Optical design of the Advanced Camera for Surveys, a third generation HST axial science instrument

The Advanced Camera for Surveys (ACS), designed to be installed in 1999 during the third servicing mission of the Hubble Space Telescope (HST), is a high performance axial bay camera. The design provides three wide-band cameras plus coronagraphic capability. The cameras cover the spectral region from 115 nm to 170 nm and from 200 nm to 1000 nm. They are the very wide Field of View (FOV) (200 by 204 are sec) Wide Field Channel (WFC) camera, the wide FOV (26 by 29 are sec) High Resolution Channel (HRC) camera, the wide FOV (26 by 29 are sec) Solar Blind Channel (SBC) ultraviolet, and the coronagraph which uses the HRC CCD. Numerous filters are provided for each of the three cameras using only three filter wheels, two of which are shared by the WFC and HRC. The optical design uses heritage from Space Telescope Imaging Spectrograph (STIS), a second generation HST instrument. To achieve full wavelength coverage the designs are all-reflective. The FOV of the two-mirror STIS design is extended for the WFC by use of a reflective Schmidt-like plate as a third mirror. The filters are placed in the corrected converging beam and are OPD-matched to maintain focus. Protected silver reflective coatings are used on the WFC mirrors to maximize throughput and minimize polarization sensitivity. The paper will described the optical design and compare predicted performance with measured performance. The use of error budgets during the design process will also be discussed.