|ACS Data Handbook v. 9.0|
1.2.1 Target AcquisitionsFor most ACS observations, target acquisition is simply specified by the observing aperture in the Phase II proposal. Once the telescope acquires its guide stars, the target will be within ~1–2 arcseconds of the required pointing. For observations using ramp filters, only the desired central wavelength is required to define the acquisition aperture.For HRC coronagraphic observations, an onboard target acquisition was necessary. The nominal accuracy of the combined target acquisition and slew procedure was ~0.03 arcseconds.WFC users (and erstwhile HRC users) must consider "packaging" their exposures to mitigate the impact of cosmic rays either by dithering their images or constructing a mosaic observing pattern to cover an extended target. SBC exposures do not suffer the effects of cosmic rays or read noise, but long integration times are often needed to obtain sufficient signal-to-noise in photon-starved bluer and/or narrow-band filters. Typical WFC and HRC observing sequences consist of a series of "CR-SPLIT" and dithered 10–20 minute exposures for each program filter. Detailed information about dither and mosaic strategies can be obtained from the ACS Dither Web Page and the DrizzlePac website.HRC coronagraphic observations required an initial target acquisition observation to permit centering of the target under the occulting mask. Observers were encouraged but not required to obtain contemporaneous images of coronagraphic PSF-reference stars in order to subtract residual scattered light and enhance the contrast of their science images.At the conclusion of each exposure, science data are read out from the detector and stored in ACS's internal buffer memory until they can be transferred to HST's solid state data recorder (and thereafter to the ground). The internal buffer memory is large enough to hold one full frame WFC image, or sixteen HRC or SBC images. Dumping a full ACS buffer requires 339 seconds; dumping a partially filled buffer (if, e.g., reading out the following exposure would over-fill it) requires proportionately less time. Buffer dumps can occur concurrently with a following exposure if its duration exceeds the buffer-dump duration (typically 339 seconds, corresponding to an entirely filled buffer). Otherwise, an observational overhead is imposed between exposures to clear the buffer for the next exposure.ACS's internal buffer stores data in a 16 bit per pixel format. This structure imposes a maximum of 65,535 counts per pixel. For the SBC, this is the maximum limit on the total number of detected photons per pixel that can be accumulated in a single exposure. For WFC and HRC gain settings above 2 e−/DN, it is the depth of the pixel well (and not the 16 bit buffer format) that limits the number of photons that can be accumulated without saturating the pixel in a single exposure.1.2.4 Parallel OperationsBefore January 2007, parallel observations with the WFC and HRC were possible with ACS for certain filter combinations. ACS can be used in parallel with the other HST science instruments with restrictions described in detail in the ACS Instrument Handbook. No pure or coordinated parallels with the SBC are allowed. The policy for applying for parallel observing time is described in the HST Call for Proposals.