The WFPC2 has two different analogue-to-digital (A/D) converters, with different gain factors. One channel has a gain of 14 electrons/DN, which significantly undersamples the CCD read noise (5 electrons/pixel RMS), and gives a digital full well of about 53,000 electrons. The other channel has a gain of 7 electrons/DN, which is comparable to the CCD read noise, and saturates at about 27,000 electrons.
The choice of the gain factor will be determined by the scientific objective. The 7 electrons/DN channel is best suited for faint object and UV imaging, where the lower CCD read noise will be most effective. For example, it should be used for UV imaging of planets or narrowband imaging of high redshift galaxies. The 14 electrons/DN channel has slightly higher effective read noise due to the quantization granularity, but can be used for programs where a signal level in excess of 27,000 electrons is required. Even when imaging faint sources, it may be desirable to retain the high signal-to-noise information on brighter field stars as a PSF reference.Hans-Martin Adorf <email@example.com>, ST-ECF