1.2.1 Target Acquisitions

For 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 reference pixel of the selected aperture. For observations using ramp filters, the desired central wavelength will be located at the specified 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.

1.2.2 Typical ACS Observing Strategies

WFC users (and erstwhile HRC users) must consider "packaging" their exposures to mitigate the impact of cosmic rays either by dithering their images (shifting the pointing of the telescope slightly between component exposures) 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 observing sequences consist of a series of dithered 5-20 minute exposures for each program filter.  Detailed information about dither and mosaic strategies can be obtained from the ACS Dither webpage 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.

1.2.3 Data Storage and Transfer

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 SBC images. Dumping a full ACS buffer requires 349 seconds and may not occur while ACS is being actively commanded; dumping a partially filled buffer (if, e.g., reading out the following exposure would over-fill it) requires proportionately less time. Of this time, 337 seconds is spent dumping the image. The buffer dump cannot be done during the next exposure if the latter is shorter than 337 seconds. If, however, the next exposure is less than 337 seconds the buffer dump will create an extra 5.8 minutes of overhead.

ACS's internal buffer stores data in a 16 bit per pixel format. This structure imposes a maximum of (2¹⁶ - 1) =  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 ≥ 2 e¯/DN, it is instead the depth of the CCD pixel well (rather than the 16 bit buffer format) that limits the number of electrons that can be accumulated without saturating the pixel in a single exposure. Even in extreme cases, charge leakage from WFC CCD full-well saturation is captured in adjacent pixels and may still be amenable to accurate aperture photometry (ACS ISR 2020-07) at gain settings  ≥ 2 e¯/DN.

1.2.4 Parallel Operations

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. Before January 2007, parallel observations with the WFC and HRC were possible with ACS for certain filter combinations.