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Instrument Science Report ACS 2012-04

Assessing ACS/WFC Sky Backgrounds
Josh Sokol, Jay Anderson, Linda Smith July 31, 2012 A
BSTRACT

This report compares the on-orbit sky background levels present in Cycle 18 ACS/WFC fullframe images against estimates provided by the Exposure Time Calculator (ETC). Backgrounds of over ~20 e- can alleviate charge-transfer efficiency (CTE) losses, which especially affect low S/N sources. HST observers can use these estimates to anticipate the natural background that should be present in their exposures, and can then determine whether they want to supplement that background with a post-flash to improve the CTE ­ at the cost of more background noise.

Introduction
Since it was installed on the Hubble Space Telescope (HST) in March 2002, the Advanced Camera for Surveys (ACS) has been continuously exposed to bombardment by high-energy cosmic rays. The intense radiation environment of space has caused an ever-increasing number of defects throughout the CCDs that act as charge traps. The traps briefly retain flux as charge is transferred along the detector during readout, causing bright sources in ACS images to exhibit smeared, vertical trails. Over time, this cumulative damage to the silicon lattice has diminished the overall charge transfer efficiencies (CTE) of the ACS CCDs. The most straightforward way to ameliorate CTE losses is to position sources close to the readout amplifiers, reducing the number of imperfect charge transfers. After an observation has been made, it is also possible to approximate the original image through the pixel-by-pixel CTE correction developed by Anderson & Bedin (2010). The ACS data reduction pipeline CALACS has incorporated this pixel-based corrective algorithm in version 2012.2, taking into account the dependence of CTE losses on temperature and time. Alternatively, photometry performed on CTE-trailed images can later be recalibrated (Chiaberge et al., 2009 & Chiaberge et al., in preparation). However, in images with low sky backgrounds, charge-transfer losses are so pathological that there is no information left for post-processing to reconstruct. A recommended pre-observational strategy is to "pave over" the charge traps with a modest
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increase in background. Background electrons can fill in many of the traps before the charge packets from sources are moved across those traps, improving the CTE for each transfer. With fewer background electrons, low S/N sources superposed on low (<~20 e-) backgrounds can lose more than 80% of their flux during readout. By contrast, similar sources imaged against brighter backgrounds (> 20 e-) are better able to hold onto their charge as it is transfered. Although the marginal transfer efficiency is still low with a 20 e- background, exceeding this threshold can help users avoid the most severe CTE losses. There are two ways observers can increase the background in their exposures. The easiest is to increase their exposure times by taking fewer individual exposures. Observers also have the option of applying a post-flash, which will add a roughly uniform number of electrons to each pixel. This new capability will be available for the first time on ACS in Cycle 20. This ISR is intended to help observers estimate the natural background in their images so that they can make the best possible decision regarding how to address CTE, be it by taking longer exposures, utilizing post-flash, or relying on post-processing. Readers are encouraged to consult the recent CTE White Paper1 (MacKenty & Smith, 2012) which summarizes our current understanding of CTE in both the WFC3/UVIS and ACS/WFC CCDs.

Data
All external full-frame ACS/WFC images taken during Cycle 18 were extracted from the Mikulski Archive for Space Telescopes (MAST). To obtain the broadest sample of sky backgrounds possible, no effort was made to exclude data based on telescope pointing or target. The resulting dataset contained 8227 images. Table 1 breaks down these data further to show the distribution of images by filter. The very few observations taken using ramp filters were excluded from this analysis.

1

http://www.stsci.edu/hst/acs/documents/CTE_White_Paper.pdf

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Table 1. Filter usage statistics and filter characteristics for full-frame Cycle 18 ACS/WFC images. Filter description, pivot wavelength, and widths are taken from the ACS website. Both PHOTBW (the RMS bandwidth) and CTW95 (95% cumulative throughput width: the width between 2.5% and 97.5% of the throughput curve integrated over wavelength) are provided. PHOT BW (е ) 298.2 420.2 28.6 163.1 360.1 672.3 415.4 36.8 34.9 434.4 652.0 525.7 Num Imgs (% ) 2.1 16.1 1.1 0.2 0.3 10.7 0.7 0.6 0.1 5.1 43.5 13.1 Total Exptime (hrs) 24.8 152.5 4.4 2.4 2.6 122.7 7.0 12.8 2.4 59.6 435.1 146.2 Median Exptime (sec) 543.0 370.0 30.0 500.0 500.0 480.0 507.0 820.0 800.0 526.0 425.0 500.0 Min Exptime (sec) 10.0 10.0 0.7 339.0 1.0 15.0 10.0 339.0 339.0 30.0 2.1 30.0 Max Exptime (sec) 1360.0 1524.0 1300.0 800.0 703.0 1430.0 800.0 1379.0 1300.0 800.0 1534.0 800.0

Filter

Description

Pivot ! (е )

CTW95 (е )

F435W F475W F502N F550M F555W F606W F625W F658N F660N F775W F814W F850LP

Johnson B SDSS g
[OIII] (1%) Narrow V Johnson V Broad V SDSS r H!(1%) [NII] (1%) SDSS i Broad I SDSS z

4328.2 4746.9 5023.0 5581.5 5361.0 5921.1 6311.4 6584.0 6599.4 7692.4 8057.0 9033.1

1017.7 1421.5 67.4 532.6 1262.8 2235.5 1396.2 89.8 49.2 1492.8 2337.6 2062.6

For each filter studied, the ETC was also queried three times to roughly span the range of background value predictions. Although the ETC does allow for greater sophistication in the specification of simulated backgrounds, we chose the "standard" normalizations for background components and scaled them using drop-down lists. Varying zodiacal light, earth shine, and air glow (respectively), we tested permutations of "low-average-low," "average-average-average", and "high-high-high" over 1000-second exposures. The resulting simulated sky backgrounds, together with the ETC's estimate of dark current, produce a cumulative background commensurate to the background levels measured from actual raw images. Table 2 presents the results of these ETC simulations.

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Table 2. ETC estimates for background levels in full-frame WFC images. zodiacal: low earthshine: average airglow: low (e-/pix /1000s) 27.6 45.6 7.6 24.7 43.5 94.3 62.9 9.4 7.5 59.0 75.9 32.8 ave ave ave (e-/pix rage rage rage /1000s) high high high (e-/pix/1000s) 61.2 107.8 9.8 54.1 102.6 235.1 153.9 14.6 9.4 145.1 189.9 76.7

Filter

F435W F475W F502N F550M F555W F606W F625W F658N F660N F775W F814W F850LP

36.4 62.1 8.2 32.6 59.3 132.5 87.7 10.8 8.0 83.1 108.0 45.3

Analysis
This analysis was done on the raw fits files from the archive using a FORTRAN routine. Before measuring the background in each quadrant of the detector, the routine first removes the average bias level from a fit to the pre-scan, then removes the horizontal bias trend using a lookup table (see Golimowski et al., 2012). The routine then takes the 2048x2048 bias-corrected pixels of the data frame for each of the four quadrants and determines a robust average value, iteratively sigma-clipping at 3.5 !. It reports this average, along with the ! and the fraction of pixels that fit within the ±3.5 ! range. In addition, it outputs the image name, date, exposure time, and filter name from the archive. The background values reported here are averages of the measurements made for each quadrant. In an attempt to exclude some spuriously high backgrounds due to extended sources, images in which less than 92% of the pixels fall within the calculated flat sky distribution have been omitted.
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In Figure 1, background levels are plotted as a function of exposure time for the most-used filter in our dataset, F814W. Plots for the other filters can be found in the Appendix. The average backgrounds measured in raw images are plotted in green, with overlaid dashed slopes representing the accumulation of background predicted by the ETC. These lines are defined as: Magenta: low zodiacal, average earthshine, low airglow Yellow: average zodiacal, average earthshine, average airglow Black: high zodiacal, high earthshine, high airglow An additional dotted black line is present at the 20 e- level, which loosely marks the minimum background level needed to mitigate CTE loss.

Figure 1. Background levels as a function of exposure time for F814W images. The plotted background is the average of the background measured by ACS's four amplifiers. Estimates of sky backgrounds from the ETC are plotted alongside the data. For reference, a dotted line marks the 20 e- limit below which CTE loss becomes increasingly problematic.

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As is clear from the images with >40 e- backgrounds at low exposure times in Figure 1, the dataset includes anomalously high "backgrounds" computed from images of extended sources. In this case, the culprits are all images of M31. Since the lower range of attainable backgrounds is more pertinent to this study, no further effort has been made to throw out images of extended sources on a case-by-case basis. Excluding these outliers, it is apparent from visual inspection that the ETC's estimates do a decent job of characterizing the actual data. To quantify the match between observations and the ETC, Figure 2 presents a cumulative percentage plot of these same F814W data. Here, the portion of F814W images with background fluxes underneath a given level is plotted. Analagous plots for the six other most popular filters, which will also help indicate the trustworthiness of the ETC, are included in the Appendix.

Figure 2. Plot of cumulative percentage within F814W images, showing the distribution of measured backgrounds against the ETC's predictions.

Taken together, these plots demonstrate that the standard specification of "low" background in the ETC still predicts backgrounds that exceed ~10-20% of the background levels measured in ACS/WFC Cycle 18 data. We therefore recommend that in situations where low background may threaten CTE, observers should use the ETC's "low" estimates conservatively; with the recognition that the ETC does not provide a lower limit.

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Further information on how the ETC models ACS sky backgrounds can be found in Boffi et al. (2003). In addition, section 9.4 of the ACS Instrument Handbook contains a general discussion of ACS sky and detector backgrounds.

Conclusions
For each ACS/WFC filter studied, we have presented plots that allow observers to predict the background level in their exposures, so that they can plan their future observations in such a way as to minimize CTE losses. The plots, combining on-orbit measurements of backgrounds in Cycle 18 images and the ETC's expected backgrounds, should indicate exposure times that are likely provide adequate backgrounds. A sufficient background will reduce charge loss from low S/N sources, while at the same time avoiding the added noise of a post-flash. However, in cases where the ~20 e- level remains out of reach, observers may want to consider applying a postflash to combat CTE loss.

Acknowledgments
We thank Sylvia Baggett for many useful discussions that enhanced our analysis and Aparna Maybhate for her helpful advice. Further thanks to Marco Chiaberge, Leondardo Ubeda, Ralph Bohlin, Roberto Avila, and Sara Ogaz for their comments on this document.

References
Anderson, J. & Bedin, L.R. 2010, PASP 122 1035-1064 Boffi, F.R. et al. 2011, Instrument Science Report ACS 2003-08 (Baltimore: STScI) Chiaberge, M. et al. 2009, Instrument Science Report ACS 2009-01 (Baltimore: STScI) Golimowski, D. et al. 2012, Instrument Science Report ACS 2012-02 (Baltimore: STScI) MacKenty, J., & Smith, L. 2012, CTE White Paper Ubeda, L. et al. 2011, ACS Instrument Handbook, Version 11.0 (Baltimore: STScI)

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Appendix

Figure 3a. Background levels shown as a function of exposure time for remaining filters. The plotted background is the average of the background read by ACS's four amplifiers. Estimates of sky backgrounds from the ETC are plotted alongside the data. For reference, a dotted line marks the 20 e- limit below which CTE loss becomes increasingly problematic.

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Figure 3b. Same as 3a.

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Figure 3c. Same as 3a.

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Figure 4a. Plots of cumulative percentage for a given filter, showing the distribution of measured backgrounds against the ETC's predictions.

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Figure 4b. Same as 4a.

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