Comparative Performance of a 30m
Groundbased GSMT and a 6.5m
(and 4m) NGST
NAS Committee of Astronomy & Astrophysics
9th April 2001
Matt Mountain
Gemini Observatory/AURA NIO
1
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
2
GSMT Science Case
“The Origin of Structure in the Universe”
Najita et al (2000,2001)
From the Big Bang… to clusters,
galaxies, stars and planets
3
Mass Tomography of the Universe
Existing Surveys + Sloan
Hints of Structure at z=3
(small area)
z~0.5
z~3
100Mpc (5Ox5O), 27AB mag (L* z=9), dense sampling
GSMT
1.5 yr
Gemini
50 yr
NGST
140 yr
4
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, 3s limit
at R=5,000
0.1”x0.1” IFU pixel
(sub-kpc scale structures)
J
26.5
H
25.5
K
24.0
5
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
S/N=100, R=100,000, >4m
Gemini
GSMT
NGST
out to 0.2pc
1.5kpc
X
sample ~ 10s
~100s

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).
6
30m Giant Segmented Mirror
Telescope concept
GEMINI
Typical 'raft', 7 mirrors per raft
1.152 m mirror
across flats
Special raft - 6 places, 4
mirrors per raft
30m F/1 primary, 2m adaptive secondary
Circle, 30m dia.
7
GSMT Control Concept
Deformable M2 : First stage
MCAO, wide field seeing
improvement and M1 shape
control
Active M1 (0.1 ~ 1Hz)
619 segments on 91 rafts
LGSs provide full sky
coverage
 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).
10-20’ field at
0.2-0.3” seeing
1-2’ field fed to the
MCAO module
Focal plane
8
GSMT Implementation concept
- wide field (1 of 2)
Barden et al (2001)
9
GSMT Implementation concept
- MCAO/AO foci and instruments
Oschmann et al (2001)
MCAO optics
moves with telescope
elevation axis
Narrow field AO or
narrow field seeing
limited port
MCAO Imager
at vertical
Nasmyth
4m
12
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)
14
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
Source noise
background
dark-current read-noise
•
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
15
Space verses the Ground
Takamiya (2001)16
Adaptive Optics enables groundbased
telescopes to be competitive
For background or sky noise limited observations:
S
N

Telescope Diameter
Delivered Image Diameter
. 
B
Where:  is the product of the system throughput and detector QE
Bis the instantaneous background flux
17
Adaptive Optics
works well
18
Modeling verses Data
GEMINI AO Data
20 arcsec
2.5 arc min.
M15: PSF variations
and stability
measured as
predicted
19
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
20
Model results
Multi-Conjugate Adaptive Optics
2.5 arc min.
MCAO
•Tomographic calculations correctly
estimated the measured atmospheric phase
errors to an accuracy of 92%
–better than classical AO
21
–MCAO can be made to work
AO Technology constraints (50m telescope)
Actuator pitch
r0(550 nm) = 10cm
S(550nm) S(1.65m)
No. of
actuators
Computer
power
(Gflops)
CCD pixel
rate/sensor
(M pixel/s)
10cm
74%
97%
200,000
9 x 105
800
25cm
25%
86%
30,000
2 x 104
125
50cm
2%
SOR (achieved)
61%
8,000
789
1,500
~2
31
4 x 4.5
Early 21st Century technology will keep AO confined to  > 1.0 m
for telescopes with D ~ 30m – 50m
22
MCAO on a 30m: summary
• MCAO on 30m telescopes should be used >.5m
• Field of View should be < 3.0 arcminutes,
(m)
1.25
1.65
2.20
Delivered Strehl
0.2 ~ 0.4
0.4 ~ 0.6
0.6 ~ 0.8
Rigaut & Ellerbroek (2000)
9 Sodium laser constellation
4 tip/tilt stars (1 x 17, 3 x 20 Rmag)
PSF variations < 1% across FOV
• 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
23
Modeled characteristics of a 30m GSMT with MCAO
(AO only, >3m) and a 6.5m NGST
Assumed encircled-energy diameter (mas) containing energy fraction 
30M
1.2m 1.6m 2.2m 3.8m 5.0m 10m 17m 20m
(mas)
23
29
41
34
45
90
154
181
34%47%6%50%54%56%57%58%
NGST 1.2m 1.6m 2.2m 3.8m 5.0m 10m 17m 20m
(mas)
100
100
82
138
182 363
617 726
70%70%50%50%50%50%50%50%
Assumed detector characteristics
m < <5.5m
Id
0.01 e/s
5.5m < <5m
Nr
qe
Id
4e
80%
10 e/s
Nr
qe
30e
40%
24
Comparative performance of a 30m
GSMT with a 6.5m NGST
10
R = 10,000
R = 1,000
R= 5
1
NGST advantage
S/N Gain (GSMT / NGST)
R 5
=
= ,0
R
1 0
R =
1 0
, 0
GSMT
advantage
Assuming a detected S/N of 10 for NGST on
a point source,
withof4x1000s
integration
Comparative
performance
a 30m GSTM
with a 6.5m NGST
0.1
0.01
1E-3
1
10
Wavelength (microns)
25
Comparative performance of a 30m
GSMT with a 4m NGST
Assuming a detected S/N of 10 for NGST on
a point source,
withof4x1000s
integration
Comparative
performance
a 30m GSTM
with a 4.0m NGST
R = 10,000
R = 1,000
R= 5
GSMT
advantage
R =
5
= 1
R
,0 0
R =
1 0
, 0
1
NGST advantage
S/N Gain (GSMT / NGST)
10
0.1
0.01
1
10
Wavelength (microns)
26
Observations with high Signal/Noise, R>30,000
is a new regime
- source flux shot noise becomes significant
4.6m Spectroscopy at R=100,000
GSM T 30m
NGST 6.5m
Comparative noise contributions after first 1,000s
1/2
(electrons)
1000
1000
Detector
B ac kgr ound
S ource
100
100
10
10
1
1
Detector
B ac kgr ound
S ource
0.1
0.1
10
Target S/N after 4,000s
100
10
Target S/N after 4,000s
100
27
High resolution, high Signal/Noise
observations
Molecular line spectroscopy S/N = 100
S/N Gain (GSMT / NGST)
10
R=10,000
R=30,000
R=100,000
Detecting the molecular gas from gaps swept
out by a Jupiter mass protoplanet, 1 AU
from a 1 MO young star in Orion (500pc)
(Carr & Najita 1998)
1
4.6
12.3
0.1
17.0
0.01
1
10
Wavelength (microns)
GSMT observation ~ 40 mins
(30 mas beam)
28
Conclusions
NGST advantage
GSMT advantage

X
NGST
6.5m 4.0m
1.
Camera
0.6 – 5 m
2.
MOS
R=1,000
1.2 – 2.5m
2.5 – 5.0 m
Camera
5 – 28 m
5 – 28 m
3.
4.
Spec. R=1500
IFU
R=5,000
1.2 – 2.5m
2.5 – 5.0 m
Comments






X



X
X
Detector noise limited for < 2.5m
D2 advantage for groundbased GSMT
For >2.5m, NGST wins even D~4m
X
D2 advantage for groundbased GSMT
For <12m

High S/N, R~100,000 spectroscopy
X

WF MOS Spectroscopy <.5m
X
X
Deep imaging from space; consistent
image quality, IR background, even
for < 2.5m if D>4.0m
NGST MOS still competitive for <
2.5m only if D~6.0m (consistent
image quality, coverage)
Clear IR background advantage
observing from space, even for D~4m
and R< 30,000
AW advantage of GSMT,technology
challenges from space (fibers)
29