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

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

Outline of lecture

Claire Max

AY 289C

UC Santa Cruz

March 6, 2008

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.

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” ?

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)

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

Ragazzoni’s Tomography Cartoon

Credit: Ragazzoni, Nature 403, 2000

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

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

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

How tomography works: some math

Assume we measure y with our wavefront sensors

Want to solve for x= value of dOPD)

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 dOPD) at each voxel in turbulent volume above telescope

x

y

- Ais a forward propagator(entries = 0 or 1)

Solve for the full turbulence above the telescope using the back-propagator

x

- y= vector of all WFS measurements
- x= value of dOPD) 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

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

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)

The multi-conjugate AO concept

Turb. Layers

Telescope

WFS

#2

#1

DM2

DM1

Atmosphere

UP

Credit: Rigaut, MCAO for Dummies

“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

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

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

MCAO Simulations, 3 laser guide stars

Strehl at 2.2m

3 NGS, FoV = 1 arc min

Strehl at 2.2m

3 NGS, FoV = 1.5 arc min

Average Strehl drops, variation over FoV increases as FoV is increased

Credit: N. Devaney

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

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)

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

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

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

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

- Review of AO tomography concepts
- AO applications of tomography
- Multi-conjugate adaptive optics (MCAO)
- Multi-object adaptive optics (MOAO)
- Ground-layer AO (GLAO)

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

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

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)

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

time

Credit: A. Tokovinin

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.

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