|HST Primer for Cycle 25|
The ACS High Resolution Channel (HRC) provided high resolution, near-UV to near-IR imaging in Cycle 11 through 14. HRC observations occupied approximately 8% of all HST science orbits during this time. Its detector was a 1K by 1K, thinned and backside-illuminated, SITe CCD with a near-ultraviolet-optimized coating and 21 x 21 μm pixels, that provided ~0.028 x 0.025 arcsec/pixel spatial resolution. This format yielded a nominal 29 x 26 arcsec field of view. The spectral response of the HRC ranged from ~1700 Å to ~11,000 Å, and it had a peak efficiency of 29% at ~6500 Å (including OTA throughput).The HRC PSF was critically sampled at 6300 Å and was undersampled by a factor 3 at 2000 Å. Well-dithered observations with the HRC led to a reconstructed PSF FWHM of 0.03" at ~400 nm, which increased towards longer wavelengths.The HRC utilized the same narrow-band and broad-band filters available with the ACS Wide Field Camera (WFC).The HRC also offered several optics specifically designed for HRC's field of view and UV sensitivity, but also permitted vignetted WFC imaging. These HRC-specific optics included UV and visible polarizers, a prism (PR200L), three medium-broad UV filters (F330W, F250W, and F220W), and two narrow-band filters (F344N and F892N). HRC could also use the FR459M and FR914M broad ramp filters, as well as the FR505N [OIII], FR388N [OII], and FR656N (H Alpha) narrow ramp filters.The coronagraphic mode of ACS HRC allowed high contrast imaging of faint circumstellar or circumnuclear sources, such as circumstellar debris disks and planetary companions, by preventing saturation of the detector and suppressing the diffraction pattern of the bright central source. The coronagraph featured a Lyot stop and two occulting disks that could be deployed into the field of view on demand. Finally, the HRC allowed low-dispersion (R ~100) slitless spectroscopy with the G800L grism (~23 Å/pixel in first order from 5500 Å to 10,500 Å) and the PR200L prism (~21 Å/pixel from ~3500 Å to the UV cut-off).