Comparing a 20m and 30m GSMT
for
5 driving science themes





2.1. Large Scale Structure
2.2. Galaxy Formation
2.3. Resolved Stellar Populations
2.4. Planet Formation Environments
2.5. High Dynamic Range Science
Matt Mountain
(a personal view)
AURA New Initiatives Office
Signal to Noise (S/N) gain for groundbased telescopes
For background or sky noise limited observations:
S
a
N

Telescope Diameter (D)

B
.
Delivered Image Diameter (q)
Speed = Time to given S/N
a
D2
q .
B
For detector noise limited observations:
S/N
a
Telescope Diameter (D)2 . 
Speed = Time to given S/N a
D4
.

Where:  is the product of the system throughput and detector QE
B is the instantaneous background flux
AURA New Initiatives Office
Theme 1: Baryonic Structure at High Redshift
The 3-D Structure of the diffuse IGM
can be probed using “tomography” via
multiple sightlines through the survey
volume
100Mpc (5Ox5O), 27AB mag (L* z=9), dense sampling
30m GSMT 1.5 yr
Gemini
50 yr
NGST
140 yr
AURA New Initiatives Office
Theme 1: Large Scale Structure
• In the seeing limited, sky noise limited regime
•
S/N gain of a 30m is 1.5 (30/20), as q is fixed for a fixed number
of fibers
100Mpc (5Ox5O), 27AB mag (L* z=9), dense sampling
20m GSMT 2.25 yr
Gemini
50 yr
NGST
140 yr
•
Or a 20m will need 2.25 times more fibers (and 1.5 times the FOV) to
complete survey in the same time as on a 30m
• If R ~ 15,000 – 20,000 spectroscopy is detector noise limited
•
S/N gain of a 30m is 2.25, or a 20m will need 5 times as many fibers to
complete a survey in the same time as a 30m and 2.25 times the FOV
AURA New Initiatives Office
Theme 2: Tomography of Individual Galaxies out
to z ~3
• Determine the gas and stellar dynamics within individual galaxies
• Quantify variations in star formation rate
– Multiple IFU spectra [R ~ 2,000 – 10,000]
30m GSMT 6 hour, 10s limit
at R=2,000
0.1”x0.1” IFU pixel
(sub-kpc scale structures)
J
26.1
H
25.3
K
23.7
(updated 7/22/02)
AURA New Initiatives Office
Theme 2: Tomography of Individual Galaxies out
to z ~3
• For a fixed IFU pixel scale of 0.1 arcsec, the S/N gain of a 30m will be
1.5, or it will take 2.25 times longer on a 20m to complete the same data
cube to the same S/N
20m GSMT 6 hour, 10s limit
at R=2,000
0.1”x0.1” IFU pixel
(sub-kpc scale structures)
J
25.6
H
24.8
K
23.3
(D mag ~ 0.44 magnitudes for R~2,000)
(D mag ~ 0.88 magnitudes for R~10,000)
This will require an MCAO system of comparable
complexity to a 30m, the DM’s (~4,000 actuators)
are ~ a factor of 2 away of what is available today
AURA New Initiatives Office
Theme 3: Resolved Stellar Populations
20”
M 32 (Gemini/Hokupaa)
GSMT with MCAO
NGST
30m MCAO simulation
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Theme 3: Stellar Populations
• For point sources, a 30m will have a S/N gain of 2.25 times that of a
20m
• In the confusion limited regime, a 30m will have an additional ~ D
advantage due to the enhanced contrast ratio
•
For this Theme, the S/N gain of a 30m over a 20m will be ~ a factor of 3.37
or the same project will take 11 times longer to complete on a 20m since the
MCAO FOV is “fixed” at ~ 2 arcmin.
30m
20m
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Theme 4: Probing Planet Formation with
High Resolution Infrared Spectroscopy
Planet formation studies in the infrared (5-30µm):




Probe forming planets in inner disk regions
Residual gas in cleared region
low t emission
Rotation separates disk radii in velocity
High spectral resolution
high spatial resolution
S/N=100, R=100,000, > 4m
Gemini
out to 0.2kpc sample
30m GSMT
1.5kpc
NGST
X
~ 10’s
~100’s

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).
AURA New Initiatives Office
Theme 4: Origins of Planetary Systems
•
In the diffraction limited, background dominated regime,
• The S/N gain of a 30m over a 20m is x 2.25, or
• The same survey will take 5 times longer on a 20m
S/N=100, R=100,000, > 4m
Gemini
out to 0.2kpc sample
20m GSMT
~ 1 kpc
~ 10’s
~30’s
i.e. for a fixed survey time, a 30m samples 3
times the volume of a comparable survey on a
20m
NGST
X
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Theme 5: High Dynamic Range Science
Fisher et al 2002
30m
The angular resolution of a GSMT should be able
to detect and resolve the thermal emission
from large planets in such systems as b Pic
b Pic. 10m and 18m
AURA New Initiatives Office
Theme 5: High Dynamic Range Science
•
For high dynamic range (high Strehl) , background
limited imaging, the contrast ratio gain of a 30m is 5
times that of a 20m (energy concentration a D4 )
•
Or the same observation to an equivalent contrast ratio
will take 25 times longer on a 20m
(hence the tremendous gain of a 100m OWL for this
science theme)
AURA New Initiatives Office
Defining “Cost Effectiveness”

Define cost effectiveness = Number of Observations / $
= 1/Tto given S/N / $
= Speed gain / $

Costs:
– 30m estimated* to be between $600M - $700M
– 20m using D2.7 is unrealistic at $200M - $230M
• No allowance for development costs (e.g AO)
• Many fixed costs reasonably insensitive to D (20m<D<30m)
(e.g. site preparation, computing infrastructure, project office)
– 20m would more realistically cost ~ $300M - $350M
* CELT and GSMT studies
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Relative Cost Effectiveness
30m/20m
Science Program
Ratio of Speed Gain
for fixed S/N
30m/20m
Cost Effectiveness of 30m
(assuming $ a D)
Large Scale Structure
(seeing limited)
2.25
1.1
Large Scale Structure
(detector noise limited)
5
2.5
2.25
1.1
11
5.5
Origins of Planetary Systems
5
2.6
High Dynamic Range
25
12.5
Tomography of Galaxies
Resolved Stellar Populations
AURA New Initiatives Office
Relative Cost Effectiveness 30m/20m
(assuming $ a D2.7 )
Science Program
Ratio of Speed Gain Cost Effectiveness of 30m
for fixed S/N
(assuming $ a D2.7)
20m/30m
Large Scale Structure
(seeing limited)
2.25
0.75
Large Scale Structure
(detector noise limited)
5
1.7
2.25
0.75
11
3.7
5
1.7
25
8.4
Tomography of Galaxies
Resolved Stellar Populations
Origins of Planetary Systems
High Dynamic Range
AURA New Initiatives Office