Hubble Space Telescope Primer for Cycle 11

4.2 Advanced Camera for Surveys (ACS)

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The ACS is designed to advance the survey capabilities of HST, defined as the product (QE x DETECTOR AREA), by a factor of ~10 in the visual and near infrared. This instrument comprises three channels, each optimized for a specific goal:

• The Wide Field Channel (ACS/WFC):
• The WFC has a 202 x 202 arcsec field of view from 3700-11,000 Å, and a peak efficiency of 44% (including the OTA). The plate scale is ~0.05 arcsec/pixel, providing critical sampling at 11,600 Å. The detector consists of a mosaic of two 2048 x 4096 Scientific Imaging Technologies (SITe) CCDs, with 15 x 15 µm pixels.
• The High Resolution Channel (ACS/HRC):
• The HRC has a 29 x 26 arcsec field of view from 2000-11,000 Å and a peak efficiency of 29%. The plate scale is ~0.025 arcsec/pixel, providing critical sampling at 6300 Å. The detector is a 1024 x 1024 Scientific Image Technologies (SITe) CCD, with 21x 21 µm pixels.
• The Solar Blind Channel (ACS/SBC):
• The SBC has a 35 x 31 arcsec field of view from 1150-1700 Å, and a peak efficiency of 6%. The plate scale is ~0.030 arcsec/pixel. The detector is a solar-blind CsI MAMA, with 25 x 25 µm pixels.

In addition to these three prime capabilities, ACS also provides:

• Grism spectroscopy: Low resolution (R~100) wide field spectroscopy from 5500-11,000 Å in both the WFC and the HRC.
• Objective prism spectroscopy: Low resolution (R~100 at 2000 Å) near-UV spectroscopy from 2000-4000 Å in the HRC.
• Objective prism spectroscopy: Low resolution (R~100 at 1216 Å) far-UV spectroscopy from 1150-1700 Å in the SBC.
• Coronography: Aberrated beam coronography in the HRC from 2000-11,000 Å with 1.8 arcsec and 3.0 arcsec diameter occulting spots.
• Imaging Polarimetry: Polarimetric imaging in the HRC and WFC with polarization angles of 0^o, 60o and 120o.

4.2.1 Comparing ACS to WFPC2 and STIS

The ACS/WFC camera covers a field of view (202x202 arcsec) larger than the WFPC2, has lower read-out noise and higher throughput over a wide spectral range and better samples the PSF compared to the WF cameras of WFPC2. Thanks to the use of protected silver mirror coatings and to the small number of reflections, WFC with a broad band filter has a throughput comparable to the unfiltered STIS CCD imaging, while retaining a FOV fifteen times larger. Thus, ACS makes it possible to carry out multi-color photometry of very faint sources previously accessible only to the unfiltered STIS CCD imaging mode.

The ACS/HRC camera provides critical sampling of the PSF in the visible and high throughput in the blue over a field of view of 29x26 arcsec. For broad-band UV imaging this channel is competitive with the STIS NUV-MAMA.

The ACS/SBC channel provides FUV imaging capability with a more extended set of filters than available with the STIS FUV-MAMA.

4.2.2 Performance Issues

In common with the WFPC2 and STIS CCDs, the ACS CCD performance will also exhibit a gradual deterioration in performance due to HST's space radiation environment. The major effects will be:

• A degradation of the WFC and HRC CCD Charge Transfer Efficiency (CTE), which will affect observations of the faintest sources (see the ACS Instrument Handbook for details).
• An increase in the number of hot pixels with time.

In order to mitigate these effects, ACS will be equipped with a system that will allow the CCDs to be preflashed with 100 to 200 electrons during later years of its operation. Observers should consider whether their science is affected by a preflash and, if so, plan to propose their science as soon as possible with the appropriate justification.