Study of the evolution of the ACS/WFC sensitivity loss

Instrument Science Report ACS 2013 01 Study of the evolution of the ACS/WFC sensitivity loss Leonardo Úbeda and Jay Anderson Space Telescope Science Institute January 28, 2013 ABSTRACT We present a study of the sensitivity loss of the ACS/WFC CCDs for one medium-band, eight broad-band, and three narrow-band filters. This study was done using a calibration field located 6.7 arcmin West of the center of globular cluster 47 Tucanae. For pre-sm4 images, a comparison of the sensitivity loss rates found in this research with those calculated using standard white dwarf stars by Bohlin, R. et al. (ISR ACS 2011-03) shows excellent agreement within the uncertainties of the two methods. We found that the sensitivity losses are less than 0.0004 mag/year. We also have a baseline of at least three years of post-sm4 observations of the 47 Tucanae calibration field. Our study shows that, on average, the sensitivity loss post- SM4 is negligible. This is a remarkable result considering that ACS is an instrument that has been in space for over ten years and subject to contamination. 1. Introduction The Advanced Camera for Surveys (ACS) Wide Field Camera (WFC) consists of two 4096 2048 charge-coupled devices (CCDs), which are butted together along their long dimension to create an effective 4096 4096 array. The spectral sensitivity of the WFC ranges from 3500Å Copyright c 2013 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. 1

to 11000Å. As part of the yearly calibration program, ACS/WFC observations are taken regularly of flux standard stars and star cluster fields to monitor the photometric performance of the camera and its filters. In this Instrument Science Report (ISR) we intend to quantify the photometric change by analyzing ten years of observations of a particular calibration field in the outskirts of globular cluster 47 Tucanae. We explore here the entire current database of ACS observations in a wide range of filters, using both science programs and special calibration proposals. According to Bohlin, R. et al. (ISR ACS 2011-03) two important events in the life of ACS have introduced discontinuities in the sensitivity evolution. The first event took place on 04 July 2006 and it is the decrease in the ACS operating temperature from 77 C to 81 C in order to lower the number of observed hot pixels. The second event is the CCD Electronics Box Replacement (CEB-R) that took place during Servicing Mission 4 (SM4) in May 2009 in an effort to remedy the ACS failure in January 2007. To account for these changes, we follow the recommendation of ISR ACS 2011-03 and separate analyses for time periods before and after the ACS failure were performed. In this ISR we report the results of our photometric study of the sensitivity of the ACS/WFC using 47 Tucanae data and compare them to the sensitivity loss rates presented in ISR ACS 2011-03 using observations of white dwarf standard stars G191B2B, GD153, and GD71. We also present new measurements of the WFC CCDs sensitivity loss using post-sm4 images. For a complete study of the ACS High Resolution Channel (HRC) please refer to Bohlin, R. et al. (ISR ACS 2011-03). Their study shows no obvious differences for the loss rates between WFC and HRC. 2. Observations The main calibration field for this study is located about 6.7 arcmin West of the center of globular cluster 47 Tucanae. This calibration field has been observed multiple times through many ACS/WFC filters for both calibration and science purposes. We downloaded the entire database of ACS observations from MAST in the following filters: F435W, F475W, F502N, F550M, F555W, F606W, F625W, F658N, F660N, F775W, F814W, and F850LP. The sensitivity of the cameras should be independent of the integration time for each exposure; however, the longer exposures provide much better signal to noise for more stars, so we focus here on the deeper exposures whenever possible All images with integration time shorter than 30 seconds were not included in this study. Some of the images in the Archive were not successful. These include several images from HST proposal 9018 (PI: G. de Marchi) which show all stars as streaks and were not suitable for this study. Most images from proposal 10771 (PI: M. Sirianni) were also discarded. The main goal of proposal 10771 was to test the ACS/WFC operating at three different tempertatures: 74 C, 77 C, and 80 C. For this study we only 2

kept the images obtained at an operating temperature of 77 C. The automated calibration pipeline CALACS (version 8.0.6, 18 Jul 2012) takes care of the basic data reduction (bias, dark, flat field corrections). We downloaded the flat fielded images (_FLT files), as well as the CTE corrected images (_FLC files). The _FLC files are data products generated using the pixel based CTE correction algorithm (PixelCTE v3.2; Anderson & Bedin, 2010). Figure 1 shows the ACS/WFC footprints for all the F606W images that were used (exposure time > 300 sec). The background is an STScI Digitized Sky Survey image depicting the center of globular cluster 47 Tucanae and its Western field. The footprints are organized according to year of observation and shown in red. In Table 1 we list the number of F606W images used for this study (second column) organized by year of observation, as an example. The third column lists the proposal identification numbers from the STScI observing programs. The fourth column gives the total integration time (in units of seconds) for all deep observations. Year Images Proposal IDs Exposure Time (count) (seconds) 2002 5 9018 9656 3735.0 2003 31 9433 9648 9656 14670.0 2004 24 10043 9600.0 2005 43 10368 10569 10737 10771 16045.0 2006 10 10730 10737 3573.0 2009 19 11397 11880 6746.0 2010 126 11677 11887 12385 12389 167114.0 2011 9 12389 12730 12734 3417.0 2012 2 12734 678.0 Table 1: F606W observations used in this study. Shown as an example. 3. Method 3.1 Aperture Photometry The main goal of this project was to compare accurately measured photometry of as many stars as possible (> 1000 stars) in a given field in different epochs and determine whether there is a trend that shows some kind of sensitivity loss. Our analysis is statistical in nature. 3

Figure 1: ACS/WFC footprints for the F606W images with exposure time > 300 seconds used for the calculation of the sensitivity loss. The background is an STScI Digitized Sky Survey image depicting the center of globular cluster 47 Tucanae and its Western field. North is up and East is left. The footprints are organized according to year of observation and shown in red. The orange outline represents the location of the sources in the F606W master initial catalog. To acomplish this task, positions and fluxes of point sources were accurately measured with the software img2xym_wfc.09x10 in the FORTRAN library of codes by J. Anderson. This code includes the best available geometric distortion solution and is documented and described in detail in Anderson, J. & King, I. (ISR ACS 2006-01). This calibration field is not excessively crowded which means that aperture photometry should suffice for our purpose. Aperture pho- 4

tometry was performed on all _FLT and _FLC files for all filters. This task produces an _xym file for each exposure containing the position and magnitude for each found source. Since astrometry is not critical here, the positions are measured with simple centroids. The fluxes are measured using an aperture of 3.5 pixels and a sky between 8 and 12 pixels. The positions are corrected for geometric distortion and the fluxes are corrected for pixel area and converted into instrumental magnitudes ( 2.5 log(flux)) where the flux is given in units of electrons. See Section 4.2 for comments on changes in the PSF encircled energy due to variations in the telescope focus. In July 2006, the temperature of the WFC detector was lowered from 77 C to 81 C in order to mitigate the impact of transfer inefficiencies and the production of hot pixels. This temperature change caused a loss in sensitivity (Mack, J. et al. ISR ACS 2007-02) which we took into account for all the observations made after the temperature change. 3.2 Initial Master Catalogs ACS was installed on board HST in March 2002 during SM3B (ACS Instrument Handbook; Úbeda, L. et al. 2012). This initial epoch is the baseline against which we measure sensitivity loss. For each filter we built photometric catalogs by combining the magnitude measurements of a set of images from these original observations. For most filters, observations from program 9018 provide a wide spatial coverage and enough well-measured stars to create the initial epoch catalogs. In most cases, images from April and May 2002 were used. For narrow band filters there were no observations available from program 9018 and images from programs 9656 (PI: G. de Marchi) and/or 9663 (PI: R. Gilliland) were used instead. Unfortunately, several F606W images from proposal 9018 failed (stars show as short streaks) and therefore, images from proposals 9433 (PI: G. Bernstein), 9648 (PI: A. Riess), and 9656 had to be considered. This means that the initial catalog for F606W consists of the combination of measurements obtained from April 2002 until March 2003. To cross-identify the stars in each exposure with the master list, we used an in-house FORTRAN program from Anderson, J. & King, I. (ISR ACS 2006-01). These cross-identified stars define a linear transformation from the distortion-corrected frame of each exposure into the master frame. We then used a different in-house program to generate a master list of stars that could be found in a minimum number of exposures through each filter. Table 2 shows the number of sources found in each initial master catalog as a function of filter. Column three lists the proposal identification numbers for the observations used and column four shows the range of observation dates assumed as initial epoch. The orange outline in Figure 1 represents the location of the stars in the F606W initial master catalog as an example. 5

Filter Sources (count) Proposal IDs Date Range F850LP 20386 9018 April 2002 F814W 30415 9018 April 2002 May 2002 F775W 20963 9018 April 2002 October 2002 F660N 7166 9656 9663 July 2002 July 2003 F658N 6415 9656 October 2002 August 2003 F625W 10033 9018 9656 April 2002 December 2002 F606W 19003 9018 9433 9648 9656 April 2002 March 2003 F555W 14561 9018 May 2002 F550M 15570 9656 9663 July 2002 July 2003 F502N 5328 9648 9656 September 2002 August 2003 F475W 8264 9656 October 2002 August 2003 F435W 9474 9018 May 2002 Table 2: Summary of the initial master catalogs. The number of stars in each catalog are listed as well as the range of observation dates assumed as initial epoch. Whenever possible, images from GO program 9018 were used. 3.3 Position/Magnitude Correlation. The Relative Photometry Once we had the master catalog of stars for each filter, we went back and cross-identified the stars in the individual exposures. We included only those stars that were bright but not saturated by selecting sources with flux (in units of electrons) such that 13.0 < 2.5 log(flux) < 9.0 or 4000 flux 150000 electrons. Although fluxes for saturated stars can be measured reliably using approaches described in Gilliland, R. L. (ISR ACS 2004-01), we focus here on the unsaturated stars, since bleeding can take flux out of our apertures. The _xym files contain many false detections, such as cosmic rays, hot pixels, or features around saturated stars. These detections are weeded out by our cross-identification procedure. 4. Results For each exposure, the cross-identification procedure above gives us a list of stars with photometry in the exposure and average photometry from the master list. Figure 2 shows an example of our typical magnitude residuals for 1300 matched stars from F775W image j8hm01yrq_flt.fits observed on 16 March 2003 and the F775W initial master catalog. This plot clearly shows a larger dispersion for fainter objects as expected. The adopted mean value was calculated using an iteratively sigma clipping flux weighted method. We used σ = 2.0 to reject possible bad outliers clearly seen in Figure 2. The final mean value is shown as a red horizontal line. The dotted lines represent the adopted error in the calculated average. This 6

average magnitude difference ( m) gives us the information that we need to measure whether a sensitivity trend exists or not. We analyze the average magnitude difference ( m) as a function of time for each filter. Since observations have different exposure times, we scaled the averages to an exposure time of 1000 seconds. We also shifted the plots so that m = 0.0 at the time of the installation of the ACS on board HST (07 March 2002) during Servicing Mission 3B. Figure 3 shows the results for broad-band filters F814W, F775W, F625W, and F606W. Figure 4 shows the results for broad-band filters F555W, F475W, and F435W and for medium-band filter F550M. Figure 5 shows the results for narrow-band filters F660N, F658N, and F502N, and for wide filter F850LP. Figure 2: Typical magnitude residual plot before scaling for exposure time. It shows the difference in magnitude of 1300 matched stars from F775W image j8hm01yrq_flt.fits (m) and the F775W initial master catalog (m cat ). The horizontal axis represents the average of the two magnitudes. The horizontal red line represents the adopted average. Each plot shows the average magnitude difference ( m) as a function of time in units of years. The plots on the left were created with the _FLT files as input. The plots on the right were created with the _FLC files as input. A dashed line through m = 0.0 is displayed to aid the eye. The symbols are represented in two shades of blue: the light blue symbols represent short exposure observations (exposure time lower that 300 seconds); the dark blue symbols represent long exposures (exposure time > 300 seconds). All plots span the lifetime of ACS/WFC: from March 2002 until the present, through failure in January 2007 and successful restoration during Servicing Mission 4 (SM4) in May 2009. Because of the electronics change in May 2009 the time period before and after the ACS failure must be analyzed separately (ISR ACS 2011 03). 4.1 Analysis and Discussion of pre-sm4 data Sensitivity changes of pre-sm4 data have already been published and presented in ISR ACS 2011 03. Table 1 in that document lists the rate of sensitivity loss in magnitudes per year for all the filters that we analyze in this ISR. The losses are less than 0.003 mag/year, and were calculated using high precision aperture photometry of three white dwarf primary standard stars, namely G191B2B, GD153, and GD71. Charge transfer inefficiencies have been taken into account in this study using a parameterized CTE correction formula from ISR ACS 2011 01. 7

The left panels of Figures 3, 4, and 5 show the results of the study performed on 47 Tuc _FLT images (no CTE correction applied). The change in sensitivity for each filter is fit with a line shown in red. The orange lines represent the white dwarf fit from ISR ACS 2011 03. The estimated slopes (in units of magnitudes/year) and formal errors are shown in Table 3. The fitting was performed by minimizing the χ 2 error statistic. The error assigned to the point that represents each image is given by 1σ of the distribution. Measurements performed using short exposures (integration time < 300 seconds) are less reliable and, for that reason, their errors were arbitrarily doubled. This method assigns a higher weight to measurements done on long exposures. The right panels of Figures 3, 4, and 5 show the results of the study performed on 47 Tuc _FLC images. These images have been corrected using the pixel-based CTE correction. The estimated slopes (in units of magnitudes/year) and formal errors are shown in Table 3 In general, the 47 Tuc sensitivity loss rates are in agreement with the WD method values within the errors. This is true specially for filters F435W, F475W, F502N, F550M, F625W, F775W, and F814W. Narrow-band filters F658N and F660N show slightly worse sensitivity losses. The study of filter F606W shows a perfect agreement with the WD method if the _FLT files are considered. If the correction for ineffective CTE is taken into account, the 47 Tuc method shows that the sensitivity loss is somewhat smaller. Filter WD Study 47 Tuc Study APPHOT FLT APPHOT FLC epsf FLC F435W -0.0028-0.0042 (0.0006) -0.0027 (0.0006) F475W -0.0028-0.0064 (0.0018) -0.0034 (0.0018) F502N -0.0028-0.0045 (0.0010) -0.0012 (0.0010) F550M -0.0029-0.0047 (0.0010) -0.0024 (0.0010) F555W -0.0029-0.0049 (0.0012) -0.0018 (0.0012) F606W -0.0028-0.0032 (0.0004) -0.0011 (0.0004) -0.0015 (0.0004) F625W -0.0027-0.0061 (0.0015) -0.0030 (0.0015) F658N -0.0025-0.0059 (0.0018) -0.0039 (0.0018) F660N -0.0025-0.0087 (0.0015) -0.0039 (0.0015) F775W -0.0014-0.0039 (0.0007) -0.0016 (0.0007) -0.0008 (0.0007) F814W -0.0009-0.0052 (0.0012) -0.0019 (0.0012) -0.0022 (0.0012) F850LP 0.0000-0.0028 (0.0006) -0.0014 (0.0006) Table 3: Estimated pre-sm4 sensitivity losses per year as a function of ACS/WFC filter for _FLT and _FLC images. The errors are between parentheses. The losses and their errors are expressed in units of magnitudes per year. The values in the second column are taken from ISR ACS 2011-03. These are the values of the fourth order polynomial fit and have a formal error of 0.0008 mag/year. 8

Figure 3: Magnitude residuals average as a function of time for filters F814W, F775W, F625W, and F606W. Dark blue symbols correspond to deep observations (exposure time > 300 sec) and short exposures are shown in light blue. The orange line represents the pre-sm4 fitting using the standard white dwarf stars from ISR ACS 2011 03. The red line is the best fit of the 47 Tucanae study. 4.2 Telescope Breathing and Changes in Focus Having selected an aperture radius of 3.5 WFC pixels (0.175) for the aperture photometry means that changes in the PSF encircled energy due to telescope focus variations will be present in our measurements. In order to quantify this effect and verify our method we repeated the study for filters F814W and F775W using epsf photometry (ISR ACS 2006 01) with the flag PERT activated. This allows the code to find a spatially constant perturbation to the library of PSFs to account for focus variations that impact the inner 5.0 pixels of the PSF. The final flux 9

Figure 4: Magnitude residuals average as a function of time for filters F555W, F550M, F475W, and F435W. Dark blue symbols correspond to deep observations (exposure time > 300 sec) and short exposures are shown in light blue. The orange line represents the pre-sm4 fitting using the standard white dwarf stars from ISR ACS 2011 03. The red line is the best fit of the 47 Tucanae study. is calculated with respect to a 10.0 pixel aperture. Figure 6 shows a comparison between aperture and epsf photometry for filter F814W. All the points correspond to deep exposures (integration time > 300 seconds). It is evident (see bottom panels) that in some cases, observations made on the same day present m variations of up to 0.01 mag. The same measurements made using epsf photometry show lower dispersion becasue the telescope focus variations are taken into account. However, a similar sensitivity loss is found for both methods. The epsf method averages out the focus variations before the linear fitting and the statistical approach presented in this ISR averages out during the χ 2 error 10

Figure 5: Magnitude residuals average as a function of time for filters F850LP, F660N, F658N, and F502N. Dark blue symbols correspond to deep observations (exposure time > 300 sec) and short exposures are shown in light blue. The orange line represents the pre-sm4 fitting using the standard white dwarf stars from ISR ACS 2011 03. The red line is the best fit of the 47 Tucanae study. Note that the CTE corrections for narrow-band filters F660N and F658N are large because of the low backgrounds detected in those images. This is specially important for post-sm4 data. statistic fitting. Figure 7 shows an equivalent study for filter F775W. Note that both deep and short exposures were considered for this filter. The lower dispersion in the epsf study is quite evident. The final fitting of the _FLC images shows virtually the same sensitivity loss. In Table 3 we report the values of the sensitivity loss estimated using epsf photometry in three filters. 11

Figure 6: Comparison between aperture (bottom) and epsf (top) photometry for images obtained with filter F814W. Figure 7: Comparison between aperture (bottom) and epsf (top) photometry for images obtained with filter F775W. Note that the epsf photometry shows lower dispersion, because this type of photometry accounts for telescope-focus variations. See text for more details. 12

4.3 Analysis and Discussion of post-sm4 data During Servicing Mission 4 (SM4) in May 2009 the CCD Electronics Box Replacement (CEB- R) took place and the gain of the new electronics was set so that the measured signal matched the signal for the same 47 Tucanae field as measured in 2002. Due to the lack of sufficient data, Bohlin, R. et al. (2011) adopt a slope arbitrarily set to zero for the post-sm4 points in the [ m, time] plane. For most filters we now have at least three years of observations of the calibration field and we are able to quantify the sensitivity of the WFC post-sm4 and provide information on the current status of the CCDs. Filter 47 Tuc Study FLC F435W 0.0004 (0.0005) F475W -0.0004 (0.0006) F502N 0.0002 (0.0003) F550M 0.0004 (0.0004) F555W -0.0001 (0.0005) F606W 0.0002 (0.0001) F625W -0.0003 (0.0008) F658N -0.0001 (0.0006) F660N 0.0008 (0.0004) F775W 0.0000 (0.0005) F814W 0.0002 (0.0003) F850LP 0.0000 (0.0004) Table 4: Estimated sensitivity losses per year as a function of ACS/WFC filter for _FLC post-sm4 data. The errors from the χ 2 error statistic fitting are between parentheses. The losses and their errors are expressed in units of magnitudes per year. We adopt the same initial master catalogs that were used for the pre-sm4 data. Figures 3, 4, and 5 show the results of the study performed on 47 Tuc images. By comparing the left plots (_FLT images) with the plots on the right (_FLC images), a first impression shows the effect of the CTE correction as a function of time. The CTE correction is larger for post-sm4 data than for pre-sm4 data as shown in Úbeda, L. & Anderson, J. (ISR ACS 2012-03). With the current study we are able to confirm those results. Moreover, it is very clear that the point distributions post-sm4 are virtually flat for all filters. The change in sensitivity for each filter was fit with a line by minimizing the χ 2 error statistic. The estimated slopes (in units of magnitudes/year) and formal errors are shown in Table 4. These results show that the loss in sensitivity post-sm4 is negligible. 13

5 Conclusions We performed a study of the sensitivity loss of the ACS/WFC CCDs using twelve filters. This study was done using a calibration field about 6.7 arcmin West of the center of globular cluster 47 Tucanae. For data obtained before SM4, a comparison of the sensitivity loss rates with those calculated using standard white dwarf stars by Bohlin, et al. (2011) shows excellent agreement within the uncertainties of both methods as can be seen in Figure 8. The only large discrepancy corresponds to filter F606W where our study shows a lower sensitivity loss ( 0.0011 mag/year as opposed to 0.0028 mag/year). The use of epsf photometry leads to the value 0.0015 mag/year. Refer to Table 3. The advantage of the pixel-based CTE correction is clearly seen in Figures 3, 4, and 5. In particular, those plots show the CTE dependency with date of observation as well as exposure time. Our results confirm the study of the CTE evolution in ISR ACS 2012-03. Figure 8: Pre-SM4 slopes in units of magnitudes per year for the twelve filters that were analyzed with the 47 Tucanae data. The fourth order fit from Bohlin, R. et al. (2011) is shown as a black line with the uncertainty shown as dashed lines. Error bars on each point are taken from Table 3. Most of the error bars cross the fit with the exception of F606W. We now have a baseline of at least three years of post-sm4 observations of the 47 Tucanae calibration field. Our study shows that, on average, the magnitude difference ( m) is stable over time. This is a remarkable result considering that ACS is an instrument that has been in space for over ten years and subject to contamination. A future research using the white dwarf standards on a longer time baseline would be able to confirm or improve the loss rates presented in this ISR. No corrections to the observed photometry are necessary because the current PHOTFLAM header keywork in the ACS/WFC images is calculated as a function of time using the formulae provided by Bohlin, R. et al. (2011). 14

Acknowledgments The authors would like to thank L. Smith (ACS Lead), R. Bohlin, A. Maybhate, V. Kozhurina- Platais, and M. Chiaberge for their comments and revision suggestions that resulted in substantial improvement of this work. The authors acknowledge J. Mack, P-L. Lim, and R. Avila for their past contributions to this project. References Anderson, J. & Bedin, L. R. 2010, PASP, 122, 1035 1064 Anderson, J. & King, I. R., 2006, Instrument Science Report ACS 2006-01 (Baltimore: STScI) Bohlin, R. & Anderson, J. 2011, Instrument Science Report ACS 2011-01 (Baltimore: STScI) Bohlin, R. et al. 2011, Instrument Science Report ACS 2011-03 (Baltimore: STScI) Gilliland, R. L. 2004, Instrument Science Report ACS 2004-01 (Baltimore: STScI) Mack, J. et al. 2007, Instrument Science Report ACS 2007-02 (Baltimore: STScI) Úbeda, L. & Anderson, J. 2012, Instrument Science Report ACS 2012-03 (Baltimore: STScI) Úbeda, L. et al. 2012, ACS Instrument Handbook, Version 12.0 (Baltimore: STScI) 15