ACS is used primarily for deep, wide-field survey imaging. Important issues for observers to consider are the “packaging” of their observations, how observations are CR-SPLIT
to mitigate the impact of cosmic rays, how sub-stepping or “dithering” of images for removal of hot pixels is implemented, and how, if necessary, to construct a mosaic pattern to map the target. It is also very important to consider the effects of charge transfer efficiency.
Narrowband observations with the WFC are more likely to be read noise limited, requiring consideration of optimum number of readouts
. Observations with the MAMA detectors are not affected by cosmic rays or read noise, but long integration times will often be needed to obtain sufficient signal-to-noise.
At the conclusion of each exposure, the science data are read out from the detector and placed in ACS’s internal buffer memory, where they are stored until they can be transferred to the HST
solid state data recorder (and thereafter to the ground). The internal buffer memory is large enough to hold one WFC image, or sixteen SBC images, and so the buffer typically must be dumped before or during the following WFC exposure. If the following exposure is longer than ~339 seconds, then the buffer dump from the preceding exposure will be performed during integration (see Section 8.2
for a more complete discussion).