The High Resolution Channel of ACS is the prime ACS camera for near-UV imaging. HRC provides high throughput in the blue and a better sampling of the PSF than either the WFC or other CCD cameras on HST. The HRC pixel size critically samples the PSF at 6300 Ċ and is undersampled by a factor 3.0 at the blue end of its sensitivity range (2000 Ċ). With this capability, the HRC functionally replaces the Faint Object Camera as the instrument able to critically sample the PSF in the V band. For this reason, most of the usage of HRC will be for UV and blue imaging. HRC can also be convenient for imaging in the red when the PSF sampling is important. As an example, better PSF sampling is probably important for accurate stellar photometry in crowded fields. We expect that the photometric accuracy achievable by the HRC will be higher than that achievable with the WFC. Well-dithered observations with the HRC should lead to a reconstructed PSF FWHM of 0.03 arcsec at ~4000 Ċ, increasing towards longer wavelengths. HRC also includes a coronagraph that is discussed in Chapter 6. The HRC CCD presents a long wavelength halo problem similar to the STIS CCD since the front-side metallization correcting the halo problem for the WFC CCDs was implemented only after the HRC CCD had been procured. Although most of the HRC imaging is likely to occur in the UV, users should be cautioned to take into account the effects of the long wavelength halo when using the HRC in combination with near-IR filters (See Section 5.6.5).
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