5.4 High-Resolution Optical and UV Imaging

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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 Å and is undersampled by a factor 3.0 at the blue end of its sensitivity range (2000 Å). 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 Å, 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).

5.4.1 Filter Set

The HRC-specific filters are mostly UV and blue. The set includes UV and visible polarizers (discussed in Chapter 6), a prism (PR200L, discussed in Chapter 6), three medium-broad UV filters (F330W, F250W, and F220W) and two narrow band filters (F344N and F892N). Use of the UV filters with the WFC is not supported because of the uncertainty of the WFC silver coating transmission below 4000 Å.

All broad, medium and narrow band WFC filters can be used with the HRC whenever a better PSF sampling is required. In general, the throughput of WFC is higher than that of HRC where their sensitivity overlaps. Only some of the WFC ramp segments can be used with the HRC since only the middle segment overlaps with the HRC FOV. In particular, HRC can use the FR459M and FR914M broad ramps, and the FR505N [OIII], FR388N [OII], and FR656N (Ha) narrow ramps.

5.4.2 Multiple Electron Events

Like the STIS CCD but unlike WFPC2, the HRC CCD is directly sensitive to UV photons and for this reason is much more effective in detecting them. However, whenever a detector has non-negligible sensitivity over more than a factor two in wavelength, it becomes energetically possible for a UV photon to generate more than one electron and be counted more than once. This effect has been seen for STIS and also during the ground testing of the HRC detector. The effect is only important shortward of 3200 Å, and reaches a magnitude of approximately 1.7 e^–/photon at 2000 Å. Multiple counting of photons has to be taken into account when estimating the detector QE and the noise level of a UV observation since multiple photons cause a distortion in the Poisson distribution of electrons.

5.4.3 Red Leaks

When designing a UV filter, a high suppression of off-band transmission, particularly in the red, has to be traded with overall in-band transmission. The very high blue quantum efficiency of the HRC compared to WFPC2 makes it possible to obtain an overall red leak suppression comparable to that of the WFPC2 while using much higher transmission filters. In Cycle 14 we obtained new calibration data to check the impact of red leaks on observations. The results are described in ACS ISR 2007-003. In Table 5.6 we show the ratio of in-band versus total flux for a few UV and blue HRC filters, where the cutoff point between in-band and out-of-band flux is defined as the filter’s 1% transmission point. The same ratio is also listed for the equivalent filters in WFPC2. Correction factors for different stellar spectral types and non-stellar spectra can be found in the ISR. Clearly, red leaks are not a problem for F330W, F435W, and F475W. Red leaks are more important for F250W and F220W. In particular, accurate UV photometry of objects with the spectrum of an M star will require correction for the redleak in F250W and will be essentially impossible in F220W. For the latter filter a red leak correction will also be necessary for K and G stars.

Table 5.6: In-band flux as a percentage of the total flux.

Stellar type	WFPC2 F218W	HRC F220W	WFPC2 F255W	HRC F250W	WFPC2 F300W	HRC F330W	WFPC2 F439W	HRC F435W	WFPC2 F450W	HRC F475W
O5 V	99.8	99.6	99.6	99.7	99.9	99.9	99.9	99.9	99.9	99.9
B1 V	99.7	99.6	99.6	99.7	99.9	99.9	99.9	99.9	99.9	99.9
A1 V	99.4	98.8	99.2	99.0	99.2	99.9	99.9	99.9	99.9	99.9
F0 V	98.5	97.0	98.8	98.3	98.8	99.9	99.9	99.9	99.9	99.9
G2 V	92.5	88.7	97.4	97.1	97.4	99.9	99.9	99.9	99.8	99.9
K0 V	71.7	60.6	95.0	95.2	95.0	99.9	99.9	99.9	99.8	99.9
M2 V	0.03	1.5	45.5	62.4	45.4	99.9	99.9	99.9	99.6	99.9