4.6	ACS/WFC CCD Detector Charge Transfer Efficiency (CTE)

4.6.1 The Issue

4.6.2 Improving CTE: Considerations Before Making the Observations

4.6.3 Improving CTE: Post-Observation Image Restoration

4.6.1 The Issue

The ACS/WFC CCD detectors operate by the simple process of converting incoming photons into electron/hole pairs, collecting the electrons in each pixel, and then transferring those electrons across the detector array during the device readout. The transfer process moves each pixel’s electrons down along the columns and then across in a transfer register to the amplifier located in the corner of the detector array.

When the detectors were manufactured, these transfers were extremely efficient (typically 0.999996 of each charge packet was transferred successfully from one pixel into the next), which means that slightly over 99% of the charge collected in a pixel would be delivered to the transfer register. Once in space, the flux of energetic particles such as relativistic protons and electrons damages the silicon lattice of the CCD detectors. This creates both “hot” pixels and charge traps. This radiation damage is cumulative and was unavoidable given current technologies for detector construction and shielding.

The charge traps degrade the efficiency with which charge is transferred from pixel to pixel during the readout of the CCD array. This is seen directly, as shown in Figure 4.24, in the “charge trails” that follow hot pixels, cosmic rays, and bright stars that can extend to over 50 pixels in length.

Figure 4.24: CTE Trails

A section (800 x 800) of an ACS frame of 47 Tucanae. Note the presence of trails extending from the stars indicating the effect of CTE on the detector.

4.6.2 Improving CTE: Considerations Before Making the Observations

The simplest mitigation of the imperfect CTE is to reduce the number of charge transfers required for a given source to reach the readout amplifier on the CCD. If the source of interest is small, placing it close to the corner of the detector will result in greatly enhanced net CTE.

CTE is a strong function of the signal level in the pixels through which a charge packet must pass on its way to the transfer register.

Observations with very low background (<100 e− for ACS) will suffer large losses for very faint sources. This is likely to be problematic for narrow band filters and observations in the UV where the background is very low. In these cases, raising the background will greatly improve the CTE and thus the S/N of these sources. For users planning to stack multiple images to reach very faint limits, they should plan to achieve a background level of ~100 e- for ACS.

The background can be increased in several ways:

1.	Longer exposure times

2.	Selection of a broader filter

3.	Selection of the other instrument

4.	the addition of internally generated photons (i.e., “post-flash”).

ACS/WFC contains LED lamps configured to illuminate the side of the shutter blade that faces the CCD detector. Designed to provide fairly even illumination at low signal level, these lamps provide a “post-flash” capability. The post-flash lamp can be used to increase the background in an image. While doing something that adds noise to an image may seem counter-productive from a S/N perspective, it will significantly increase the efficiency of charge transfer for low-level sources when the background is otherwise very low. As a result, it can increase the signal much faster than it increases the noise.

4.6.3 Improving CTE: Post-Observation Image Restoration

The ACS team has developed and implemented a post-observation correction algorithm based upon the Anderson and Bedin (2010PASP..122.1035A) methodology. This empirical algorithm first develops a model to reproduce the observed trails, then inverts the model to convert observed pixel values in any image to an estimate of the original pixel values, undoing the effects of the degraded CTE.

The original version of the code worked very well for intermediate to high flux levels (> 200 electrons). Improvements in the new version of the code have made corrections more effective at low flux levels (< 100 electrons), and employs more accurate time- and temperature-dependent corrections for CTE over ACS’s lifetime. Recent developments on this subject can be found in Bohlin, R. & Anderson, J. (ACS ISR 2011-01); Ubeda, L. & Anderson, J. (ACS ISR 2012-03); and Sokol, J. et al. (ACS ISR 2012-04).

While this algorithm does a good job of removing trails behind stars, cosmic rays, and hot pixels, it has one serious and fundamental limitation: it cannot restore the lost S/N in the image. Besides this problem, the reconstruction algorithm provides the best understanding of the “original” image before the transfer, and also helps understand how the value of each pixel may have been modified by the transfer process. This algorithm is available in the ACS pipeline; standard calibrated products are now available both with and without this correction.

Figure 4.25: An Example of the Pixel-Based CCD Corrections

(Left) A 1000 x 1000 pixel region at the top of the chip 1 extension in image jbmncoakq_flt.fits. CTE vertical trails are clearly visible. (Right) The reconstructed CTE-corrected flc.fits image after the execution of calacs.

An alternate method for post-observation restoration involves a simple recalibration of the photometry using correction curves that have been provided in Chiaberge M. (ACS ISR 2012-05). This can be effective for isolated point sources on flat backgrounds, but is less effective for extended sources or sources in crowded regions. Please refer to Section 5.1.5 for more details

These empirical corrections, available as an online tool at the ACS Web site, are also useful for planning observations: they allow an estimate of the CTE losses for point-like sources that can be expected in a near-future observation for a given background and source flux. The expected losses should be taken into consideration for a decision on the best CTE mitigation strategy, and if necessary, to adjust integration times to meet signal to noise requirements.