Science Impact of Sensor Effects or How well do we need to understand our CCDs? Tony Tyson

Cosmic shear vs redshift

LSST Cosmic Shear power spectra Gold Sample galaxy shear shot noise useful range baryons

Correcting PSF systematics The shape of the PSF must be known (measureable and stable) to a part per ten thousand in each exposure at each position in the CCD. Software corrections to its effects on faint galaxies will be made: below are the shear-shear correlation residuals in a simulation of LSST observing.

The Problem Imaging data from fields are not independent! The same CCD systematics pattern is imprinted on the sky for each dithered field Thus CCD systematic errors in shear-shear correlation do not average down over many fields Must reach full survey systematics specs in each ~100 visit dithered field!

CCD additive shear systematic astrometric bias due to charge transport anomalies around edges of device and at bloom stop.

Andrew Bradshaw Edge astrometric bias drives a shear bias

What if we don’t correct for systematics? Simulated LSST survey with only CCD effects on  (mass overdensity) map made from residual CCD shear systematics, in dithered and rotated observations.

Zoom into systematic patterns

Effect on cosmic shear correlation error of dropping 2 or 4 lines/columns James Jee

CCD multiplicative shear systematic and one more additive systematic Stars in the field are used as PSF calibrators Brighter-fatter effect causes a PSF error Isotropic BF effect Anisotropic BF effect fixed in CCD coords! BF effect depends on star-sky contrast!

Electrostatic model BF effect Craig Lage Calibrate model to 3E-3 for isotropic BF, and 3E-4 for anisotropic BF

Multi step CCD systematics calibration Use everything we know about these CCDs from lab measurements Develop physics-based model for charge transport effects Model astrometric bias and correct it in processing; then drop 2 lines and columns Rely on >20,000 stars per exposure from LSST operations to update and fine tune

Meeting the BF shear precision goal The raw uncorrected BF bias in the PSF FWHM is 1- 2%. Lab measurements will tell us how to correct for that as a function of star intensity, wavelength, seeing, voltages. We could do that to better than 1% precision. This in itself could meet the 3E-3 & 3E-4 goals. Moreover, each exposure will have >20,000 stars in the focal plane which will be in the linear BF region. The lab derived model could then be fit to these stars as a way of further improving the correction for the PSF bias.

Required lab precision To develop physics-based model for charge transport effects: Subpixel dithered spot array data 0.1 micron resolution vs x, y, voltages and f/1.2 typical seeing dithered star array data 0.1 micron resolution vs x, y, voltages and  -> Derivative of astrometric bias to 3E-4 shear Validate on new f/1.2 lab data from complex masks

Characterization priorities in 2016 Sub-pixel studies vs position, voltages… Study CCD effects vs sky background All vs wavelength Develop models of charge transport Null tests; confirm removal of PSF systematics Undertake lab simulated observing Can we measure the science signals? What is the residual error and B mode?

Discussion

DETF FoM(t) during ten year survey LSST FoM > 800 FoM = Stage II DETF Stage IV FoM = 110 DETF Stage IV = 10x Stage II

Pixellation effects

Beating down the atmosphere: stellar PCA PSF correction

Shear signal and noise spectra Galaxy ellipticity shot noise and cosmic variance combine to give a roughly constant noise floor at ~3E-7 over useful scales

DES PSF modeling

Reduced sensitivity to systematic error Combining WL and BAO breaks degeneracies. Joint analysis of WL & BAO is far less affected by systematics. p/  = w 0 + w a (1- a)

LSST f1.2 beam simulator Illuminates full CCD. arXiv:

Project a grid of 40,000 stars