odelling to Mitigate the Threat

Radiation damage on Gaia CCDs odelling to Mitigate the Threat Thibaut Prod homme Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011

1. What is CCD Radiation Damage? 2. Modelling to Mitigate the Threat! 3. Examples of Results 4. Mitigation at the Image Processing Level 5. Conclusion Thibaut Prod homme - Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011

. What is CCD Radiation Damage? Thibaut Prod homme - Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011

adiation environment Gaia 2013 6 1.5 10 km L2 Solar wind 1. What is CCD Radiation Damage? 5

adiation environment NASA Solar Dynamics Observatory kev < E protons < MeV 1. What is CCD Radiation Damage? 6

isplacement damage Collision Proton - Si atom Vacancy - Interstitial atom Vacancy can bind with impurities (O, P) Vacancy - Impurity complex introduce energy level in semiconductor band-gap Energy levels trapped the transferred charges in the CCD increase the Charge Transfer Inefficiency CTI 1. What is CCD Radiation Damage? 7

TI effects on the images CTI-free Photoelectrons Counts 10 # 10 " 10! Damaged { 18 12 6 pix. 12 pix. 10 AL Transfer 1. What is CCD Radiation Damage? 8

ardware CTI countermeasures Empty traps Electron release Filled traps charge injections Periodical Charge Injections 1. What is CCD Radiation Damage? 9

ardware CTI countermeasures Along scan Across scan Normal buried channel 3 or 4!m Supplementary buried channel Anti-blooming drain Anti-blooming drain shielding Channel potentials (positive) Clock low Clock high Voltage level at which pixel is full Excess charge V ABD Supplementary Buried Channel = notch 1. What is CCD Radiation Damage? 10

. Modelling to Mitigate the Threat! Thibaut Prod homme - Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011

ow models supported CTI mitigation? BY >> Understanding experimental data Deepening our understanding of CTI Characterizing in detail the CTI effects Calibrating for CTI in the on-ground data processing Testing the CTI mitigation strategy 2. Modelling to Mitigate the Threat! 12

his required a variety of models: Publication date Author name level type computational load 1998 L. Lindegren trap MC physical high 2005 A. Short trap MC physical high 2007 A. Short image analytic physical moderate 2008 L. Lindegren CDM01 image analytic phenomenological low 2009 A. Short CDM02 image analytic semi-physical low 2010 G. Seabroke pixel architecture analytic physical high 2010 A. Short CDM03 image analytic semi-physical low 2011 T. Prod homme trap MC physical high 2011 B. Holl statistical analytic phenomenological low 2. Modelling to Mitigate the Threat! 13

. Examples of Results Thibaut Prod homme - Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011

his required a variety of models: Publication date Author name level type computational load 1998 L. Lindegren trap MC physical high 2005 A. Short trap MC physical high 2007 A. Short image analytic physical moderate 2008 L. Lindegren CDM01 image analytic phenomenological low 2009 A. Short CDM02 image analytic semi-physical low 2010 G. Seabroke pixel architecture analytic physical high 2010 A. Short CDM03 image analytic semi-physical low 2011 T. Prod homme trap MC physical high 2011 B. Holl statistical analytic phenomenological low 3. Examples of Results 15

his required a variety of models Why so many different models and why not using what already existed? Different operation of the CCD than e.g., HST: TDI Time-delayed integration = CCD constantly readout => Very low background ( 1e-/pixel at readout), => Very low signal level even for bright stars for a part of the transit No full frame data, 1D images 3. Examples of Results 16

nderstanding experimental data + Deepening our understanding of CTI TDI TDI igs courtesy A. Short (ESA/ESTEC) Discrepancy in image location bias and charge loss between Sira (now Surrey) and Astrium first tests at same signal level 3. Examples of Results 17

TDI Fig courtesy A. Short (ESA/ESTEC) model 2005 A. Short (trap level MC physical) showed that: A difference in the background level of a few electrons makes a big difference CTI models must be density driven not volume driven 3. Examples of Results 18

nderstanding experimental data Imaging mode SIRA Imaging mode Astrium model 2011 T. Prod homme (trap level MC physical) SBCs are not functioning properly in the upper CCD half 3. Examples of Results 19

nderstanding experimental data pixel architecture model 2010 G. Seabroke (Silvaco) mask alignment errors in the CCD fabrication process 20

BC issue Along scan Across scan Normal buried channel 3 or 4!m Supplementary buried channel Anti-blooming drain Anti-blooming drain shielding Channel potentials (positive) Clock low Clock high Voltage level at which pixel is full Excess charge V ABD Supplementary Buried Channel 1. What is CCD Radiation Damage? 21

haracterizing in detail the CTI effects We assessed the impact of CTI on Gaia astrometry Prod homme et al. 2011b, Holl et al. 2011 I. on the image location estimation using 2011 T. Prod homme MC physical Generating a large set of synthetic CTI-free and damaged Gaialike images (~40 000 images) using CEMGA Computing the theoretical limit to image location Applying the Gaia image parameter determination algorithms 3. Examples of Results 22

mage location window α flux β background κ image location 23

mage location requirements per CCD observation at G= 13 image location accuracy < 20 atoms of Si sitting next to each other 24

ntrinsic loss of image location accuracy due to decrease in S/N induced by charge loss SBC mitigation effect 2011 T. Prod homme MC physical Charge injection mitigation effect Can only be prevented by avoiding trapping Need for Hardware CTI countermeasures 25

trong bias in the image location due to image distortion CI delay ~ 1 s CI delay ~ 27 s Charge injection mitigation effect SBC mitigation effect Hardware CTI countermeasures useful but not enough Need for CTI calibration 26

haracterizing in detail the CTI effects We assessed the impact of CTI on Gaia astrometry Prod homme et al. 2011b, Holl et al. 2011 II. on the astrometric solution AGIS using 2011 B. Holl (statistical level analytic phenomenological) AGIS + image locations for all (single) stars = astrometric parameters for each stars Solution for 1 million stars (semi-realistic star distribution in G) CTI errors vary as func. of G, t since last CI, (prev.) Solar Cycle 3. Examples of Results 27

TI induced parallax errors G = 13.3 requirements Equatorial projection 3. Examples of Results 28

rrors vs Solution Residuals 3. Examples of Results 29

rrors vs Solution Residuals 3. Examples of Results 30

. Mitigation at the Image Processing Level Thibaut Prod homme - Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011

orward modelling approach α, κ, β AGIS sampling illumination history comparison conv image sampled image CDM modelled counts PSF model CDM parameters observed counts iterative parameter update 4. Mitigation at the Image Processing Level 32

mage location residual bias before after Better than a factor ten decrease in the bias 4. Mitigation at the Image Processing Level 33

inal astrometric accuracy 4. Mitigation at the Image Processing Level 34

inal astrometric accuracy 35

. Conclusion Thibaut Prod homme - Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011

hanks to important modelling efforts upported by experimental tests e were able to demonstrate that: CCD radiation damage is not a threat to Gaia anymore; we can calibrate for it! however... 4. Conclusion 37

mplementation of the CTI mitigation strategy n the on-ground data processing emains a challenge & only first data from Gaia in 2013 will tell us if our predictions were right The END 4. Conclusion 38

hank you! Thibaut Prod homme - Leiden Observatory Edinburgh - Following the photons Workshop - 12 Oct 2011