Advance Camera For Surveys ========================== Instrument Overview =================== The information in this document is taken from Gonzaga et al. (2005). The Advanced Camera for Surveys (ACS), a third-generation instrument, was installed in the Hubble Space Telescope on March 7, 2002. Its primary purpose is to increase HST imaging discovery efficiency by about a factor of 10, with a combination of detector area and quantum efficiency that surpasses previous instruments. In addition, coronagraphic, polarimetric, and grism capabilities make this a versatile and powerful instrument. ACS is a versatile instrument that can be applied to a broad range of scientific programs. The three prime capabilities of ACS are: - Deep, wide-field imaging from visible to near-IR wavelengths. - High spatial resolution imaging from near-UV to near-IR wavelengths. - Solar blind UV imaging. ACS has three independent cameras that provide wide-field, high resolution, and ultraviolet imaging capabilities espectively, using a broad assortment of filters designed to address a large range of scientific goals. The three channels are each optimized for a specific goal: - Wide Field Channel (WFC): 202 x 202 arcsecond field of view from 3500 A to 11,000 A, and peak efficiency of 48% (including the Optical Telescope Assembly (OTA)). The plate scale of ~0.05 arcseconds/pixel provides critical sampling at 11,600 A. - High Resolution Channel (HRC): 29 x 26 arcsecond field of view from 1700 A to 11,000 A, and peak efficiency of 29%. The plate scale of ~0.027 arcseconds/pixel provides critical sampling at 6300 A. - Solar Blind Channel (SBC): 35 x 31 arcsecond field of view from 1150 A to 1700 A, plate scale of ~0.032 arcseconds/pixel, and peak efficiency of 7.5%. Calibration =========== The calibration accuracy is summarized in the table below. Attribute WFC HRC SBC Limiting factor ------------------------------------------------------------------ Distortion solution 0.1 pix 0.1 pix 0.25 pix Calibration & stability accuracy of geometric distortion Absolute astrometry 0.5-1 in 1 in 1 in Guide Star Catalog uncertainties Absolute photometry 3% 2% 5% Absolute calibration, standards Relative photometry 1% 1% 2% Flat-field or of the same star geometric distortion Transformation to 0.02 m to SDSS n/a DQE curve standard magnitude 0.025 m to WFPC2 determination system 0.03 m to BVRI Color terms Polarimetry 1% 1% n/a Wavelength 20 A 12 A 1 pix Accuracy of dispersion Spectrophotometry 6% 10% 20% ------------------------------------------------------------------ Detectors ========= ACS uses the following detectors in each channel: - The WFC employs a mosaic of two 4096 x 2048 Scientific Imaging Technologies (SITe) CCDs. The 15 x 15 um pixels provide ~0.05 arcseconds/pixel spatial resolution, with critical sampling at 11,600 A, resulting in a nominal 202 x 202 arcsecond field of view (FOV). The spectral sensitivity of the WFC ranges from 3500 A to 11,000 A, with a peak efficiency of 48% at ~7000 A (including OTA). - The HRC detector is a 1024 x 1024 SITe CCD, with 21 x 21 micron pixels that provide ~0.028 x 0.025 arcsecond/pixel spatial resolution with critical sampling at 6300 A. This gives the HRC a nominal 29 x 26 arcsecond field of view. The spectral response of the HRC ranges from ~1700 A to 11,000 A, and it has a peak efficiency of 29% at ~6500 A (including OTA). - The SBC detector is a solar-blind CsI microchannel plate (MCP) with Multi-Anode Microchannel Array (MAMA) readout. It has 1024 x 1024 pixels, each 25 x 25 micron in size. This provides a spatial resolution of ~0.034 x 0.030 arcseconds/pixels, producing a nominal FOV of 35 x 31 arcseconds. The SBC UV spectral response ranges from 1150 A to 1700 A with a peak efficiency of 7.5% at 1250 A. Filters ======= ACS has three filter wheels: two shared by the WFC and HRC, and a separate wheel dedicated to the SBC. The WFC/HRC filter wheels contain the major filter sets. Each wheel also contains one clear WFC aperture and one clear HRC aperture (see Chapter 5 of Gonzaga et al. 2005 for more on filters). Parallel WFC and HRC observations are possible for some filter combinations, unless the user disables this option, or if adding the parallel observations cannot be done due to timing considerations. Because the filter wheels are shared, it is not possible to independently select the filter for WFC and HRC parallel observations. The filters used in ACS are summarized in the table below. Filter description Camera ----------------------------------------------------------------- Broadband: Sloan Digital Sky Survey (SDSS): F475W, F625W, WFC/HRC F775W, F850LP B, V, Medium V, Wide V, I: F435W, F555W, F550M, WFC/HRC F606W, F814W Near-UV: F220W, F250W, F330W HRC No Filter: CLEAR WFC/HRC Narrowband: Halpha (2%), [OIII] (1%), [NII] (1%): F658N, WFC/HRC F502N, F660N NeV (3440 A): F344N HRC Methane (8920 A): F892N HRC/[WFC1] Ramp: 2% bandpass (3700-10,700 A): FR388N, FR505N, WFC/HRC FR656N 2% bandpass (3700-10,700 A): FR423N, FR462N, WFC FR716N, FR782N, FR853N, FR931N, FR1016N, FR551N, FR601N 9% bandpass (3700-10,700 Å): FR459M, FR914M WFC/HRC 9% bandpass (3700-10,700 Å): FR647M WFC Spectroscopic Grism: G800L WFC/HRC Prism: PR200L WFC/HRC Polarizers: Visible: (0, 60, 120): POL0V, POL60V, POL120V HRC/[WFC1] Near-UV (0, 60, 120): POL0UV, POL60UV, POL120UV HRC/[WFC1] Medium Band Lyman-Alpha: F122M SBC Long Pass: MgF2, CaF2, BaF2, Quartz, Fused Silica: F115LP, F125LP, F140LP, F150LP, F165LP SBC Prisms: LiF, CaF2: PR110L, PR130L SBC ----------------------------------------------------------------- References ========== Gonzaga, S., et al. 2005, ACS Instrument Handbook, Version 6.0, Baltimore: STScI. References ========== Gonzaga, S., K. Sembach, J. Biretta, R. Bohlin, M. Chiaberge, and 19 others, ACS Instrument Handbook, Version 6.0, Baltimore: STScI, 2005.