| DESCRIPTION |
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 ): FR459M, FR914M WFC/HRC
9% bandpass (3700-10,700 ): 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.
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