Cross-Talk in the ACS WFC Detectors. I: Description of the Effect Mauro Giavalisco August 10, 2004 ABSTRACT Images acquired with the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) are affected by cross talk between the four CCD quadrants that correspond to the four amplifiers of the detector array. The effect is observed as (mostly) negative ghost images generated by relatively bright sources located in adjacent quadrants. Their position in the quadrant is mirror-symmetric relative to the positions of the generating sources. The strength of the cross talk is small, i.e. the ghost images have apparent surface brightness of only a few electrons per pixel, and it appears to be mostly a cosmetic problem, with little measurable effects on the photometry of affected sources. Here we describe the phenomenology of the cross talk, and in a companion ISR (Giavalisco 2004) we suggest an observing strategy to minimize its effects on the images. 1. Introduction The Wide Field Channel of the Advanced Camera for Surveys provides HST with large area, sensitive imaging capabilities at optical wavelengths. The camera delivers high quality, stable images with relatively modest undersampling of the telescope Point Spread Function (PSF) across most of its wavelength range, and achieves very good photometric performance (Sirianni et al. 2004). Only a few imperfections and blemishes affect ACS images. Among these is the electronic cross talk between the four amplifiers of the two CCD detectors of the WFC. This effect is the cause of negative ghost images, i.e. depressions against the sky background, that are most easily observed in deep images. It is a
known phenomenon that shows itself in multichannel CCD amplifiers due to a number of different causes (e.g., see Janesick 2001). The nature of the specific cross talk of WFC is not known; however experimentations with the CCD electronics of the Ultra-Violet Imager and Spectrograph (UVIS) channel of the Wide Field Camera 3 (WFC3), which adopts identical electronics as ACS for the very similar array of two CCD detectors, offer hope that the nature of the effect will be understood and, possibly, minimized or eliminated (Baggett, Hartig, & Cheung 2004; see also Giavalisco 2004). In this Instrument Science Report (ISR) we provide a phenomenological description of the cross talk as derived from the analysis of images taken with WFC, and study its consequences on photometric measures of affected sources. An accompanying ISR shows that using the gain setting GAIN=2 provides a way to significantly reduce the cross talk strength, while providing no negative side effects on the data (Giavalisco 2004). 2. General Description The effects of the cross talk appear in the ACS WFC CCD frames as artificial ghost images located in mirror-symmetric positions relative to those of the source images, namely real astronomical images in adjacent quadrants of the CCD array that cause the ghosts to appear. Figure 1 shows an example of the ghosts produced by the cross talk in one of the images obtained during the Great Observatories Origins Deep Survey (GOODS) program, an exposure with total integration time Texp=2120 sec in the Chandra Deep Field South taken through the F850LP passband. Source images G1, G2, G3, and G4, which are recorded on quadrant C, produce negative ghost images G1, G2, G3, G4, G1, G2, G3, G4, and G1, G2, G3, G4 in quadrants A, B and D, respectively. Other ghosts are clearly visible throughout the image. Figure 2 shows a similar image of the same field taken through the F606W passband, with a total integration time Texp=1060 sec. Ghost images similar to those in Figure 1 are observed here as well, but they have fainter apparent surface brightness. Figure 3 and 4 show two images of the Tadpole Galaxy with identical exposure time (Texp=1215 sec) and taken through the same passband (F475W). The two images differ in that the sky background is lower in the image in Figure 3 (35 e-/pixel) than that in Figure 4 (70 e-/pixel), because this has been taken at smaller Earth limb angle. A cross-talk ghost image of the galaxy, recorded in quadrant A, is observed in quadrant C in the former image, while no obvious ghost seems to appear in the latter. A close inspection of Figures 1 through 4 reveals a number of interesting features that characterize the cross talk observed in the WFC. 1) The ghost images are rather faint in absolute value. For example, the ghosts of galaxies G1 and G2 in the original.flt files are depressions of about 2 e-/pix relative to the back- 2
ground, which is ~12.2 e-/pixel (the individual.flt images have exposure time Texp=530 sec), or approximately 16%. Figure 1: An exposure in the F850LP passband with Texp=2120 sec obtained during the GOODS ACS program. The cross-talk ghost images of bright sources in quadrant C (e.g. galaxies G1, G2, and G3) are clearly observed in the other quadrants. The stellar image S1, which is brighter than all the other galaxies, produces a very faint ghost in quadrant D and no obviously visible ghosts in A and B. The red arrow points at the faint galaxy GG, which is entirely embedded in the ghost of galaxy G1. A G1 G2 G3 G2 G1 G3 B G4 G4 G4 G4 GG G3 G3 C G1 G2 S1 S1 G2 G1 D 2) The ghost images, when they are observed, are negative, i.e. they appear as depressions relative to the general sky background, and do not appear to bear an obvious imprint of the light profile of the originating source. In other words, if there is a correlation between the surface brightness of the sources and that of the ghosts, it must be relatively weak. Further evidence in favour of this possibility comes from comparing some of the source images. Two of the brightest ghost images, G1 and G2 in Figure 1, are generated by galaxies G1 and G2, whose peak surface brightness is ~6.3 and 4.2 e-/pix respectively. Sources G3 and G4, which have peak surface brightness of ~3.0 and 2.2 e-/pix, respectively, similar (to 3
within a factor of 2) to that of the previous galaxies, produce significantly fainter ghosts. On the other hand, source S1, which is a stellar-like object and has a peak surface brightness of 63.2 e-/pix, also has a very weak ghost, in fact weaker than any of the previous ghosts, in quadrant D and no obviously detected ghost in A and B. Thus, the suspicion here is that the correlation between surface brightness of sources and of ghosts is not only weak, but that it must be somewhat noisy, too. Also, as the case of the star suggests, the absolute value of the surface brightness of the source or its flux (the star is brighter than the three galaxies considered above) are unlikely to be the only relevant parameters in determining the surface brightness of the ghosts. The morphology (spatial extent) seems to be playing a role, too. Figure 2: An exposure in the F606W passband with Texp=1060 sec obtained during the GOODS ACS program. The field of view is the same as Figure 1. Note that the strength of the cross-talk ghosts in this image is significantly reduced compared to the F850LP one. This is due to a marked dependence of the cross talk on the sky background, which is higher in the V-band than it is in the z-band. A B C D 4
3) The light profile of the ghosts does not seem to bear any obvious resemblance to that of the sources. The ghosts appear to be relatively structureless images, i.e. characterized by approximately constant surface brightness. While this observation is qualitative in nature and might be driven by the relatively low S/N of the ghost images, we note that the stellar source considered above has peak surface brightness one order of magnitude higher than the brightest of the galaxies (source G1), and yet its ghost in Figure 1 is similar to those of the galaxies, albeit smaller in size. 4) Sources hosted in some quadrants seem to be generally more effective in generating cross talk than sources located in other quadrants, and some quadrants seem more affected than others by the cross talk. For example, a number of sources in quadrant C and D in Figure 1 have clearly visible ghosts in quadrant D and C, respectively. However, except for the brightest sources of quadrant C, which generate ghosts in all other quadrants, no other ghosts are apparently observed in quadrant A and B, and there seem to be no ghosts of sources in quadrant A and B appearing in C and D. In fact, a bright early-type looking galaxy, which has similar apparent magnitude as some of the cross-talk inducing galaxies of quadrant C, and a nearby stellar source (the brightest one of the whole frame), do not produce any obvious ghost anywhere. Also, a visual inspection suggests that, with the exception of the brightest sources in C, sources in quadrant D are actually more effective in producing ghosts in C than sources in C producing ghosts in D. 5) The cross talk seems more prominent at some wavelengths than others. Figure 2 shows an exposure of the same field as in Figure 1 taken through the F606W passband. A simple comparison shows that the intensity of the ghost images is significantly smaller in the F606W image than in the F850LP. As we will discuss later, this is most likely not a wavelength dependence of the cross talk, but rather the result of the fact that the cross talk strength depends on the background of the image, the sense of the correlation being that the cross talk is stronger in images with fainter background. The sky background of the F850LP image is indeed smaller than that of the F606W one. 6) Sources in images that are identical in terms of target observed, exposure time, and passband may show different cross talk, such as the case of Figure 3 and Figure 4 where a ghost generated by the Tadpole galaxy is observed in the former but not in the latter. A closer inspection of these images shows that the one in Figure 3 has sky background (35 e- /pix) half as strong as the image in Figure 4 (~70 e-/pix), because the latter image was acquired with HST placed at a smaller Earth limb angle. This fact is another manifestation of the dependence of the cross talk on the sky background mentioned in point 5). Baggett et al. (2004) describe the phenomenology of the cross talk observed in the Ultraviolet and Visual (UVIS) channel of Wide Field Camera 3 (WFC3). This instrument 5
is very similar in design to ACS WFC and has similar electronics, although different detectors. Baggett et al. find that WFC3 UVIS images also are affected by cross talk, with characteristics that are very similar to those discussed here. In particular, they find a lack of correlation between the light profile of the sources and that of the ghosts, and the fact that the ghosts light profile is relatively featureless and spanning a limited range of surface brightness (typically, a few electrons). They also find that the strength of the cross talk depends on such things as the electronic bias level, as well as gain setting. Figure 3: An image of the Tadpole Galaxy (UGC10214) in the F475W band (Texp=1215 B A S2 C S1 sec). The ghost images of the galaxy are clearly seen in quadrant B and C, and, possibly, in D. Note Stellar source S1, which in this image is entirely within the ghost image of quadrant C. Stellar source S2 is unaffected by cross talk. D 6
B A S2 C S1 D Figure 4: An image of the Tadpole Galaxy identical to that shown in Figure 3, except that here the sky background is twice as high, because the image was acquired with HST at a much closer Earth limb angle. As a result of the higher sky background, no cross talk ghosts of UGC 10214 are visible in this image. 3. Trends and Correlation In an attempt to provide a quantitative characterization of the cross talk, we have investigated a number of correlations between the cross talk s strength and other variables, as we are about to detail in this section. A first, important analysis to carry out is to measure the correlation, if one exists, between the flux or surface brightness of the ghosts as a function of analogous properties of the sources. Figure 5 shows the relationship between the flux of sources and the average surface brightness (top panel) and the flux (bottom panel) of ghosts. The measurements were 7
carried out as follows. A quadrant was extracted from an.flt image and was used as a source image. A victim image was created by extracting an adjacent quadrant and transposing it so that its pixels are mirror-symmetric with respect to those of the source image. Source photometry was then carried out in dual-image mode with the SExtractor package on both the source and ghost image, using the source image as the detection image. With this procedure, sources are detected in the source image and their isophotes, defined during this detection phase, are used as fixed apertures on the second image to carry out matched photometry. Since there is no need to push the detection to the faintest objects in the frame (only relatively bright sources give origin to cross-talk ghosts), we have set the faintest isophotal level at 1.5 times the rms fluctuations (1 σ) in excess of the sky background, and we also requested that sources had a minimum isophotal area of at least 9 contiguous pixels. In this way, flux is measured at the expected positions of ghosts based on the positions of sources. This provides an accurate measure of the flux of ghosts in absence of intervening sources in the ghost image (in other words, if no accidental sources are recorded within the ghost isophotal apertures defined above, this method provides a measure of the ghost images flux). If accidental sources are present in some of the apertures, they will add scatter to the correlation between source and ghost flux; however, the number of accidental sources within the ghost isophotal apertures should be small. The top panel of Figure 5 shows that there is a correlation between the flux of the sources and the average surface brightness (isophotal flux divided by the isophotal area) of the ghosts. Note that the range of ghost surface brightness is only a few electrons per pixel while the corresponding range of source fluxes spans about three orders of magnitude. Thus, the correlation is relatively weak, and it is also characterized by considerable scatter. This is in general agreement with the discussion in point 2) of the previous section. The bottom panel of the figure shows the correlation between the flux of the sources and that of the ghosts. At faint source fluxes, there is essentially no correlation between sources and ghosts fluxes (Figure 6 shows a blown-up portion of the bottom panel of Figure 5 that illustrates this point). Only bright sources, with fluxes in excess of ~10 5 e - can produce ghosts with appreciable flux. Note that these sources have to be rather big, i.e. have large spatial extension, in order to produce a bright ghost (i.e. ghosts with fluxes well in excess of the rms scatter observed in Figure 6), because the surface brightness of the ghosts remains relatively low in absolute value for any value of the source flux. In other words, the bright ghosts are not produced by sources with high surface brightness, but by sources that are large. And they are bright not because they have high surface brightness, but because they have large area. We conclude this paragraph noticing that the limited dynamic range of the ghosts surface brightness and its absolute value, its week depen- 8
dence on the sources flux, and the fact that this is largely driven by size are all in quantitative agreement with what was observed for WFC3 UVIS (Baggett et al. 2004). Figure 5: The average surface brightness and the flux of cross-talk ghost images as a function of the flux of the sources. Units are e - /pixel for the surface brightness and e - for the flux. The data are from the image shown in Figure 1. Note the very small dynamic range of the ghost surface brightness (a few e - /pixel), despite the run of more than three orders of magnitudes of the source flux. The dotted line represents the global sky background in the victim quadrant. The local sky value has not been subtracted when carrying out the measure of the average surface brightness of the ghosts, but it has been subtracted when measuring their flux. Another measure of the extent of the cross talk is provided by the correlation between the flux in the individual pixels of the source image and the flux in the pixels of the victim image at the corresponding mirror-symmetric positions. One way to measure this correlation from the data is to look at the distribution of flux values in the victim pixels for a 9
given value of the flux in the source pixels. Only pixels that recorded sky background in the victim image are useful for this analysis. The mode and the median of such a distribution are relatively unaffected by pixels that recorded sources, and provide an estimate of the effective background in the presence of cross talk in the victim pixels, i.e. the true sky background of these pixels plus the additive component due to the cross talk, as a function of the source pixels flux. Figures 7 and 8 show both the mode and the median of such a distribution measured from images obtained during the GOODS survey (note that the mode is a noisier estimator of the sky background than the median). We used two repeated exposures, each with Texp=530 sec, of the same area of the sky through the F850LP passband. The two exposures are identical except for the fact that the first image has somewhat higher sky background than the other, because it was acquired at higher Earth limb angle (and also the positions of the two pointings differ by a few pixels due to dithering, but this is inconsequential). Cross talk is clearly observed in both cases as a trend of the victim pixel sky that decreases as a function of the source pixel flux. Interestingly, the cross talk is stronger in Figure 7, which shows the image with fainter sky, as shown by the steeper decrease of the sky background as a function of the source flux. This is better observed in the right panels, which show the case of quadrant D being the source image and C being the victim one. This provides quantitative support of the observation discussed earlier that 10
sources observed in quadrant D seem to be the most effective in producing cross-talk ghosts. Figure 6: A blown-up view of the left portion of the bottom panel of Figure 5. Note that even for sources with fluxes as high as ~10 4 e - the cross talk is not yet detected above the noise of the measure of the ghost flux. The dependence of the strength of the cross talk on the sky background appears to be a general characteristic of this phenomenon. Figure 9 shows the cross talk strength expressed in terms of the difference between the average sky background in areas unaffected by cross talk and areas affected by it as a function of the unaffected background. The data points are the average of a number of sky measures in 100x100 pixel areas, and clearly show that the cross talk decreases when the sky background increases. These data are from the images reproduced in Figures 3 and 4, which show the Tadpole Galaxy (UGC 10214) observed in the F475W passband. The two images, acquired as part of the ACS GTO program of observation (PROP ID=8992), have Texp=1215 sec and are identical 11
except that the former has half the sky background than the latter because it was acquired at smaller Earth limb angle, as discussed earlier. A similar trend with the sky background is seen upon comparison of Figures 7 and 8. Figure 7: The mode and the median of the distribution of ghost pixels as a function of the flux in source pixels. Cross talk is observed as a decreasing trend of both statistics as a function of the source pixels flux. Data are from the image shown in Figure 1. Note that sources hosted in quadrant D are more effective in generating cross-talk ghosts than those in quadrant C. Also note that the mode is a significantly noisier statistics than the median. The dashed horizontal lines represent the global sky background of the victim quadrants (estimated as the mode of the pixel value distribution) and the rms fluctuation below and above it. The dynamic range of the surface brightness of the cross-talk ghosts is small, and the pronounced dependence of the cross-talk strength on the sky background explains the apparent dependence of the cross talk on the wavelength. What happens is that images of a given source taken through different passbands would, in principle, produce very similar 12
ghost images, regardless of the color of the source, because of the shallow correlation between source flux and ghost surface brightness. However, different passbands also result in greatly varying degree of sky background. For example, for a given exposure time an image taken with the F606W passband will have a background roughly 6.7 times larger than an image taken with the F850LP filter. As we have seen, the value of the sky background deeply affects the surface brightness of cross-talk ghosts, much more so than the flux of their sources, and this is the reason of the apparent dependence of the cross talk on the wavelength, as shown in the comparison of Figures 1. We explicitly note that Baggett et al. (2004) report a strong dependence of the cross-talk strength of WFC3 UVIS, which has very similar electronics to that of ACS WFC, with the level of electronic bias used to operate the camera. We do not know if such a dependence is related to that on the sky background discussed here for ACS WFC. The pronounced dependence of the strength of the cross talk on the sky background suggests a possible observing strategy to minimize the effects of the cross talk. This would be to obtain exposures with an exposure time as long as possible, compatible with the need to have enough frames to remove cosmic rays and cosmetic defects, and to provide adequate dithering. The higher value of the sky background in such exposures should result in weaker cross-talk ghost images. Finally, in a companion ISR (Giavalisco et al. 2004), we report the finding that operating ACS WFC with the gain setting GAIN=2 results in significantly weaker cross talk ghosts. Using such a gain setting has no negative consequences on the data quality, and it is as well calibrated as the default setting GAIN=1. A combined 13
strategy of longer exposures with using the GAIN=2 setting seems to be a promising one to keep the incidence of cross talk in the ACS WFC images to a minimum. Figure 8: Similar to Figure 7, except that the data are from an image with the same passband and exposure time, but with higher sky background, because it was acquired with HST at smaller Earth limb angle. Note the smaller cross talk compared to Figure 7. 14
Figure 9: The dependence of the cross talk expressed as the difference of the sky background inside and outside of a cross talk ghost as a function of the sky background in the victim quadrant. The data are averages measured in three different F475W images of the Tadpole Galaxy (GTO ID=8997). 4. Effect on Photometry From an operational point of view it is fundamental to provide a quantitative description of the effects of the cross talk on the photometric performance of the WFC. An example of a question that one would like to answer is whether or not the flux of a source that happens to be located within a cross-talk ghost would be different from the flux that the source would have if it were located outside of the ghost. A major step in this direction is to understand if the cross talk is additive or multiplicative in nature, namely if the areas of depressed sky background that give origin to the ghost images are produced when a component (negative in this case) is added to the image at their pixel positions or if they are the result, for example, of gain variations which in turn 15
cause variations in the count rate of the sky background. In the former case it is expected that the cross talk has no consequences on source photometry, provided that the relative positions of the source under analysis and of the cross-talk ghosts is such that an accurate subtraction of the local sky background is still possible. In the latter case, of course, the photometry of the source would be altered proportionally to the fractional gain variation. Since we have seen that the areas affected by the cross talk can have variations of the sky background up to ~15%, this would provide a bias of unacceptable magnitude in the photometric performance of ACS WFC. To test the additive hypothesis versus the multiplicative one, let s consider a pair of images with the same exposure time and passband where the same compact, bright source (requirements necessary to provide a measure with good S/N ratio) is observed in one case within a cross-talk ghost, and in the other case outside of it. Let s call this a type-a pair. This is obtained, for example, when two images of the same area of the sky have been taken at different position angles, or with a sufficient large dithering step (because the ghosts are mirror images of the sources, their apparent motion as a result of the dithering is different from that of real images). Alternatively, a pair of identical images for which one is affected by cross talk and the other is not, for example, because of higher sky background, can also be used. Let s call this a type-b pair. Figure 3 and 4 provide an example of a type-b pair, whose case we are considering now. In Figure 3 the cross-talk ghost of the Tadpole galaxy is clearly observed in quadrant C. Inside the ghost there is stellar source S1. In Figure 4, which has been acquired without dithering the telescope relative to the position of Figure3, the ghost is not observed, because, as we have discussed earlier, the image has higher sky background. The average sky within the ghost in Figure 3 is 30.0 e - /pixel, and it is 31.4 e - /pixel outside of it, a fractional variation of 4.7%. Aperture photometry of the stellar source S1 shown in Figure 3 shows that the source has flux of 106,203.6, 108,845.3 and 110,150.5 e - in circular apertures of 5, 6 and 7 pixels in radius, respectively. The values of the flux of S1 in Figure 4 are 106,206.6, 108,803.6 and 110,139.5 e - in the same apertures, respectively, which is well within the fractional 1-σ poisson noise of 0.31% for this source. The predicted count differences if the cross talk were multiplicative are 4,988.6, 5,157.4 and 5,188.1 e - for the three apertures, respectively, about a factor of 15.5 larger than the 1-σ poisson noise (~326 e - ). Thus, based on this analysis, cross-talk induced variations of gain (the multiplicative hypothesis) seems to be ruled out at the ~15-σ level! The comparison of photometry of sources in type-a pairs gives similar results. The bottom panel of Figure 10 shows six growth curves of star S1 observed from six individual frames obtained during the program of observations of the Tadpole Galaxy (the image 16
reproduced in Figure 3 is among them), all obtained through the F475W passband. In three of the frames, due to the adopted dithering strategy, the star is located within the Tadpole Galaxy ghost image (the annulus around the photometric aperture is also within the ghost image), while in the remaining three frames the star is outside the ghost image. The observed photometric scatter is typical for a star with this brightness of post-pipeline processed data (.flt files) with no correction for geometrical distortions and no CR removal. In particular, photometric variations of 4-5%, predicted in this case if the cross talk were a multiplicative effect, are not observed. As a comparison, the upper panel shows the growth curves of star S2 (see Figure 3) in the same frame, which has comparable apparent magnitude, but is located in a region completely unaffected by cross talk. The photometry of S2 is characterized by the same amount of scatter as S1, showing that the case of S1 is not by 17
any means peculiar due its being, in three frames, inside a cross-talk ghost. This is consistent with an additive nature of the cross talk and inconsistent with a multiplicative one. Figure 10: Aperture photometry of the stellar sources S1 and S2 shown in Figures 3 and 4 from six dithered exposures in the F475W passband. In three of the frames star S1 is inside the Tadpole Galaxy ghost image, while it is outside of it in the others. Star S2 is unaffected by cross talk in all six exposures. The photometric scatter is typical of post pipeline.flt WFC frames, and no difference is observed in the photometry of the two sources, which have similar apparent magnitude. Finally, Figure 11 shows a similar case for the faint galaxy GG in Figure 1, observed during the GOODS survey. Again, no variation of the photometry of the galaxy could be observed (within the normal photometric error for galaxies of this apparent magnitude) when the galaxy was located within and outside a major cross talk ghost (this observational configuration was possible thanks to the periodic 90-degree rotation of the GOODS fields). 18
Figure 11: Aperture photometry of galaxy GG shown in Figure 1. The three curves correspond to three different images of the same field of view rotated by 45 degree from each other. Galaxy GG falls inside the cross talk of galaxy G1 only in one image (that of Figure 1), and the corresponding curve is colored in red. The photometric scatter is typical of a galaxy of this apparent magnitude. 5. Conclusions We have shown that the cross talk observed among the amplifiers (quadrants) of the ACS WFC produces very faint ghost images whose position mirrors that of the source images. These ghost images appear as depressions of the sky background in their quadrant, and their surface brightness is a very weak, non-linear function of the flux of the source images. Similar findings were also reported by Baggett et al. (2004) to occur in the WFC3 UVIS instrument, which has very similar electronics as ACS WFC. The dynamic range of the ghosts surface brightness cover a range of only several e - /pixel, despite the fact that 19
the sources flux ranges over three orders of magnitudes. The cross talk also seems to be affected by some degree of stochasticity, in that there is a fair degree of noise in the correlation between ghosts surface brightness and sources flux. As a general rule, fairly extended sources produces larger ghosts than brighter sources (even very bright sources) nearly independently of the source s surface brightness. The cross talk strength seems to be a very strong function of the sky background, the sense being that a higher background results in a weaker cross talk. A frame with a sky background of ~70 e - /pixel that we have studied has no obvious presence of cross talk. Baggett et al. (2004) report the finding of similar trend with the level of electronic bias of the CCD of WFC3 UVIS. An observing strategy that produces fewer frames with longer exposure time, and thus higher background (consistently with the need to have enough frames to remove cosmic rays and produce adequate sampling of the telescope s PSF) should minimize the effects of cross talk. Very importantly, the cross talk ghosts are, to within the accuracy of our measures, the results of an additive phenomenon (as opposed to a multiplicative one). As such, it has no practical consequences on photometric measures in the vast majority of cases, as long as the local sky background around the sources can be removed accurately. We also note that we have tried to correct for the effects of the cross talk. The correction consists of adding to each pixel of a victim quadrant an amount of e - calculated from a regression fit to data similar to those presented in the top panel of Figure 5 applied to the value of the corresponding source pixels. Visually, such an algorithm seems to work well in that it eliminates the most obvious ghosts that affect the images. However, we do not encourage at this time the adoption of such corrections. The reason is that the cross talk is a non-linear effect that it is characterized by a significant amount of noise. It also affects only a very small fraction of the pixels in an image. Since it appears to provide additive artifacts to the images, which in most photometric applications are removed when performing the local sky subtraction, we believe that it is preferable not to modify an image by adding to it a correction image that contains a relatively high pixel-to-pixel noise introduced by the intrinsic stochasticity of the cross talk and whose value is derived from a non-linear fit. Finally, in a companion ISR (Giavalisco 2004) we report the finding that operating ACS WFC with the setting GAIN=2 significantly reduces the strength of cross talk, while introducing no negative counter effects on the data quality. A combined strategy of observing with GAIN=2 and of obtaining frames with as long an integration time as possible is likely the most effective way to keep the effects of the cross talk to a minimum. 20
7. Acknowledgments We would like to thank Ron Gilliland for the numerous discussions on the cross talk of the detectors of ACS WFC, and Marco Sirianni for his assistance with the data of program GTO PROPID=8892. 8. References Baggett, S., Hartig, G., & Cheung, E. 2004, Instrument Science Report, WFC3-ISR 2004-11 Giavalisco, M. 2004, STScI Instrument Science Report, ACS-ISR 2004 - XX. Giavalisco et al. 2004, ApJ, 600, 1 Gilliland, R., 2004, STScI Instrument Science Report, ACS-ISR 2004-01. Janesick, J.R., 2001, Scientific Charge-Coupled Devices, SPIE Press. 21