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 using ramp filters, one must specify the desired central wavelength for the observation. For HRC coronagraphic observations, onboard target acquisition was necessary. The nominal accuracy of the combined target acquisition and slew procedure was ~0.03 arcseconds, comparable to that achieved by STIS.
Observers must consider the “packaging” of their observations, i.e., how observations should be divided to mitigate the impact of cosmic rays, whether dithering of images is required, and how, if necessary, to construct a mosaic pattern to cover an extended target. Detailed information about dither and mosaic strategies can be obtained from the
ACS Dither Web Page and the
MultiDrizzle Handbook.
HRC images and narrow-band WFC observations are likely to be read noise limited and require judicious “CR-SPLIT” times. SBC MAMA observations do not suffer from cosmic rays or read noise, but long integration times are often needed to obtain sufficient signal-to-noise in photon-starved ultraviolet bands. A typical ACS observing sequence consists of a series of “CR-SPLIT” and dithered ~10 to 20 minute exposures for each program filter. Coronagraphic observations required an initial target acquisition observation to permit centering of the target under the occulting mask. Observers are generally not required to obtain calibration exposures.
At the conclusion of each exposure, science data are read out from the detector and placed in ACS’s internal buffer memory, where it’s stored until it 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 WFC image, or sixteen HRC or SBC images. Therefore, the buffer typically needs to be dumped following a 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 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.
Parallel observations with the WFC and HRC used to be 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.
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.
