At the conclusion of each exposure, the science data are read out from the detector and placed in ACS's internal buffer memory, where it is stored until it 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 HRC or SBC images, and so the buffer will typically need to 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).
ACS's internal buffer stores the data in a 16 bit-per-pixel format. This structure imposes a maximum of 65,535 counts per pixel. For the MAMA detectors this maximum is equivalent to a limit on the total number of detected photons per pixel which can be accumulated in a single exposure. For the WFC and HRC, the 16 bit buffer format (and not the full well) limits the photons per pixel which can be accumulated without saturating in a single exposure when GAIN
=1 for WFC, and GAIN
=2
for the HRC is selected. See Chapter 4 and Chapter 7 for a detailed description of ACS instrument operations.
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