Maximizing GSMT Science Return with Scientific Figures of Merit
Maximizing GSMT Science Return
with
Scientific Figures of Merit
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
Quantifying value
Components of value
• Performance– Requirements– Goals
• Cost– Build– Operations
• Schedule– First light– Operating life
R
I
S
K
$$$
Science
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
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
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
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
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
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
Imaging FOM inputs: telescope
• D, primary mirror diameter
• TPtel ( ), throughput
( , , t ), delivered image quality • S ( , , t ) , Strehl ratio
( ) , emissivity
• Etel , operating efficiency
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
Imaging FOM inputs: multiplex advantages
, total solid angle field of view
• n, number of simultaneous spectral channels
Imaging FOM inputs: other science value factors
• Timeliness
• First light
• Other facilities
• Competition
• Access
• To facility
• To data
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
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
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
Formulation of image quality
arcsec
, arcminutes
Delivered image quality vs field angle and conditions
Poor conditions
Good conditions0.5
1.0
0 10 20
Optimizing /
Time
/
/
detection
photometry
1 2 3 4
Weighting function for
0
1
we
igh
t
, arcminutesMCAO regime
Tel, atmos rolloffs
0 20
Enhanced native seeing imager trades
Some performance (and cost) trades:
– D,
,
– TPtel () (coatings)
– n (instrument complexity)
(optics complexity, coatings choices)
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
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
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
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?
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
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 )
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
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
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?
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