How Post-Flashing Can Improve CTE in Some UVIS images Jay Anderson John MacKenty Sylvia Baggett Kai Noeske

Background on CTE Understanding evolving, improving – Photometric corrections – Pixel-based corrections – Corrections never perfect Better… – Minimize losses in first place Place source close to amplifier Maximize signal (fewer exposures) Avoid low backgrounds UVIS charge-injection Post-flash options

CTE TRAILS: ACS-vs-UVIS (May 2012) WP

WP LOSSES DOWN CHIP: ACS UVIS The magic of short-long dark studies… 1000s 100s −5% −75%

Finer- scale CTE-loss trends

Flat? monotonic Linear growth N traps α N q

Way to think about it… 12 in trail 18 total 6 in head ✕ Transfer out of board Number of traps encountered is proportional to the cross-section area Want to spread out Prefer to squeeze rather than go up Prefer to go up and squeeze more

Way to think about it… 18 total Above bkgd 6 in bkgd 12 in trail 18 total 6 in head

New Observations Omega Cen center, from SM4-calibration --- F336W Two Kinds of Obsns 1) 10s + 700s at same pointing with var bkgd 2) three 9pt dither stacks: (a) 700s deep (b) 10s (no bkgd) (c) 10s (12 e - bkgd)

Deep Stack (700s) A: 22 electrons above sky B: 28 electrons above sky C: 65 electrons above sky D: 13 electrons above sky

Short Unflashed Stack (10s)

Short Post-Flashed Stack (10s)

Deep Stack (700s) A: 22 electrons above sky B: 28 electrons above sky C: 65 electrons above sky D: 13 electrons above sky

Trends from ~5×5- pixel Aperture Photometry on ×2-Sampled Stacks

Losses as a Function of Background: Survival of a 100 e- source at top of chip = sweet spot!

Next steps… UVIS – Help users re-plan Phase-II’s to hit sweet spot – Use observations to calibrate losses-vs-bkgd – Constrain model better Take more long/short darks-vs-background ACS – Following in UVIS’s footsteps – No sweet spot: compromise Add noise, preserve signal More complicated analysis for us and users – Post-flash issues: 50% variation across field 25% dependence on shutter (A or B)