Advanced Camera for Surveys - AbsoluteAstronomy.com

The Advanced Camera for Surveys (ACS) is a third generation axial instrument aboard the Hubble Space Telescope

Hubble Space Telescope

The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

(HST). The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University

Johns Hopkins University

The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...

. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp. and the Goddard Space Flight Center

Goddard Space Flight Center

The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...

and underwent a final flight-ready verification at the Kennedy Space Center

Kennedy Space Center

The John F. Kennedy Space Center is the NASA installation that has been the launch site for every United States human space flight since 1968. Although such flights are currently on hiatus, KSC continues to manage and operate unmanned rocket launch facilities for America's civilian space program...

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

STS-109

STS-109 was a Space Shuttle mission that launched from the Kennedy Space Center on 1 March 2002. It was the 108th mission of the Space Shuttle program, the 27th flight of the orbiter Columbia and the fourth servicing of the Hubble Space Telescope...

) and installed in HST on March 7, replacing the Faint Object Camera

Faint Object Camera

The Faint Object Camera was a camera installed on the Hubble Space Telescope from launch in 1990 until 2002. It was replaced by the Advanced Camera for Surveys.The camera was built by Dornier GmbH and was funded by the European Space Agency...

(FOC), the last original instrument.

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

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

to the near-infrared regions of the spectrum

Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

, a large detector area and quantum efficiency

Quantum efficiency

Quantum efficiency is a quantity defined for a photosensitive device such as photographic film or a charge-coupled device as the percentage of photons hitting the photoreactive surface that will produce an electron–hole pair. It is an accurate measurement of the device's electrical sensitivity to...

, resulting in an increase in HST's discovery efficiency by a factor of ten, a rich complement of filter

Filter (optics)

Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....

s, and coronagraphic, polarimetric

Polarimetry

Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves...

, and grism

Grism

A grism is a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through. The advantage of this arrangement is that one and the same camera can be used both for imaging and spectroscopy without having to be moved...

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

Hubble Ultra Deep Field

The Hubble Ultra-Deep Field is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from September 24, 2003, through to January 16, 2004...

, and encompass a wide range of astronomical phenomena, from comets and planets in our Solar System

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

to the most distant quasars known.

On 25 June 2006 ACS went out of action due to electronic failure. It was powered up successfully after switching to its redundant (Side-2) set of electronics. The instrument sub-systems, including the CCD detectors, all seemed to be working well and after some engineering tests, ACS resumed science operations on July 4, 2006. On 23 September 2006, the ACS again failed, though by 9 October the problem had been diagnosed and resolved. On January 27, 2007 the ACS failed due to a short circuit in its backup power supply. The instrument's Solar Blind Channel (SBC) was returned to operation on 19 February 2007 using the side-1 electronics. The Wide Field Channel (WFC) was returned to service by STS-125

STS-125

STS-125, or HST-SM4 , was the fifth and final space shuttle servicing mission to the Hubble Space Telescope .Launch occurred on 11 May 2009 at 2:01 pm EDT...

in May 2009. The High Resolution Channel (HRC), however, remains offline.

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 device

Charge-coupled device

A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

s (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.

High-Resolution Channel (HRC)

The HRC, currently disabled due to an electrical fault, used two light suppression options to mask out bright astronomical sources. 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. 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).

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"×25". The ACS SBC is in fact a flight spare from the Space Telescope Imaging Spectrograph

Space Telescope Imaging Spectrograph

The Space Telescope Imaging Spectrograph is a spectrograph, also with a camera mode, installed on the Hubble Space Telescope. It operated continuously from 1997 until a power supply failure in 2004. After repairs, it began operating again in 2009...

(STIS), which failed in August 2004.

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

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

Sloan Digital Sky Survey

The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...

(SDSS). Five linear ramp filters divided into three individual segments each provide continuous imaging capability from 380 nm to 1070 nm and so ensure 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.

External links

the ACS Web site at Johns Hopkins University
Johns Hopkins University
The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...

, 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)
All Publicly released ACS images
a collection of HST images, including ACS, at the Gallery of the Hubble Site
ACS at ESA/Hubble