4.2 Quantum Efficiency

The Loral CCDs are thick, front-side illuminated devices. This lowers their intrinsic QE, due to the absorption of incident light by the polysilicon electrode structure on the front-side surface of the CCD. We also note that due to its MPP operation, the QE of the Loral devices is stable without maintenance such as UV flooding.

The front surfaces of the CCDs are overcoated with a Lumogen phosphor, which serves as the primary detection medium for photons shortward of about 4800Å, down-converting these to 5100Å - 5800Å. Its long wavelength cutoff (4800Å) is also well matched to a CCD's intrinsic sensitivity. The QE of the four flight WFPC2 CCDs is shown in Figure 4.2, which demonstrates the uniform UV response of 10-15% and a peak optical QE of 40%.

This phosphor coating also produces an enhancement of DQE at visual wavelengths, since it acts as an anti-reflection coating.

Figure 4.2: Pre-flight DQE Measurements on WFPC2 CCDs. The differences between the chips are probably due to systematic measurement error, and do not reflect a real difference in sensitivity.