March 31, 2000 SPIE CONFERENCE 4007, MUNICH

1

Principles, Performance and Limitations of Principles, Performance and Limitations of Multi-conjugate Adaptive OpticsMulti-conjugate Adaptive Optics

F.Rigaut F.Rigaut 11, B.Ellerbroek , B.Ellerbroek 11 and R.Flicker and R.Flicker 22

11Gemini ObservatoryGemini Observatory22Lund ObservatoryLund Observatory

Talk available at www.gemini.edu (adaptive optics documents page)

MCAO

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

2

A Reality CheckA Reality Check

Acquisition Cam:Acquisition Cam:V Band, 30 sec

Seeing limited (0.85 arcsec)22’’x22’’ subimage

M13 with Gemini

Hokupa’a+Quirc:Hokupa’a+Quirc: K’ filter15mn. FWHM ~ 92 mas

K’K’limlim=24.1, 5=24.1, 5 in 1hr in 1hr

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

3

TomographyTomography

Altitude Layer (phase aberration =

++)Ground Layer = Pupil (phase aberration = OO)

WFS#1 WFS#2

Tomography = Tomography = StereoscopyStereoscopy

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

4

Multiconjugate AO Set-upMulticonjugate AO Set-up

Telescope

DM1 DM2

Turb. Layers#2 #1

Atmosphere

WFS

UP

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

5

Effectiveness of MCAOEffectiveness of MCAO

Numerical simulations:• 5 Natural guide stars• 5 Wavefront sensors• 2 mirrors• 8 turbulence layers• MK turbulence profile• Field of view ~ 1.2’ • H band

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

6

Classical AO

MCAO

No AO

165’’

MCAO Performance MCAO Performance SummarySummaryEarly NGS results, MK ProfileEarly NGS results, MK Profile

2 DMs / 5 NGS

320 stars / K band / 0.7’’ seeing

1 DM / 1 NGS

Stars magnified for clarity

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

7

Example of MCAO Example of MCAO PerformancePerformance

• 13x13 actuators system• K Band• 5 LGSs in X of 1 arcmin

on a side• Cerro Pachon turbulence

profile• 200 PDE/sub/ms for

H.Order WFS• Four R=18 TT GS 30” off

axis (MCAO)• One R=18 TT GS on

axis(AO)

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

8

• Robustness

• Sensitivity to noise is fairly better than with AOProp noise AO / Prop noise MCAO sqrt( NGS )

• Predictive algorithms possible ?

Other nice features of MCAOOther nice features of MCAO

• Robustness

Avera

ge S

treh

l (

tria

ng

les)

1

.5

+ ++

++

+

Str

ehl S

t. d

ev a

cross

FoV

% (

+)

2

4

Profile number

0

• Robustness

• Sensitivity to noise is fairly better than with AO Prop noise AO / Prop noise MCAO sqrt( NGS )

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

9

Generalized Fitting Generalized Fitting (Finite number of DMs)

Highest spatial frequencies projected out of the command

c(h) = (h)-(h).h

dact

Geometry of the problem

d

Geometry of the problem

4 130

Altitude [km]

Ave

rage

Str

ehl i

n F

ov

Simulations

Model4 1

30

.h / dact G

ener

aliz

ed F

itti

ng/A

O F

itti

ng

< c(h) 2 > vs .h

Error [rd2] (.h)5/30.23(dact /r0)5/3

5/3

1.75

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

10

Generalized Fitting Generalized Fitting (Finite number of DMs)

Error [rd2] (.h)5/3

Design Criteria e.g. Error balanced hmax(,dact)

DM Spacing = 2 x hmax

dact FoV [arcmin] hmax [m] NDM/GS

0.5 1 3000 3

0.2 1 1200 5- 6

0.2 10 120 50

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

11

Generalized AnisoplanatismGeneralized Anisoplanatism(Finite number of Guide Star)

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

12

FoV = 70”FoV = 100”

Generalized Anisoplanatism Generalized Anisoplanatism (Finite number of Guide Star)

• Turbulence altitude estimation error• OK toward GS, but error in between GS: Strehl “dips”• Maximum FoV depends upon DM pitch.• Example for 7x7 system

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

13

Generalized Anisoplanatism Generalized Anisoplanatism goes down with increasing goes down with increasing aperturesapertures

2D info only

3D info2D info only

3D info

Aperture

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

14

LGS Related Problems: Null LGS Related Problems: Null modesmodes

• Within these modes, 5 “Null Modes” not seen by LGS (Tilt indetermination problem) Need 3 well spread NGSs to control these modes

• Detailed Sky Coverage calculations (null modes modal control, stellar statistics) lead to approximately 15% at GP and 80% at b=30o

• Tilt Anisoplanatism : Low order modes > Tip-Tilt at altitude Dynamic Plate Scale changes

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

16

On MCAO for ELTsOn MCAO for ELTs

• Generalized Fitting FoV 5/3 for a fixed DM configuration, irregardless of D

NGSs or LGSs ?• NGSs -> FoV of 15-20 arcmin to get S.C > 50% with 4 stars• Gen.Fitting error blows up the error budget, unless many

DMs are used• Many DMs mean many GS -> 20 arcmin not enough

-> NGS do not work for ELTs. Need LGS.

March 31, 2000 SPIE CONFERENCE 4007, MUNICH

17

Available on Available on Earth sooner Earth sooner than you think ?than you think ?