With the WFPC2 CCD sensors, images may be obtained in any spectral region defined by the chosen filter with high photometric quality, wide dynamic range, and excellent spatial resolution. The bright end of the dynamic range is limited by the 0.11 seconds minimum exposure time, and by the saturation level of the analog-to-digital converter (ADC) at the chosen gain, which is roughly 53000 (gain=14, though called ADT-GAIN=15 in RPS2) or 27000e- (gain=7) per pixel. The maximum signal-to-noise ratio corresponding to a fully exposed pixel will be about 230. The faint end of the dynamic range is limited by photon noise, instrument read noise and, for the wide-band visible and infra-red filters, the sky background.
Table 2.2
Table 2.2: WFPC2 Dynamic Range in a Single Exposure
Figure 2.4: WFPC2 + OTA System Throughput. These measurements made on orbit are much more accurate than the pre-launch estimates, and are used consistently throughout this Handbook.
The visible and red sensitivity of the WFPC2 is a property of the silicon from which the CCDs are fabricated. To achieve good ultraviolet response, each CCD is coated with a thin film of Lumogen, a phosphor. Lumogen converts photons with wavelengths less than 4800Å into visible photons with wavelengths between 5100Å and 5800Å, which the CCD detects with good sensitivity. Beyond 4800Å, the Lumogen becomes transparent and acts to some degree as an anti-reflection coating. Thus, the full wavelength response is determined by the MgF2 field flattener cutoff on the short-wavelength end and the silicon band-gap in the infrared at 1.1 eV (~11000Å).
