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

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Presentation on theme: "Science Impact of Sensor Effects or How well do we need to understand our CCDs? Tony Tyson."— Presentation transcript:

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

2 Cosmic shear vs redshift

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

4 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.

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6 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!

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

8 Andrew Bradshaw Edge astrometric bias drives a shear bias

9 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.

10 Zoom into systematic patterns

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

12 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!

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

14 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

15 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.

16 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

17 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?

18 Discussion

19 DETF FoM(t) during ten year survey LSST FoM > 800 FoM = 11 2011 Stage II DETF Stage IV FoM = 110 DETF Stage IV = 10x Stage II

20 Pixellation effects

21 Beating down the atmosphere: stellar PCA PSF correction

22 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

23 DES PSF modeling

24 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)

25 LSST f1.2 beam simulator Illuminates full CCD. arXiv:1411.5667

26 Project a grid of 40,000 stars


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