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Comparative Performance of a 30m Groundbased GSMT and a 6.5m

(and 4m) NGST

NAS Committee of Astronomy & Astrophysics9th April 2001

Matt MountainGemini Observatory/AURA NIO

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Overview• Science Drivers for a GSMT• Performance Assumptions

– Backgrounds, Adaptive Optics and Detectors• Results

– Imaging and Spectroscopy• compared to a 6.5m & 4m NGST

– A special case, • high S/N, R=100,000 spectroscopy

• Conclusions

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GSMT Science Case“The Origin of Structure in the Universe”

From the Big Bang… to clusters, galaxies, stars and planets

Najita et al (2000,2001)

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Mass Tomography of the Universe

z~0.5

Existing Surveys + Sloan

z~3

Hints of Structure at z=3(small area)

100Mpc (5Ox5O), 27AB mag (L* z=9), dense samplingGSMT 1.5 yrGemini 50 yrNGST 140 yr

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Tomography of Individual Galaxies out to z ~3

• Determine the gas and mass dynamics within individual Galaxies• Local variations in starformation rate Multiple IFU spectroscopy R ~ 5,000 – 10,000

GSMT 3 hour, 3 limit at R=5,000

0.1”x0.1” IFU pixel(sub-kpc scale structures)

J H K 26.5 25.5 24.0

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Probing Planet Formation with High Resolution Infrared Spectroscopy

Planet formation studies in the infrared (5-30µm):

Planets forming at small distances (< few AU) in warm region of the disk

Spectroscopic studies:

Residual gas in cleared region emissions Rotation separates disk radii in velocity High spectral resolution high spatial resolution

8-10m telescopes with high resolution (R~100,000) spectrographs can detect the formation of Jupiter-mass planets in disks around nearby stars (d~100pc).

S/N=100, R=100,000, >4m

Gemini out to 0.2pc sample ~ 10sGSMT 1.5kpc ~100sNGST X

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30m Giant Segmented Mirror Telescope concept

Typical 'raft', 7 mirrors per raft

Special raft - 6 places, 4 mirrors per raft

1.152 m mirror across flats

Circle, 30m dia.30m F/1 primary, 2m adaptive secondary

GEMINI

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GSMT Control ConceptLGSs provide full sky coverage

Deformable M2 : First stage MCAO, wide field seeing improvement and M1 shape control

10-20’ field at 0.2-0.3” seeing

1-2’ field fed to the MCAO module

M2: rather slow, large stroke DM to compensate ground layer and telescope figure, or to use as single DM at >3 m. (~8000 actuators) Dedicated, small field (1-2’) MCAO system (~4-6DMs).

Focal plane

Active M1 (0.1 ~ 1Hz)619 segments on 91 rafts

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GSMT Implementation concept- wide field (1 of 2)

Barden et al (2001)

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GSMT Implementation concept- wide field (2 of 2)

20 arc minute MOSon a 30m GSMT

• 800 0.75” fibers• R=1,000 350nm – 650nm• R=5,000 470nm – 530nm• Detects 13% - 23% photons hitting 30m primary

1m

Barden et al (2001)

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Spot Diagrams for Spectrograph

R=1000 case with 540 l/mm grating.

R=5000 case with 2250 l/mm grating.

350 nm 440 nm 500 nm 560 nm 650 nm

470 nm 485 nm 500 nm 515 nm 530 nm

On-axis

On-axis

Circle is 85 microns equal tosize of imaged fiber.

Barden et al (2001)

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GSMT Implementation concept- MCAO/AO foci and instruments

MCAO opticsmoves with telescope

Narrow field AO ornarrow field seeing limited port

MCAO Imagerat vertical Nasmyth

elevation axis

4m

Oschmann et al (2001)

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Spot diagrams for MCAO + Imager

Diffraction limited performance for 1.2m – 2.2 m can be achieved

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MCAO Optimized Spectrometer

• Baseline design stems from current GIRMOS d-IFU tech study occurring at ATC and AAO– ~2 arcmin deployment field– 1 - 2.5 µm coverage using 6 detectors

• IFUs– 12 IFUs total ~0.3”x0.3” field– ~0.01” spatial sampling R ~ 6000 (spectroscopic OH suppression)

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Quantifying the gains of NGST compared to a groundbased telescope

• Assumptions (Gillett & Mountain 1998)• SNR = Is . t /N(t): t is restricted to 1,000s for NGST

• Assume moderate AO to calculate Is , Ibg

• N(t) = (Is . t + Ibg. t + n . Idc .t + n . Nr2)1/2

• For spectroscopy in J, H & K assume “spectroscopic OH suppression”

• When R < 5,000 SNR(R) = SNR(5000).(5000/R)1/2

and 10% of the pixels are lost

Source noise background dark-current read-noise

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Space verses the Ground

Takamiya (2001)

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Adaptive Optics enables groundbased telescopes to be competitive

For background or sky noise limited observations:

S Telescope Diameter .

N Delivered Image Diameter

Where: is the product of the system throughput and detector QE is the instantaneous background flux

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Adaptive Opticsworks well

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Modeling verses Data

20 arcsec

M15: PSF variations and stability measured as predicted

GEMINI AO Data

Mod

el R

esu l

ts

2.5 arc min.

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Quantitative AO Corrected Data

• AO performance can be well modeled• Quantitative predictions confirmed by observations

• AO is now a valuable scientific tool:

• predicted S/N gains now being realized

• measured photometric errors in crowded fields ~ 2%

Rigaut et al 2001

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•Tomographic calculations correctly estimated the measured atmospheric phase errors to an accuracy of 92%

–better than classical AO–MCAO can be made to work

Multi-Conjugate Adaptive Optics

MCAO

2.5 arc min.

Mod

el r

esul

ts

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AO Technology constraints (50m telescope)

r0(550 nm) = 10cm No. of Computer CCD pixel Actuator pitch S(550nm) S(1.65m) actuators power rate/sensor

(Gflops) (M pixel/s) 10cm 74% 97% 200,000 9 x 105 800

25cm 25% 86% 30,000 2 x 104 125 50cm 2% 61% 8,000 1,500 31 SOR (achieved) 789 ~ 2 4 x 4.5

Early 21st Century technology will keep AO confined to > 1.0mfor telescopes with D ~ 30m – 50m

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MCAO on a 30m: summary• MCAO on 30m telescopes should be used m• Field of View should be < 3.0 arcminutes,

• Assumes the telescope residual errors ~ 100 nm rms• Assumes instrument residual errors ~ 70 nm rms

– Equivalent Strehl from focal plane to detector/slit/IFU > 0.8 @ 1 micron– Instruments must have:

• very high optical quality• very low internal flexure

(m) Delivered Strehl

1.25 0.2 ~ 0.4 1.65 0.4 ~ 0.6 2.20 0.6 ~ 0.8

9 Sodium laser constellation4 tip/tilt stars (1 x 17, 3 x 20 Rmag)

PSF variations < 1% across FOV

Rigaut & Ellerbroek (2000)

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Modeled characteristics of a 30m GSMT with MCAO (AO only, m) and a 6.5m NGST

Assumed detector characteristics

m <m 5.5m <m

Id Nr qe Id Nr qe

0.01 e/s 4e 80% 10 e/s 30e 40%

Assumed encircled-energy diameter (mas) containing energy fraction

30M 1.2m 1.6m 2.2m 3.8m 5.0m 10m 17m 20m(mas) 23 29 41 34 45 90 154 181Strehl 0.40 0.56 0.73 0.85 0.91]

NGST 1.2m 1.6m 2.2m 3.8m 5.0m 10m 17m 20m (mas) 100 100 82 138 182 363 617 726

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Comparative performance of a 30m GSMT with a 6.5m NGST

1 101E-3

0.01

0.1

1

10

Comparative performance of a 30m GSTM with a 6.5m NGST

S/N

Gai

n (G

SMT

/ NG

ST)

Wavelength (microns)

R=5 R=1,000 R=10,000

Assuming a detected S/N of 10 for NGST on a point source, with 4x1000s integration

GSM

T ad

vant

age

NG

ST a

dvan

tage

R = 10,000 R = 1,000 R = 5

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Comparative performance of a 30m GSMT with a 4m NGST

1 10

0.01

0.1

1

10

Comparative performance of a 30m GSTM with a 4.0m NGST

S/N

Gai

n (G

SMT

/ NG

ST)

Wavelength (microns)

R=5 R=1,000 R=10,000 R = 10,000 R = 1,000 R = 5

Assuming a detected S/N of 10 for NGST on a point source, with 4x1000s integration

GSM

T ad

vant

age

NG

ST a

dvan

tage

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Observations with high Signal/Noise, R>30,000 is a new regime

- source flux shot noise becomes significant

10 1000.1

1

10

100

1000

10 1000.1

1

10

100

1000

GSMT 30m

Com

para

tive

nois

e co

ntrib

utio

ns a

fter f

irst 1

,000

s(e

lect

rons

)1/2

Target S/N after 4,000s

Detector Backgr ound Source

4.6m Spectroscopy at R=100,000

NGST 6.5m

Target S/N after 4,000s

Detector Backgr ound Source

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High resolution, high Signal/Noise observations

1 10

0.01

0.1

1

10

17.0

12.3

4.6

Molecular line spectroscopy S/N = 100

S/N

Gai

n (G

SMT

/ NG

ST)

Wavelength (microns)

R=10,000 R=30,000 R=100,000

Detecting the molecular gas from gaps sweptout by a Jupiter mass protoplanet, 1 AU from a 1 MO young star in Orion (500pc) (Carr & Najita 1998)

GSMT observation ~ 40 mins (30 mas beam)

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Conclusions

6.5m 4.0m Comments

1. Camera 0.6 – 5 mDeep imaging from space; consistent image quality, IR background, even for < 2.5m if D>4.0m

2.MOSR=1,000

1.2 – 2.5m2.5 – 5.0 m

NGST MOS still competitive for < 2.5m only if D~6.0m (consistent image quality, coverage)

3.CameraSpec. R=1500

5 – 28 m5 – 28 m

Clear IR background advantage observing from space, even for D~4mand R< 30,000

4. IFU R=5,000

1.2 – 2.5m2.5 – 5.0 m

Detector noise limited for < 2.5m D2 advantage for groundbased GSMTFor >2.5m, NGST wins even D~4m

D2 advantage for groundbased GSMTFor <12m

A advantage of GSMT,technology challenges from space (fibers)

NGST advantage GSMT advantage X

X

X

X

NGST

NG

S T I n

s tru

me n

t

High S/N, R~100,000 spectroscopy

WF MOS Spectroscopy m

XXX X