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How accurate do we need to be?. The Forward Process. During observation, a galaxy image is convolved with a PSF: making it bigger and changing its ellipticity. The process of shear measurement. The Inverse…. During data analysis, shear measurement methods seek to undo these changes to

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Presentation Transcript
the process of shear measurement

The Forward Process

During observation, a galaxy image is

convolved with a PSF: making it bigger

and changing its ellipticity

The process of shear measurement

The Inverse…

During data analysis, shear measurement

methods seek to undo these changes to

recover the true shape

…problem

An imperfect shear measurement method may not measure (any of) these quantities well. It instead obtains an inaccurate measurement, denoted by a hat.

previous figures of merit

STEP

Obtain imperfect shear measurements

where

Previous figures of merit

GREAT10

galaxy challenge

GREAT10

star challenge

Generically expect things to

be easier for large galaxies

G08

But can’t compare true against measured shear

from images containing a spatially varying signal,

and still average over intrinsic galaxy shapes.

quantifying shear measurement accuracy from a power spectrum

From imperfect shear measurements

with constant m and c

form 2-point correlation function

Quantifying shear measurement accuracy from a power spectrum

Multiplicative systematics

Fourier transform into power spectrum

where

Additive systematics

But if systematics vary spatially,

where

requirement on multiplicative systematics

To ensure bias/error<1

(Kitching et al. 2010)

Requirement on multiplicative systematics

Multiplicative systematics

Can break this down into separate requirements

on accuracy of PSF modelling (G10star) and galaxy

shape measurement (G10galaxy).

Constraints on dark energy parameter w

quantifying shear measurement accuracy from a power spectrum1

From imperfect shear measurements

with constant m and c

form 2-point correlation function

Quantifying shear measurement accuracy from a power spectrum

Fourier transform into power spectrum

where

Additive scatter systematics

But if systematics vary spatially,

where

G10

“ ”

req mt on additive scatter systematics

To ensure bias/error<1

(Amara & Refregier 2009)

But this depends strongly

on l-range, z-binning.

Req’mt on additive scatter systematics

Can break this down into separate requirements

on accuracy of PSF modelling (G10star) and galaxy

shape measurement (G10galaxy).

possible figures of merit

STEP

Simple & direct to useful quantities

Cannot be used with variable shear field

where

Single number

Bigger is better

Only really sensitive to c

Cannot be used with variable shear field

G08

Possible figures of merit

G10

Single number sensitive to both m and c

A(l) and M(l) can vary with scale

Systematics prob’ly positive definite anyway

Ugly absolute value not really a variance

Normalisation depends on assumed l-range

denominator

better?

and similar

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