Lecture 16 The applications of tomography: MCAO, MOAO, GLAO

1. Lecture 16The applications of tomography:MCAO, MOAO, GLAO Claire Max AY 289C UC Santa Cruz March 6, 2008

2. Next week (the last week of class!) • Tuesday March 11th lecture: • AO for imaging the living human retina. • Jason Porter, Univ. of Houston • Thursday March 13th lecture: • Extreme AO for imaging planets around nearby stars • Bruce Macintosh, Lawrence Livermore National Lab • Final exam: take-home, open-book • Distributed at lecture Thursday March 13th • Due in my office (or my computer) on or before Thurs March 20th at noon. This is a hard deadline.

3. The final homework assignment • Will be on web by end of this week • Due Thurs March 13 • Will be intended to help you review the whole course and get ready for the exam • What approaches have you found useful in reviewing classes like this? What makes the material “stick” ?

4. Outline of lecture • Review of AO tomography concepts • AO applications of tomography • Multi-conjugate adaptive optics (MCAO) • Multi-object adaptive optics (MOAO) • Ground-layer AO (GLAO)

5. What is Tomography ?1. Cone effect 90 km “Missing” Data Credit: Rigaut, MCAO for Dummies

6. What is Tomography ?2. Wider field of view, no cone effect Tomography lets you reconstruct turbulence in the entire cylinder of air above the telescope mirror 90 km Credit: Rigaut, MCAO for Dummies

7. Ragazzoni’s Tomography Cartoon Credit: Ragazzoni, Nature 403, 2000

8. Concept of a metapupil • Can be made larger than “real” telescope pupil • Increased field of view due to overlap of fields toward multiple guide stars

9. How tomography works: from Don Gavel kZ kX Fourier slice theorem in tomography (Kak, Computer Aided Tomography, 1988) • Each wavefront sensor measures the integral of index variation along the ray lines • The line integral along z determines the kz=0 Fourier spatial frequency component • Projections at several angles sample the kx,ky,kz volume 9

10. How tomography works: from Don Gavel kZ kX • The larger the telescope’s primary mirror, the wider the range of angles accessible for measurement • In Fourier space, this means that the “bow-tie” becomes wider • More information about the full volume of turbulence above the telescope 10

11. How tomography works: some math Assume we measure y with our wavefront sensors Want to solve for x= value of dOPD) The equations are underdetermined – there are more unknown voxel values than measured phases Þ blind modes. Need a few natural guide stars to determine these. • where • y= vector of all WFS measurements • x= value of dOPD) at each voxel in turbulent volume above telescope x y • Ais a forward propagator(entries = 0 or 1)

12. Solve for the full turbulence above the telescope using the back-propagator x • y= vector of all WFS measurements • x= value of dOPD) at each voxel in turbulent volume above telescope y • ATis a back propagatoralong rays back toward the guidestars x • Use iterative algorithms to converge on the solution. y

13. LGS Related Problems: Null modes • Tilt Anisoplanatism : Low order modes (e.g. focus) > Tip-Tilt at altitude  Dynamic Plate Scale changes • Five “Null Modes” are not seen by LGS (Tilt indetermination problem)  Need 3 well spread NGSs to control these modes Credit: Rigaut, MCAO for Dummies

14. Outline of lecture • Review of AO tomography concepts • AO applications of tomography • Multi-conjugate adaptive optics (MCAO) • Multi-object adaptive optics (MOAO) • Ground-layer AO (GLAO)

15. What is multiconjugate AO? Turbulence Layers Deformable mirror Credit: Rigaut, MCAO for Dummies

16. What is multiconjugate AO? Deformable mirrors Turbulence Layers Credit: Rigaut, MCAO for Dummies

17. The multi-conjugate AO concept Turb. Layers Telescope WFS #2 #1 DM2 DM1 Atmosphere UP Credit: Rigaut, MCAO for Dummies

18. “Star Oriented” MCAO Guide Stars High Altitude Layer Ground Layer Telescope DM2 DM1 WFC WFSs • Each WFS looks at one star • Global Reconstruction • n GS, n WFS, m DMs • 1 Real Time Controller • The correction applied at each DM is computed using all the input data. Credit: N. Devaney

19. Layer Oriented MCAO Guide Stars High Alt. Layer Ground Layer Telescope DM2 DM1 WFC1 WFC2 WFS1 WFS2 • Layer Oriented WFS architecture • Local Reconstruction • x GS, n WFS, n DMs • n RTCs • Wavefront is reconstructed at each altitude independently. • Each WFS is optically coupled to all the others. • GS light co-added for better SNR. Credit: N. Devaney

20. MCAO Performance PredictionsNGS, Mauna Kea Atmospheric Profile Classical AO MCAO No AO 2 DMs / 5 NGS 1 DM / 1 NGS 165’’ 320 stars / K band / 0.7’’ seeing Stars magnified for clarity Credit: Rigaut, MCAO for Dummies

21. MCAO Simulations, 3 laser guide stars Strehl at 2.2m 3 NGS, FoV = 1 arc min Strehl at 2.2m 3 NGS, FoV = 1.5 arc min Average Strehl drops, variation over FoV increases as FoV is increased Credit: N. Devaney

22. Results from ESO’s Multiconjugate AO Demonstrator (MAD) Single Conjugate Multi Conjugate

23. Gemini South MCAO Science Path NGS WFS Path LGS WFS Path DM9 DM0 ADC SCIBS OAP2 OAP1 WFSBS TTM DM4.5 LGS WFS Zoom Focus Lens OAP3 ADC WFS Credit: Eric James & Brent Ellerbroek, Gemini Observatory

24. Outline of lecture • Review of AO tomography concepts • AO applications of tomography • Multi-conjugate adaptive optics (MCAO) • Multi-object adaptive optics (MOAO) • Ground-layer AO (GLAO)

25. Distinctions between multi-conjugate and multi-object AO ? 1-2 arc min • DMs conjugate to different altitudes in the atmosphere • Guide star light is corrected by DMs before its wavefront is measured • Only one DM per object, conjugate to ground • Guide star light doesn’t bounce off small MEMS DMs in multi-object spectrograph

26. Science with MOAO: multiple deployable spatially resolved spectrographs • A MEMS DM underneath each high-redshift galaxy, feeding a narrow-field spatially resolved spectrograph (IFU) • No need to do AO correction on the blank spaces between the galaxies

27. Why does MOAO work if there is only one deformable mirror in the science path? • Tomography lets you measure the turbulence throughout the volume above the telescope 90 km

28. Why does MOAO work if there is only one deformable mirror in the science path? • Tomography lets you measure the turbulence throughout the volume above the telescope • In the direction to each galaxy, you can then project out the turbulence you need to cancel out for that galaxy 90 km

29. Outline of lecture • Review of AO tomography concepts • AO applications of tomography • Multi-conjugate adaptive optics (MCAO) • Multi-object adaptive optics (MOAO) • Ground-layer AO (GLAO)

30. Ground layer AO: do tomography, but only use 1 DM (conjugate to ground) MCAO single DM conjugated to ground layer GLAO GLAO uses 1 ground-conjugated DM, corrects near-ground turbulence Credit: J-M Conan

31. Correcting just the ground layer gives a very large isoplanatic angle • Strehl = 0.38 at  = 0 0is isoplanatic angle 0is weighted by high-altitude turbulence (z5/3) • If turbulence is only at low altitude, overlap is very high. • If you only correct the low altitude turbulence, the isoplanatic angle will be large (but the correction will be only modest) Common Path Telescope

32. Ground Layer AO (GLAO) typically decreases natural “seeing” by a factor of 1.5 to 2 • Example: GLAO calculation for Giant Magellan Telescope (M. Johns) • Adaptive secondary conjugation at 160 m above primary mirror. • Performance goals: • > 0.8 mm • Field of view: >10’ • Factor of 1.5-2 reduction in image size. Modeled using Cerro Pachon turbulence profile. (M-L Hart 2003)

33. Many observatories have ambitious GLAO projects planned • Near term on medium sized telescopes: SOAR (4.25m), William Herschel Telescope (4.2m), MMT (6.5m) • Medium term on VLT (8m), LBT (2x8m) • Longer term on Giant Magellan Telescope etc. • Is it worth the large investment “just” to decrease “seeing” disk by factor of 1.5 to 2 ? • Depends on whether existing or planned large spectrographs can take advantage of smaller image • Potential improved SNR for background-limited point sources

34. time Credit: A. Tokovinin

35. Credit: A. Tokovinin

36. Summary • Tomography: a way to measure the full volume of turbulence above the telescope • Once you have measured the turbulence and know its height distribution, there are several ways to do the wavefront correction to get wider field of view • Multi-conjugate AO: multiple DMs, each optically conjugate to a different layer in the atmosphere. • Multi-object AO: correct many individual objects, each over a small field. • Ground-layer AO: correct just the turbulence close to the ground. Gives very large field of view but only modest correction. Should work in both the visible and the IR.