3.4 Basic Instrument Operations

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3.4.1 Target Acquisitions

For the majority of ACS observations target acquisition is simply a matter of defining the appropriate aperture for the observation. Once the telescope acquires its guide stars, your target will be within ~1 to 2 arcseconds of the specified pointing. For observations with the ramp filters, one must specify the desired central wavelength for the observation. For the special case of coronagraphic observations, an onboard target acquisition will need to be specified. The nominal accuracy of the onboard target acquisition process is ~6 milliarcseconds.

3.4.2 Typical ACS Observing Sequence

ACS is expected to be used primarily for deep, wide-field survey imaging. Important issues for observers to consider will be 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. HRC observations and narrowband observations with the WFC are more likely to be readnoise limited, requiring consideration of optimum number of readouts. Observations with the MAMA detectors are not affected by cosmic rays or readnoise, but long integration times will often be needed to obtain sufficient signal-to-noise.

A typical ACS observing sequence is expected to consist of a series of 10 to 20 minute dithered exposures for each program filter. Coronagraphic observations will require an initial target acquisition observation to permit centering of the target under the occulting mask. Observers will generally not take their own calibration exposures. See Chapter 7 for more details about observing strategies.

3.4.3 Data Storage and Transfer

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.