M. Clampin, H. Ford, P. Bely, Burrows, G. Hartig, M. Postman,
W. Sparks, Rick White
Space Telescope Science Institute
University of California, Santa Cruz, Department of Astronomy
T. Broadhurst, D. A. Golimowski, P. Feldman, Z. Tsvetanov
The Johns Hopkins University
E. Cheng, R. Kimble, S. Neff, D. Leviton
Goddard Space Flight Center
CASA, University of Colorado
The Advanced Camera for Surveys (ACS) will be installed in the Hubble Space Telescope (HST) in the 1999 Servicing Mission. The ACS was awarded to the Johns Hopkins University (JHU) in December 1994, and the P.I. is Holland Ford. In designing the Advanced Camera, the ACS science team focused on scientific goals which would be enabled by improvements in field of view, sensitivity and spatial resolution, in particular, a wide field survey with HST to address a number of outstanding problems in cosmology. In this paper we will describe the scientific goals of ACS and highlight the relevant performance capabilities of ACS.
The unique characteristics and high efficiency of the ACS will be employed in a focused attack on our three major science themes. We will address the formation and evolution of galaxies and clusters of galaxies, and the nature and large scale distribution of dark matter. Approximately 75 percent of our guaranteed time will be devoted to two major surveys aimed at solving outstanding problems in cosmology. In the largest survey we will take contiguous deep V- and I-band WFC images of 0.7 square degrees of sky in the CVZ. We expect to map the large scale distribution of dark matter around approximately 20 rich clusters of galaxies. This survey data will be used to establish the evolution of galaxies and clusters of galaxies in the early universe and determine the cluster and supercluster environment around one or more high redshift radio galaxies.
In the second survey, we will use Surface Brightness Fluctuations (SBF) in deep wide field I-band images of early type galaxies to map large scale flow. This method will allow us to obtain distances to galaxies which are independent of their redshifts and will contribute to the measurement of H and to the mapping of large scale flows. Mapping the infall pattern around the ``Great Wall'' of galaxies will provide us with constraints on the mass distribution on very large scales, which is presumably dominated by dark matter dynamics.
The high spatial resolution capabilities of ACS will be used in narrow band and polarimetric HRC and WFC images to address QSOs and AGNs, our second major science area. These objects are not only unique probes of early universe conditions and potential tracers of the geometric evolution of the universe, but are also important testbeds for identifying their nuclear engines. Our observations of AGNs will delineate the structure in the Narrow Line Region of AGNs and provide morphological data on stellar populations and related ionized gas. HRC observations will elaborate on the relationship between the host galaxies and associated radio sources. Imaging polarimetry data will be used to study proposed Unified Schemes for understanding active galaxies. We will use a novel polarimetric technique to measure geometric distances to galaxies, and provide independent measures of H and q. Finally, a variety of observations using narrow band imaging with the SBC will address specific questions in solar system science, our third science theme. The SBC will provide high spatial resolution narrow band data on aurorae of the giant planets. The high resolution camera will be used to study protoplanetary discs and brown dwarfs.
The ACE is an axial bay instrument which features three wide band
channels, covering the spectral regions from 115 nm to 170 nm and from
200 nm to 1000 nm. A schematic showing the optical design of the ACS and
the configuration of the cameras is shown in Figure 1. The Wide Field Camera
(WFC) is a survey camera designed for the task of deep imaging, the High
Resolution Camera is a near-UV camera, designed for high spatial resolution
imaging and the solar blind channel (SBC) is designed for high efficiency FUV imaging.
The key parameters of these three channels are summarized in Table 1,
Table 1: Advanced Camera for Surveys - key performance parameters
Figure: A schematic showing the ACS optical design
The WFC is a very wide FOV imager optimized for highest throughput in the optical I-band and is designed to accomplish the survey science program. The WFC plate scale is 0.05" pixel, so the image is half critically sampled at wavelengths greater than 500 nm. The optical design for the wide field, aberration-correcting camera, uses only three mirrors to relay the HST field image to the CCD for highest throughput and lowest scatter. The mirrors are coated with silver to provide maximum reflection efficiency at red wavelengths. The WFC uses a mosaic of two 4096x2048 CCDs, with 15 m square pixels. The CCDs are manufactured by Scientific Imaging Technology (SITe) and are backside thinned and AR coated to optimize their performance in the 400 to 1000 nm spectral region. The WFC is a survey instrument and so we have oriented the WFC CCDs in the focal plane so that survey scans across the sky can be tiled efficiently. The quantum efficiency (QE) of the WFC CCDs, shown in Figure 2, peaks at 700 nm where it is 82%. The two major features of the WFC are its wide field field capability and the high system throughput. In designing the ACS, the science team specified the product of quantum efficiencyfield of view as the instrument's discovery efficiency. The discovery efficiency should be a minimum of 10WFPC2 in the I band where the ACS survey will be conducted. The overall system efficiency is shown in Figure 3 to emphasize this point. The peak thoughput of the WFC, including the HST optical telescope assembly will be 43% at 650 nm and 35% at 800 nm.
Figure: Quantum efficiency curves for the WFC and HRC CCDs. The WFC's QE was determined from measurements of 1024x1024 CCDs with the same coating. The HRC QE is from measurements made at UA on flight-like devices.
The HRC imager is optimized for highest throughput across the spectral band from 200 nm to 1000 nm. The image is critically sampled for wavelengths greater than 500 nm, with an average plate scale of 0.025" pixel. The HRC uses a blue optimized CCD, and its mirrors are coated to allow use of these optics for the SBC also. In order to maintain the highest efficiency, the camera uses only two powered mirrors and a fold mirror to relay the HST field image to the CCD. The HRC CCD is a 1024 by 1024 STIS CCD with 21 m square pixels. To provide the desired spectral response from 200 to 1000 nm, the CCD is coated with a passivated Pt flashgate (PPtF) and a HfO coating, designed by Mike Lesser at the University of Arizona. The QE for the HRC CCD is shown in Figure 2, and illustrates the high near-UV QE delivered by the PPtF process. The three main elements of the HRC are high spatial resolution (0.025" pixels) and high efficiency in the visible and near-UV. The overall performance of the HRC's high efficiency design is shown in Figure 3.
Figure: The overall efficiencies of the WFC and HRC, with WFPC2 shown for comparison. The HST optical telescope assembly is included.
In addition to the key design goals of each camera, the other main element which determines the quality of science ACS will produce is the filter complement. The ACS optical design allows both the WFC and HRC to use the same two filter wheels which results in a net saving on mechanisms and filters, while maximizing the number of filters available to each camera. Filters which are intended for use with the HRC have been sized accordingly. A total of 30 bandpass filters, a grism and a prism, and polarizers will be mounted in the two filter wheels, with an additional four filter slots left clear. Each of the two filter wheels will contain a WFC size and a HRC size clear slot positioned at 153.5^o to each other. This arrangement will permit pairs of filters in both wheels separated by the appropriate angular distance to each other to be used in parallel. The complement of WFC/HRC filters is shown in Table 2.
Table 2: WFC/HRC filter complement
The science team has chosen to adopt the Sloane Digital Sky Survey (SDSS) filter set (Fukugita et al. 1995) as the primary broadband filter system for AC. This filter system provides extensive wavelength coverage from 300 nm to 1100 nm with five filters. The individual filter bandpasses are 1500Å wide and, therefore, ideal for the primary science goals of AC which focus on faint object imaging. The zero point of this system is the AB system of Oke & Gunn (1983), which allows immediate conversion of measured magnitudes to fluxes. We anticipate that by the launch of the 1999 Servicing Mission, the Sloane Sky Survey will have amassed a significant database of calibration data in this photometric system. The SDSS filter set will be matched to the field of view of the WFC, however, the filter bandpass will be determined by the the cutoff of the WFC's silver coated optics and consequently, differs from the standard SDSS filter (Fukugita et al. 1995).
In addition to the SDSS filter set, we have selected standard U, B, and V filters for WFC photometric programs requiring the Johnson-Cousins UBV photometric system. For very deep imaging with the WFC, two very broad bandpass filters are provided, a V filter similar to the WFPC2 F606W employed for the Medium Deep Survey, and an I band filter similar to the WFPC2 filter F814W. These two filter sets offer the additional benefit of providing a link to previous HST calibrations of WFPC2 and WFPC1. For high efficiency near-UV imaging in the HRC channel, two broadband HRC filters are provided, one centered at 220 nm and one at 250 nm.
With its superb imaging quality, low dispersion slitless spectroscopy with ACS will provide otherwise unobtainable UV and optical spectral information on a large numbers of faint sources. This spectral information can be used to estimate the redshifts of faint galaxies from the position of the Lyman break. Alternatively, surveys can be made for emission line objects in distant galaxies (e.g., planetary nebulae and supernova remnants), faint quasars, and galaxies with strong emission lines in high redshift clusters. A grism is, therefore, provided for visible WFC spectroscopy with a spectral resolution of R50 and a UV prism for the HRC with R50.
In contrast to the WFPC2, the AC has a smaller set of narrowband filters. For wide field narrowband imaging, AC has H and [OIII] filters, each with a bandpass of 1%. To complement these two filters, there is a medium band continuum filter F550M, which is centered at 550 nm with a bandpass of 10%. It is anticipated that the majority of narrowband imaging will be undertaken with a suite of ramp filters similar to those used in WFPC2. The major difference, compared to WFPC2's ramp filters, is that the AC ramp filters will offer a significantly larger monochromatic field of view ( 7030 arcsec) at any given wavelength. The four ramp filters will cover the wavelength region 365--985 nm, with a bandpass of 2% at any given wavelength. Each ramp filter will have three strips which are arranged parallel to the rotation direction of the filter wheel. This will permit the central strip in each filter wheel to image onto the HRC as well as the WFC. In order to use the ramp filters with the HRC, it will also be necessary to partially rotate the filter wheel to select the required central wavelength. Two additional 2% bandpass narrowband filters are included to provide HRC imaging in the [NeV] line, for imaging of AGN nuclei and Planetary Nebulae, and the methane band (892 nm) for imaging of the solar system planets.
The Advanced Camera team is committed to an imaging polarimetric capability and so the AC filter complement has two sets of polarizers optimized for UV and visible band polarimetry with the HRC. The UV polarizers will be optimized for the 200--400 nm region and are located in filter Wheel 1 to permit them to be crossed with the UV filters in Wheel 2. The visible band polarizers are located in filter Wheel 2 and are designed to be crossed with broad and narrow band filters in Wheel 1. Each set of polarizers comprises three separate elements which give relative polarizer angles of 0^o, 60^o and 120^o, respectively. A polarimetric imaging sequence will require only one filter move between each observation. It will not be necessary to move the target, or roll the telescope between observations.
The SBC is a wide FOV imager optimized for highest throughput from 115 nm to 170 nm and employs just two mirrors in its optical chain. The relay optic and corrector mirror mechanism are shared with the HRC camera. The SBC uses a STIS-based PCA photon counting detector with an opaque CsI photocathode and a C-plate (or two chevron ) micro-channel plate(s). It is intended that this should be a flight spare detector from the STIS program. With a QE of 25% recently demonstrated on STIS CsI photon counting detectors, the overall peak efficiency of the SBC channel will be 15% at Lyman. The SBC has its own filter wheel which provides 11 spectral filter locations are provided in the single 12-position filter wheel.
Table 3: Filter complement for the SBC
A narrowband Lyman- filter will allow Lyman- emission to be isolated for studies of auroral emission on planets and other not redshifted objects. A set of long-wavelength pass filters will allow narrow-to-medium band imaging in [CIV] of the objects such as nuclei of AGN of PNe. Longpass filters, designed for use in pairs to isolate a specific bandpass, will provide coverage of the spectral region from 120 nm to 160 nm where the SBC has good sensitivity. Two prisms are also included, a lithium fluoride (LiF) prism and a CaF prism. LiF provides high transmission, high efficiency and relatively constant dispersion from 120 nm to 180 nm. A CaF prism performs a similar function, but with a cutoff wavelength which blocks Lyman- emission.
Fukugita, M., Ichikawa, I., Gunn, J. E., Doi, M., Shimasaku, K. & Schneider, D. P. 1995, AJ, submitted
Oke, J. B. & Gunn, J. E. 1983, ApJ, 266, 173
M. Clampin, H. Ford, P. Bely, C. Burrows, G. Hartig, M. Postman, W. Sparks, R. L. White, G. Illingworth, T. Broadhurst, D. A. Golimowski, P. Feldman, Z. Tsvetanov, E. Cheng, R. Kimble, S. Neff, D. Leviton, G. Miley, F. Bartko, and R. WoodruffClampin et al.The Advanced Camera for Surveys