1.1.4 CCD Detector Technology

The WFPC2 CCDs are thick, front-side illuminated devices made by Loral Aerospace. They support multi-pinned phase (MPP) operation which eliminates quantum efficiency hysteresis. They have a Lumogen phosphor coating to give UV sensitivity. Details may be summarized as follows:

• Read noise: WFPC2 CCDs have ~5e^- RMS read noise which provides good faint object and UV imaging capabilities.
• Dark noise: Inverted phase operation yields low dark noise for WFPC2 CCDs. They are being operated at -88°C and the median dark current is about 0.0045 e^- pixel^-1 s^-1.
• Flat field: WFPC2 CCDs have a uniform pixel-to-pixel response (<2% pixel-to-pixel non-uniformity) which facilitates accurate photometric calibration.
• CTE: Low level charge traps are present in the WFPC2 devices at the present operating temperature of -88°C. For bright stellar images, there is a ~4% signal loss when a star image is clocked down through all rows of the CCD. Fainter images show a larger effect which also appears to increase with time. The effect can be as large as tens of percent for faint stars (few hundred electrons) seen against a low background (<0.1 DN) in data taken during later Cycles. For most typical applications, CTE is either negligible or can be calibrated, and pre-flash exposures are not required. This avoids the increase in background noise, and the decrease in operational efficiency that results from a preflash.
• Gain switch: Two CCD gains are available with WFPC2, a 7 e^- DN^-1 channel which saturates at about 27000 e^- (4096 DN with a bias of about 300 DN) and a 14 e^- DN^-1 channel which saturates at about 53000 e^-. The Loral devices have a full well capacity of ~90,000 e^- and are linear up to 4096 DN in both channels.
• DQE: The peak CCD DQE in the optical is 40% at 7000Å. In the UV (1100-4000Å) the DQE is determined by the phosphorescent Lumogen coating, and is 10 - 15%.
• Image Purge: The residual image resulting from a 100x (or more) full-well over-exposure is well below the read noise within 30 minutes. No CCD image purge is needed after observations of very bright objects. The Loral devices bleed almost exclusively along the columns.
• Quantization: The systematic Analog-to-Digital converter errors have been largely eliminated, contributing to a lower effective read noise.
• QEH: Quantum Efficiency Hysteresis (QEH) is not a significant problem in the Loral CCDs because they are front side illuminated and use MPP operation. The absence of any significant QEH means that the devices do not need to be UV-flooded and the chips can be warmed monthly for decontamination purposes without needing to maintain a UV-flood.
• Detector MTF: The Loral devices do suffer from low level detector MTF (Modulation Transfer Function) perhaps caused by scattering in the front side electrode structure. The effect is to blur images and decrease the limiting magnitude by about 0.5 magnitudes.