ACS uses the following detectors in each channel:
The ACS CCDs are thinned, backside-illuminated full-frame devices cooled by thermo-electric cooler (TEC) stacks housed in sealed, evacuated dewars with fused silica windows. The spectral response of the WFC CCDs is optimized for imaging at visible to near-IR wavelengths, while the HRC CCD spectral response is optimized specifically for near-UV wavelengths. Both CCD cameras produce a time-integrated image in the ACCUM data-taking mode. As with all CCD detectors, there is noise (readout noise) and time (read time) associated with reading out the detector following an exposure.
The minimum exposure time is 0.1 seconds for the HRC and 0.5 seconds for the WFC. Between successive identical exposures, the minimum time is 45 seconds for the HRC, and 135 seconds for the WFC, for full-frame readouts. However, this can be reduced to as little as ~35 seconds for WFC subarray readouts. The dynamic range for a single exposure is ultimately limited by the depth of the CCD full well (~85,000 e- for the WFC and 155,000 e- for the HRC), which determines the total amount of charge that Section 7.2 can accumulate in any one pixel during an exposure without saturation. Cosmic rays will affect all CCD exposures. CCD observations should be broken into multiple exposures whenever possible to allow removal of cosmic rays in post-observation data processing. During Phase II you can use the CR-SPLIT
optional parameter or dithering to do this (See Section 4.3.6).
The SBC MAMA is a photon-counting detector which provides a two-dimensional ultraviolet capability. It can only be operated in ACCUM
mode. In order to preserve its functionality, the SBC MAMA detector is subject to both scientific and absolute brightness limits. At high local ( 50 counts/second/pixel) and global (> 200,000 counts/second) illumination rates, counting becomes nonlinear in a way that is not correctable. At only slightly higher illumination rates, the MAMA detectors are subject to damage. We have therefore defined absolute local and global count rate limits, which translate to a set of configuration-dependent bright-object screening limits. Sources which violate the screening limits in a given configuration cannot be observed in those configurations, as discussed in Section 4.6 and Section 7.2.
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