We have the first, direct measure of photometric lossdue to imperfect CTE on ACS

The measurementsare made employinglarge scale dithers (WFC) and varyingselection of read-outamplifier (HRC)

Permutations:F606W, F775W, F502N1100 sec, 400 sec, 30 secto sample a wide-rangeof sky and stellar flux(total counts)

Example:Difference magsfor individual starsversus differentialtransfers

A linear loss trendwith parallel transferis clear at low flux,Indicating degradedCTE.Not so for serialtransfers

WFC: parallel CTE loss has strong dependence on stellar flux,Weak dependence on sky (negligible at r=5, 7)

Correction formulae derived using power law

WFC

Power law fitting formula, time dependence uncertain,but cosmic rays tails consistent with linear degradation

extrapolation

M31 faint-end CMD

SN Ia at peak, z~1.8

PSF flux=zeropoint½ orbit integration

Predicted Photometric Losses for WFC from Parallel CTE

3 example programs: source in middle of chip, y=1024

3e

30e

100e

WFC: no evidence of serial transfer losses, versus sky, or fluxor in any explored configuration

HRC example: parallel CTE loss apparent, no serial transfer loss seen

HRC: parallel CTE loss has similar dependence onstellar flux and sky level

Correction formulae derived using power law

HRC

HRC: no evidence of serial transfer losses, versus sky, or fluxor in any explored configuration

Internal data: charge deferred tails in dark frames

Indications appear consistent with direct data and providesfirst guess at time dependence: linear

1) For WFC, post-flashing may be ineffective at mitigating CTE This statement is a direct implication of the weak dependence of photometric loss on sky background. However, it is too soon to know for sure if this holds at sky levels much higher than those studied here but are readily achieved by post-flashing. Perhaps sky levels of a few hundred electrons will mitigate CTE (though such behavior would appear to conflict with the extrapolation of the WFC correction formula), but if the sky levels required are too high, the added shot noise may make such post-flashing undesirable. 2) The future photometric losses for WFC can now be predicted and are expected to grow faster than for WFPC2 Assuming the linear time-dependence justified by the internal data is correct, predictions can be made. In N years, the typical/worst case losses will be N*(2%/10%). By the end of life for HST (2010), 8 years after launch, we can anticipate typical case losses of 16% and worst case losses of 50%-80% (here the range of predictions reflects the difference given by linear and power-law time dependence). For comparison WFPC2 had typical/worst case losses of 6%/40% 7 years after launch (Whitmore et al 2000). Such a faster rate of degradation for WFC is expected from the greater number of transfers edge-to-amplifier (2048 for WFC versus 800 for WFPC2).

Implications