The full well capacity for the ACS CCDs is given in Table 4.1 as 84,700 e- for the WFC and 155,000 e- for the HRC. This is somewhat dependent on the position on the chip, with variations of the order 10% and 18% across the field of view for the WFC and the HRC, respectively. If the CCD is over-exposed, blooming will occur. This happens when a pixel becomes full, so excess charge flows into the next pixels along the column. However, extreme overexposure is not believed to cause any long-term damage to the CCDs, so there are no bright object limits for the ACS CCDs. When using GAIN=2 on the WFC and GAIN=4 on the HRC, it has been shown that the detector response remains linear to well under 1% up to the point when the central pixel reaches the full well depth. On-orbit tests have demonstrated that when using aperture photometry and summing over the surrounding pixels affected by bleeding, linearity to 1% holds even for cases in which the central pixel has received up to 10 times the full well depth (see
ACS ISR 2004-01
for details).
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