ACS WFC Flat-Field Changes Temperature change from -77 C to -81 C on July 4, 2006 leads to expected changes for flat fields. Are L-flat measures stable before δT, and is there a change after cooldown? (Yes/yes) Are pixel-to-pixel measures stable before δT, and is there a change after cooldown? (No/yes, but small.) This summarizes two recent ISRs: WFC L-Flats Post Cooldown (Gilliland, Bohlin and Mack). Pixel-to-Pixel Flat Field Changes on the WFC (Gilliland and Bohlin).

Data available and processing approach. Internal tungsten lamp exposures in F435W, F625W and F814W have been routinely acquired to provide flat field tracking. Twenty epochs available pre- and three epochs post-cooldown. Standard set: 3 exposures to 50,000 e- each in each of three filters providing Poisson limit near for co-added precision. Pipeline.flt files have bias and dark subtraction, local processing to remove cosmic rays. A mean flat field over time, separately for pre- and post-cooldown is formed. The ratio of post- to pre-cooldown means is formed. A spatial median filter 65x65 pixels, stepped by 16 pixels to form 256x256 L-flats is formed for each filter.

Ratio of Pre- to Post-cooldown, compared to stability. Same as at left, but rms over all pre- cooldown epochs. Changes across δT are much larger than inherent epoch-to-epoch changes. L-flat changes across cooldown to range, white corresponds to sensitivity loss. Schematic used to define comparison areas. Filter Pattern change F435W-0.61% F625W-0.38% F814W-0.15%

Delivery of new L-flats. We have updates directly available for F435W, F625W, and F814W -- interpolation used for other filters in this range. These “δ-L-flats” at each filter are applied as multiplicative corrections to all existing pipeline LP-flats. The 47 Tuc monitoring program data were used to show that stellar photometry from two epochs close to, and pre-/post-cooldown showed relative changes similar to those derived from the internals. A full set of such corrected flats was made available in early November with use-after date of July 4, See ISR for further details. Accuracies of these newly delivered post-cooldown flats are limited by errors following from the original laboratory flats and pre-cooldown L- flat adjustments, rather than limitations in these differential corrections.

Pixel-to-pixel Flat Field Changes Can we simply do the same thing as before? That is create separate pre- and post-cooldown internal flats, suppress L-flat changes and ratio the two to derive pixel-to-pixel flat changes as a result of the δT? A hint a why this doesn’t work appeared in Bohlin and Mack, The Internal ACS Flat Fields, ISR which obviously discussed only pre-cooldown data: “The only ubiquitous change observed in these flat fields is an excess of pixel responses that are low.” “Replacing the pre-launch baseline with one of the flight flats does not change the essential features” The first task is to look further into the stability of flat fields at the individual pixel response level, and then assess whether this allows a reasonable path to providing updated flats, or the need for such.

Pixel-to-pixel response changes in time. Started with 18 epochs of F435W internals pre-cooldown and 6 epochs post-cooldown. Removed the effect of changing L-flats by dividing out spatial median filtered image. Formed median over all pre-cooldown epochs. Tabulated how many pixels deviated by more than 1, 2 and 4% both + and - from the global median in time. Figure at right shows number of pixels at <-4% per epoch. Values have been scaled up by 32/Number of days since anneal to project to number expected at end of anneal cycles. The ‘x’ are prior to WFC cooldown, the ‘o’ are after. Linear correlation coeff over 14 points post-SMOV, pre-cooldown is r = allowing only about a part per million chance of resulting from uncorrelated data.

Pixel-to-pixel deviations correlated with time since anneal. Figure again shows the number of pixels at <-4% deviant response in each epoch of internal flat data. Linear trend shown in previous figure: -- time since launch-- has been divided out with normalization to value at adopted. This shows that the number of low response pixels grows smoothly within anneal cycles, and then resets with the vast majority of such pixels no longer deviant at this level. r = 0.958, chance of not being a real correlation is less than one in ten million. The number of pixels that are low by 4% in any epoch is highly correlated with both time since an anneal, and with overall time that the WFC has been on-orbit.

More details on low response pixels. Number at -2% is > X10 the number at -4%. Number at -1% (still 3.8 σ) is X5 that at -2%, maybe doesn’t anneal as well. The low response pixels are generally isolated, single pixels, are unique sets each epoch (not a set that ‘telegraph’ back and forth) and are not correlated in space with hot pixels, traps or other known pixel detects. Number at positive deviations negligible compared to negative.

Wavelength dependence of low response pixels. Plot for a random pre-cooldown epoch. Individual pixel values in normalized (to median over all pre- cooldown epochs) F435W internal flat plotted against the same for F814W. Points within 1% of (1,1) are not plotted, only one of every 25 out to 2.5% of (1,1) is shown. All epochs checked show similar -- pixels that show deviations are about twice as low in F435W as F814W (and F625W is half way in between).

Low response effect is multiplicative, not additive. At a few of the epochs a short exposure in F660N was substituted for one of the F814W exposures. This provides internal flats with mean exposure level of 170 electrons, instead of standard 50,000. We select all pixels showing <-4% deviation in one epoch and form a stack over all such pixels in both F435W and F660N, normalize in each epoch by the local mean and evaluate medians within +/- 2 pixels over this stack. A pure multiplicative effect should show the same depths, modulo wavelength dependence. An additive effect would be much larger in the shallow exposure. The result at F660N, compared to F435W used to flag the source pixels, is fully explained by wavelength dependence. The effect is multiplicative, which seems to rule out temporary charge traps as the cause. Central pixels are surrounded by suppressed values consistent with charge diffusion.

Recovery of Low Response Pixels Mean values by epoch for the set of pixels at <-4% in 9th overall epoch. Values are stable before epoch of anomaly and recover to an asymptotic level ~90% of the way back to nominal. Similar recoveries follow for - 2% and -1% deviation cases.

Absolute Quality of Pixel-to-Pixel Flats The monthly growth of low response pixels is sufficient to add ~0.3% noise to the flat fields -- comparable to intrinsic precision. With anneals it will be 15 years before the current growth rate of low response pixels increases enough to double the noise to 0.6%. Without anneals the noise would double in only 120 days from the low response pixels. A comparison of the pre-cooldown median flat to the reference flat shows that the number of deviant pixels is small compared to the number developing within each anneal cycle: our reference flats are excellent. A comparison of post-cooldown flats to pre-cooldown medians shows a number of deviant pixels at each threshold that is still smaller than the monthly growth number, but not by a large margin. One measure of current quality comes through estimating errors on stellar photometry: 2.6% of stars would now show errors of 0.34%, 0.02% of stars with errors > 1.2% in F435W. In F814W errors only half of this. We’ve concluded that a flat field update is not currently needed (data doesn’t exist to support except for F435W), but that we should restore the cadence of internal monitors to 6 times per year to improve tracking and allow updates if such seem desirable in a year.