Part II: ACS Data Handbook

1.2 Basic Instrument Operations

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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 coronagraphic observations, an onboard target acquisition will need to be specified. The nominal accuracy of the combined target acquisition and slew procedure is ~0.03 arcseconds, comparable to that achieved by STIS.

1.2.2 Typical ACS Observing Sequence

An important issue for observers to consider is 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. For an online library of pointing patterns, refer to the ACS dither web page:

http://www.stsci.edu/hst/acs/proposing/dither

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 are often needed to obtain sufficient signal-to-noise in the photon-starved ultraviolet.

A typical ACS observing sequence consists of a series of CR-SPLIT and dithered ~10-20 minute exposures for each program filter. Coronagraphic observations require an initial target acquisition observation to permit centering of the target under the occulting mask. Observers generally need 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 the 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 HST Call for Proposals.

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