavefront sensing for Multi Conjugated Adaptive Optics I will focus hereafter on this

16 Conventional WFSensing

17 Conventional WFSensing

18 Conventional WFSensing

19 Conventional WFSensing??? Uncertainty in the position Along the optical axis of the defect

20 Tomographic or wide field WFSensing Unambiguos detection of where the Defect is located along the optical axis

21 It is a phase-contrast Microscope, after all! Canary, fall 2001

22 PWFS SH

23 Pupil vs focal plane WFSensors Extended sources are actually seen in the focal plane so they can be handled better with the proper software Pupil plane allow for direct optical tomography Most pupil planes WFSensors are more sensitive Sampling can be changed easily and in-flight (no need to change lenslet array, just different zooming or rebinning of detector) Calibration is generally easier

24 Reality is more complex

25 Turbulent layers #2 #1 Telescope Atmosfera UP Multi Conjugated Adaptive Optics: MCAO WFS DM1 DM2

26 In the Star-Oriented concept, each star is associated to a WFS.

27 In the Layer-Oriented concept, WFSs are coupled to a given altitude layer.

28

29 Focusing mechanisms CCD conjugated to the ground layer CCD conjugated to the high altitude layer Beam splitter Pupil re-imaging optics XY stages for Stars positioners Star enlargers Entrance focal plane

30 In the case of Artificial Stars sources are NOT localized along z They are generated at the frequency of excitation of the Sodium doublet and sent along the same line of sight of the telescope, usually from an auxiliary smaller telescope

31 Z-invariant WaveFront Sensing MIBB naturally fit with Z-invariant WFS, conceived for Rayleigh LGSs on 8m class telescopes First prototype of a reflective rod in MPIA lab

32 The hard life of an Astronomer While ophtalmologists convey in Dubai or in San Paulo del Brasil The French-Italian base at 3000m height on the Anctartic plateau

33 Why not substituting the set of pyramids with a continuous sinusoidal transmission filter?

34 This light depends upon the local tilt on the subaperture (averaged in the layer where the detector is conjugated) Sum of half of the whole starlight

35 A simulated example

36 and the light modulation outcome! (~50% of modulation)

37 A concept for a 3D-scanning WFSensor? Array of spots source

38 A concept for a 3D-scanning WFSensor? Array of spots source

39 A concept for a 3D-scanning WFSensor? Array of spots source

40 A concept for a 3D-scanning WFSensor? Ehi!!! This looks like a SH spots array!!! Array of spots source

41 Alternatives After a frustrating night and before discovering the control system of the Deformable Mirror was broken in a subtle way. Thanks are due to Pablo Artal, all the Congress Organization to have organized this wonderful Meeting and, of course, to the belly dancers. but you need to play very fast!