1.2 Basic Instrument Operations
1.2.1 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, the target will be within ~1-2 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 coronographic observations, an onboard target acquisition will need to be specified. As of writing this document, the nominal accuracy of the onboard target acquisition process is expected to be 0.05 arcseconds, comparable to that achieved by STIS.
1.2.2 Typical ACS Observing Sequence
ACS is expected to be used primarily for deep, wide field survey imaging. The important issues for observers to consider will be the "packaging" of their observations, i.e. how observations are CR-SPLIT to mitigate the impact of cosmic rays, whether sub-stepping or "dithering" of images is required, and how, if necessary, to construct a mosaic pattern to map the target. Keeping in mind the ~500 sec time required between successive exposures, observers should plan a dither strategy to maximize the efficiency of their observations. (Refer to the ACS Instrument Science Report 01-07, ACS Dither and Mosaic Pointing Patterns, and to the HST Dither Handbook for a detailed discussion.)
HRC observations and narrowband observations with the WFC are more likely to be read-noise limited, requiring consideration of the optimum CR-SPLIT times. Observations with the MAMA detectors do not suffer from cosmic rays or read noise, but long integration times will often be needed to obtain sufficient signal-to-noise in the photon-starved ultraviolet.
A typical ACS observing sequence is expected to consist of a series of CR-SPLIT and dithered ~10-20 minute exposures for each program filter. Coronographic 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.
1.2.3 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 they are 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 during the following WFC exposure, assuming it is longer than 340 seconds. For shorter exposures an extra overhead of this length is imposed.
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 full well (and not the 16 bit buffer format) limits the photons per pixel which can be accumulated without saturating in a single exposure when GAIN > 1 and GAIN > 2, respectively, are selected.
1.2.4 Parallel Operations
Parallel observations with the WFC and HRC are possible with ACS for certain filter combinations. ACS can be used in parallel with any of the other science instruments on HST within restrictions described in detail in ACS Instrument Handbook. There are significant constraints on the use of the MAMA detectors in parallel. The policy for applying for parallel observing time is described in the Call for Proposals.
