maximizing gsmt science return with scientific figures of merit
TRANSCRIPT
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Maximizing GSMT Science Return
with
Scientific Figures of Merit
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Maximizing value
• Who are the interested parties?– Scientist users– Funding agencies
• What constitutes value to them?– Scientific return– Cost
• What gives greatest value?
MAXIMUM SCIENTIFIC RETURN FOR COST
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Quantifying value
Components of value
• Performance– Requirements– Goals
• Cost– Build– Operations
• Schedule– First light– Operating life
R
I
S
K
$$$
Science
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Science merit function
Science merit function = ( Wi x FOMi )
• Figure of Merit (FOM)– For each capability, embodied as instrument + telescope– Quantitative, with analytical and numerical components– Function of instrument and telescope properties
• Weight (W)– Scientific judgment call
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Example 1. GSMT spectroscopic capability
Parameter Resolved stellar
populations Star formation
Aperture 30 m 30 m Field of view 2-3 arcmin < 10 arcmin Spatial resolution 10 milliarcsec 15 milliarcsec
Wavelength coverage 0.3-1.2 microns 0.3-2.2 microns
Spectral resolution 1000’s > 2000
Instrument type OIR multislit OIR MOS
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Example 2: CELT IR AO system emissivity
• Cryogenic AO system at prime focus• Ultimate performance for emissivity• Negative impacts on telescope design, enclosure cost
• Cryogenic AO system at Nasmyth focus• Quantifiably almost as good• Expect lower total observatory cost
• Warm AO system at Nasmyth focus• Dramatically reduced performance• Low cost, maintains spatial resolution advantage• Trades against space platform sensitivity advantage
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What is the science mission?
Type of mission impacts FOM, weights
• Design reference mission
– Total science program specified
• Timely science mission
– Maximize science achieved in initial period
• Scientific capability mission
– Instrument capabilities for wide range of potential science
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Example: UKIRT WFCAM program
• WFCAM: widefield 1-2 m camera on 3.8 m telescope
• Several large scale surveys over ~10 years (DRM)
• Quick shallow surveys first (STM)
• Selected deep fields done repeatedly (STM + DRM)
• Instrument permits installation of custom filters (SCM)
http://www.ukidss.org
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GSMT sample imaging capabilities
• Enhanced seeing widefield imager– Gaussian profile– Tens of arcmin FOV
• Narrow field coronagraph– Highest possible Strehl and dynamic range– FOV is arcseconds
• Moderate field, diffraction limited imaging– Moderate Strehl over arcminute FOV
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Imaging FOM inputs: telescope
• D, primary mirror diameter
• TPtel ( ), throughput
( , , t ), delivered image quality • S ( , , t ) , Strehl ratio
( ) , emissivity
• Etel , operating efficiency
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Imaging FOM inputs: instrument
• TPinstrl ( ), throughput
• DQE( ), detector quantum efficiency
, pixel sampling , , wavelength coverage and resolution
• R, D, read noise and dark current
• Sc, scattered light susceptibility
• Etel , system efficiency
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Imaging FOM inputs: multiplex advantages
, total solid angle field of view
• n, number of simultaneous spectral channels
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Imaging FOM inputs: other science value factors
• Timeliness
• First light
• Other facilities
• Competition
• Access
• To facility
• To data
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Enhanced native seeing imager
• Science– Distribution of high redshift galaxies– Integrated properties of galaxies
• Programmatic– Use at wavelengths where diffraction limit can’t be achieved– Use in less favorable conditions, e.g. thin cirrus
• Implications for FOM– Slightly extended sources with some central concentration– Wavelength coverage is 1 m
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Enhanced native seeing imager
Background limited, uncrowded field case
Neglect Emissivity Strehl ratio Read noise, dark current Scattered light Programmatic terms
Gather terms into a Figure of Merit for (integration time)-1
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Enhanced native seeing imager
Background limited, uncrowded field FOM
1/time [ (D2/2) • TPtel () • Etel] •
[ • DQE • TPtinstr() • Etinstr • f(/) • f(n) • f(, ) ]
• Track telescope, instrument separately
• Some factors require simulations to determine appropriate formulations
• Some factors may include weighting functions
Telescope
Instrument
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Formulation of image quality
arcsec
, arcminutes
Delivered image quality vs field angle and conditions
Poor conditions
Good conditions0.5
1.0
0 10 20
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Optimizing /
Time
/
/
detection
photometry
1 2 3 4
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Weighting function for
0
1
we
igh
t
, arcminutesMCAO regime
Tel, atmos rolloffs
0 20
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Enhanced native seeing imager trades
Some performance (and cost) trades:
– D,
,
– TPtel () (coatings)
– n (instrument complexity)
(optics complexity, coatings choices)
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Narrow field coronagraphic imager
• Science– Discovery and characterization of planetary systems
• Programmatic– Diffraction limited, very high Strehl at first light– Use in best seeing conditions
• Implications for FOM– Wavelength coverage is 1 5 m– Treatment of systematic effects important– Independent of telescope design, AO implementation details
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Coronagraphic imager FOM additional inputs
• d, subaperture size of primary
• n, number of actuators on deformable mirror
, residual wavefront rms error
, speckle lifetime (site characteristic)
• g, gain, ratio of peak intensity to halo level
• R, amplitude reduction of primary core and halo by
coronagraph
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Coronagraphic imager FOM
Comparison with enhanced seeing imager:
Neglect traditional seeing measure
Include Strehl ratio S, emissivity
Use additional terms to describe AO, coronagraph
impacts
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Coronagraphic imager sensitivity FOM
FOM for sensitivity (SNR):
sensitivity [ D2 • TP • E • • DQE • -1 • f(/) • f(n) • f(, ) ]½
• [ S / (1-S) ] • [ D / d ]2 • [ 1/R ]
• Includes “traditional” components, Strehl and gain advantages
• Not yet in right units!
• How to account for systematic effects?
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Coronagraphic imager systematics
SNR limited by speckle structure in uncorrected halo
– Pointlike
– 100% amplitude modulation
– Persist for time
Variety of solutions
– Decorrelation (large n, kHz AO update rate)
– Simultaneous differential imaging (NICI)
– PSF engineering, e.g. speckle sweeping
– Data taking and reduction methods
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Coronagraphic imager final FOM
• Characterize time – SNR relation by parameter
= 2 for photon noise limited system, less if residual systematic errors are significant
1/time ( previous expression )
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Narrow field coronagraphic imager trades
• Mirror segment size d
• Speckle lifetime (site characteristics)
• Emissivity and Strehl ratio S
error budget allocations
/ with
• Suppression of systematic error
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Wide field – narrow field comparisons
Wide field Narrow field < 1 m 1 – 5 m
FOV 20 arcmin 2 arcsec
DIQ ~0.5 arcsec ~0.005 arcsec
Tel geometry uncritical important
Tel optics fast, complex slow, simple
Secondary large small
Emissivity irrelevant important
AO system Active secondary Ditto + DM w/
~10E3 actuators ~10E4 actuators
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Maximizing value, redux
Return to performance, cost, schedule, risk mix:
Is there a similar approach to maximizing value?
Performance-cost index
PCI = Science merit function / total cost (capital + ops)
How to do optimization?
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Maximizing value, redux
• Evaluate a few plausible approaches
– Telescope type
– Instruments
• Trade studies for key parameters
– Effect on SMF
– Effect on cost
• Creative tension between Scientist, Engineer, and Manager