Advanced Camera for Surveys

ACS is a highly versatile instrument that became the primary imaging instrument aboard HST. It offered several important advantages over other HST instruments: three independent, high-resolution channels covering the ultraviolet to the near-infrared regions of the spectrum, a large detector area and quantum efficiency, resulting in an increase in HST's discovery efficiency by a factor of ten, a rich complement of filters, and coronagraphic, polarimetric, and grism capabilities. The observations undertaken with ACS provided astronomers with a view of the Universe with uniquely high sensitivity, as exemplified by the Hubble Ultra-Deep Field, and encompass a wide range of astronomical phenomena, from comets and planets in the Solar System to the most distant quasars known.

Channels and detectors

ACS includes three independent channels (one now disabled), each optimized for specific scientific tasks:

Wide Field Channel (WFC)

The WFC is the most utilized channel of ACS. Its detector consists of two butted 2048x4096, 15 μm/pixel charge-coupled devices (CCDs) for a total of 16 megapixels manufactured by Scientific Imaging Technologies (SITe). The WFC plate scale is 0.05″ per pixel and it has an effective field-of-view of 202″×202″. The spectral range of the WFC detector is 350–1100 nm.

An example of a use of this channel was SWEEPS, which found 16 candidate exoplanets in the Galactic core.

High-Resolution Channel (HRC)

The HRC, which has been permanently disabled since 2007 due to an electrical fault, provided ultra-sharp views over a smaller field-of-view.

The HRC detector was a 1024×1024 SITe CCD which had a smaller field-of-view (26"×29") than the WFC but twice the spatial sampling (0.025" per pixel). This detector was also significantly more sensitive than the WFC at near-ultraviolet wavelengths (<350 nm).

The channel used two light suppression options for imaging faint objects around bright stars, improving the contrast of targets close to bright sources by tenfold. The first was a commandable coronagraphic mask that included two occulting spots, one of diameter 1.8" at the center of the field and the other of diameter 3.0" nearer to a corner. The first spot was the most popular of the two, for example, for imaging circumstellar disks around nearby bright stars or the host galaxies of luminous quasars. The second was the so-called Fastie Finger, 0.8" in width and 5" in length, located at the entrance of the HRC dewar window.

Solar Blind Channel (SBC)

The Multi Anode Microchannel Array (MAMA) of the SBC is a low-background photon-counting device optimized for the ultraviolet in the wavelength range of 115–170 nm. It consists of a photocathode, a microchannel plate, and an anode array. Its spatial sampling is 0.034"x0.030" per pixel and its field-of-view is 34.6"×30.0". The ACS SBC is in fact a flight spare from the Space Telescope Imaging Spectrograph (STIS).