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Mitigation of CTE Losses in ACS/WFC: ! Overview of Methods!
Linda Smith, Jay Anderson, Roberto Avila, Ralph Bohlin, Marco Chiaberge, David Golimowski, Shireen Gonzaga, Norman Grogin, Ray Lucas, Aparna Maybhate, Matt McMaster, Sara Ogaz, Josh Sokol, Leonardo Ubeda! ! Space Telescope Science Institute, Baltimore MD! !
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(CTE) of the ACS/WFC CCD dete! ors is declining with time due to the ct ! damage. The methods that have recently become available to observers reviewed. These include post-observation corrections, adjusting the ze CTE losses, and the use of a short post-flash to increase the The photometric CTE correction formula of Chiaberge (2012) is compared with the pixel-based CTE correction in CALACS in Fig. 1. The two methods agree well for typical ACS backgrounds but the photometric correction formula produces more accurate results for faint stars when the background is very low.! A web-based photometric CTE correction tool will shortly be available on the ACS webpage.!

Putting the electrons back where they belong!

ABSTRACT!

The charge transfer efficiency cumulative effects of radiation to mitigate CTE losses are observing strategy to minimi background.! ! !

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INTRODUCTION!
! The Advanced Camera for Surveys (ACS) was installed on the Hubble Space Telescope (HST) in March 2002. The charge transfer efficiency (CTE) of the two CCDs in the Wide Field Channel (WFC) is steadily declining with time due to the harsh radiation environment. Energetic particles damage the silicon lattice of the CCD detectors and create charge traps, leading to the characteristic charge trails seen extending away from bright stars, hot pixels and cosmic rays in ACS images. CTE losses are a strong function of the background level because many of the charge traps are filled for higher backgrounds as the pixels are read out.! ! The declining CTE impacts the science that can be obtained by altering the photometric, astrometric and morphological characteristics of sources, particularly those farthest from the readout amplifiers. ! ! CTE losses can be mitigated using two different approaches: ! ! (1) post-observation processing using either a pixel-based correction algorithm to restore the charge in the pixels, or correcting the photometry to account for the losses ! (2) adjusting the observing strategy to increase the charge transfer efficiency during the CCD readout process.! ! !

ADJUSTING THE OBSERVING STRATEGY!
Several strategies can be employed to mitigate CTE losses:! a) Moving the Source Position! The simplest technique is to place the source near a readout amplifier to reduce the number of transfers. This can be accomplished by using the aperture WFC1-CTE.! b) Increasing the Exposure Time! If using the WFC1-CTE aperture is not possible (e.g. if the source extends > 5 arcsec), then the sky background should be estimated using the information given in Sokol et al. (2012) [See poster 334-07]. Empirical sky backgrounds are provided for all ACS filters as a function of exposure time and compared to Exposure Time Calculator (ETC) estimates. ! Observers should determine if the sky background is above 20 e- for a given exposure time. If the background is lower than this value, CTE losses are likely to be severe. The simplest way to increase the background is to increase the exposure time. ! c) Using Post-Flash! If it is not possible to move the source position or to increase the exposure time, then a short (< 5 s) post-flash exposure can be added at the end of an exposure to increase the background. The ACS Team is investigating the use of the post-flash capability and details are provided in Ogaz et al. [Poster 334-09].! Calibration observations of a field in Omega Cen were taken with different post-flash exposures to measure the improvement in CTE losses. The sky background was varied from 8-215 e- for a series of short (6 s) post-flashed exposures and the fluxes in faint stars were compared to long exposures where CTE losses are minimal. As shown in Poster 334-09, there is a large improvement in CTE losses for a sky background between 25-40 e-.! The use of the post-flash will, however, degrade the signal-to-noise of the data, depending on the level of the required background. Observers should also bear in mind that the post-flash illumination is nonuniform with a gradient of ~50% across the image. While the post-flash illumination pattern will be subtracted in CALACS using a scaled reference file, the improvement in CTE losses and S/N degradation will vary across the image.! ! Further information on the mitigation of CTE losses can be found in MacKenty & Smith (2012) for WFC3 and ACS.! Updates and recommendations on the ACS post-flash capability will be available on the ACS web page (http://www.stsci.edu/hst/acs).! !

POST-OBSERVATION PROCESSING!
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The CALACS pipeline now employs a pixel-based CTE correction algorithm based on the work of Anderson & Bedin (2010). This correction method works well when CTE losses are not severe. Extensive testing has shown that for typical ACS backgrounds (> 50 e-), the correction algorithm has a 75% reconstruction accuracy. For very low backgrounds (< 20 e-), 90% of the charge can be lost as the CCD is read out, and these large losses cannot be reconstructed.!

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An alternative technique for point sources is to apply a formula to correct the measured photometry for CTE losses, depending on CCD position, background, flux and observation date. An entirely new correction formula has been derived for post-SM4 data (Chiaberge 2012). Formulae of this type are also not able to estimate accurate fluxes for objects that have been severely trailed beyond recognition. ! !

Anderson, J. & Bedin, L. R. 2010, PASP, 122, 1035­1064! Chiaberge, M, 2012, ACS ISR 2012-05! MacKenty, J. & Smith, L.J. 2012, CTE White Paper! http://www.stsci.edu/hst/wfc3/ins_performance/CTE/CTE_White_Paper.pdf! Sokol, J, Anderson, J. & Smith L, 2012, ACS ISR 2012-04! !
Figure 1: C correction in transfers for box) but the omparison of the new photometric CTE correction formula (Chiaberge 2012) with the pixel-based CTE CALACS for observations of stars in 47 Tuc. The y-axis shows the residual magnitude losses at Y=2000 stars in different flux bins. The two methods agree well for relatively high background levels (right hand photometric correction formula produces more accurate results at very low sky levels.!

REFERENCES!