4.3 Dynamic Range

Linear full well capacity for these devices, clocked appropriately for the MPP mode, is approximately 90,000e^- pixel^-1. Flight qualified ADCs with higher dynamic range (>12 bits) were not available, so WFPC2 operates the two available ADCs at different gain factors, to take partial advantage of both the low read noise and large available full well depth.

One channel has a gain of 14e^- DN^-1, which significantly undersamples the CCD read noise (5 e^- pixel^-1 RMS), and gives a digital full well of about 53,000e^-. The other channel has a gain of 7e^- DN^-1 which is comparable to the CCD read noise, and saturates at about 27,000e^-. The choice of gain factor is determined by the scientific objective. The 7 e^- DN^-1 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 e^- DN^-1 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 27,000e^- 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.

Use of the 14 e^- DN^-1 channel also allows reasonable recovery of counts for isolated, saturated point sources by summing over the saturated pixels (assuming that the charge bleeding does not extend to the edges of the CCD). See Gilliland (1994).