Advanced Camera for Surveys

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The Advanced Camera for Surveys (ACS) is a third generation axial instrument aboard the Hubble Space Telescope (HST). The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively before launch at Ball Aerospace & Technologies and the Goddard Space Flight Center and underwent a final flight-ready verification at the Kennedy Space Center before integration in the cargo bay of the Columbia orbiter. It was launched on March 1, 2002 as part of Servicing Mission 3B (STS-109) and installed in HST on March 7, replacing the Faint Object Camera (FOC).

ACS is a highly versatile instrument that has quickly become the primary imaging instrument aboard HST. It offers 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 provide us with an unparalleled 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 our Solar System to the most distant quasars known.

In June 2006 ACS went out of action due to electronic failure. On the morning of 30 June 2006 ACS was powered up successfully after switching to its redundant (Side-2) set of electronics. The instrument sub-systems, including the CCD detectors, all seem to be working well. No anomalous telemetry was observed for any ACS sub-system on Side-2. After some engineering tests, ACS resumed science operations on July 4, 2006. However, on September 29 2006, the ACS again failed, and options for restoring the camera are currently being considered. [1]

[edit] Channels and Detectors

ACS includes three independent channels, each optimized for specific scientific tasks. Here, we provide a brief description of the channels and their detectors.

Wide Field Channel (WFC) The WFC is the most utilized channel of ACS. Its detector consists of two butted 2048x4096 charge-coupled devices (CCDs) for a total of 16.7 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 200"×204". The spectral range of the WFC detector is 350-1100 nm.

High-Resolution Channel (HRC) The HRC has two light suppression options to mask out bright astronomical sources. The first is a commandable coronagraphic mask that includes 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 is 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 is the so-called Fastie finger, 0.8" in width and 5" in length, located at the entrance of the HRC dewar window. The HRC detector is a 1024×1024 SITe CCD (1.0 Megapixel) which has a smaller field-of-view (26"x29") than the WFC but twice the spatial sampling (0.025" per pixel). This detector is also significantly more sensitive than the WFC at near-ultraviolet wavelengths (<350 nm).

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 0.030" per pixel and its field-of-view is 25"x25". The ACS SBC is in fact a flight spare from the Space Telescope Imaging Spectrograph (STIS), which failed in August 2004.

[edit] Filters and Dispersers

ACS possesses a set of 38 filters and dispersers distributed among three wheels. Two of these wheels are shared by the HRC and WFC light paths while the third is dedicated to the SBC. The HRC and WFC elements consist of eleven broad-band filters, one medium-band filter, five narrow-band filters, three visible and three ultraviolet polarizers, one prism for the HRC, and one grism (580-1100 nm). Four of the filters have bandpasses in the near-ultraviolet and so can be used with the HRC only. The primary broad-band filters are equivalent to the u, g, r, i, and z filters of the ground-based Sloan Digital Sky Survey (SDSS). Five linear ramp filters divided into three individual segments each provide continuous imaging capability from 380 nm to 1070 nm and so insure adequate sampling of emission lines over a large range in redshift. Only the middle segment is accessible to the HRC. The SBC wheel is populated with one medium-band filter (Lyα), five long-pass filters, and two objective prisms.

[edit] External links

• the ACS Web site at Johns Hopkins University, which includes a complete description of the instrument, the ground calibration campaigns, the detectors, and the filters.
• the ACS Web site at the Space Telescope Science Institute (STScI)
• a collection of HST images, including ACS, at the Gallery of the Hubble Site