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