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, your target will be within ~1 to 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 onboard target acquisition process is ~6 milliarcseconds.
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