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1.4 The Previous vs. Current Generation: WF/PC-1 vs. WFPC2

• Field format: WF/PC-1 contained 8 cameras and CCDs, each CCD having 800 x 800 pixels. Four were used in the Planetary Camera mode (0.043" pixels), and four were used in the Wide Field Camera mode (0.10" pixels). The two pixel formats were selected by rotating the pyramid mirror by 45 degrees . WFPC2 budget and schedule constraints forced a reduction from 8 to 4 camera channels in August 1991. WFPC2 contains only 4 CCDs; the pyramid mirror is fixed and the 4 cameras are physically located in the bays occupied by the WF/PC-1 WFC.

• Aberration correction: WF/PC-1 contained no correction for spherical aberration of the OTA primary mirror. Only about 15% of light from a stellar target fell into the core of the PSF (diameter ~0.1" ). WFPC2 incorporates corrective figures on the Cassegrain secondary mirrors inside the relay cameras, and as a result places ~60% of the light from a star inside a diameter of 0.1" . Precise alignment of the OTA pupil on these mirrors is required to attain full correction of the spherical aberration. Hence the pick-off mirror (POM) is steerable in WFPC2, and three of the fold mirrors contain tip-tilt actuators.

• CCD Technology: Many properties of WF/PC-1 and WFPC2 CCDs are compared in Table 4.1 on page 65. Many differences derive from the fact that the WF/PC-1 CCDs were thinned, backside illuminated devices whereas the WFPC2 CCDs are thick, frontside illuminated devices. WF/PC-1 CCDs were thinned and backside illuminated. The active silicon layer was a free-standing membrane somewhat less than 10 microns thick, with photons impinging directly on the silicon layer, without attenuation in the polysilicon gate structure built on the other ('front') side of the device.

• Quantum Efficiency Hysteresis (QEH): The WF/PC-1 CCD's required a UV flood procedure and continuous cold temperatures to avoid QEH and hence non-linearity. A UV flood was performed early in the WF/PC-1 mission, but could not be repeated due to problems with the HST magnetometers. This in turn limited the temperature range allowable during decontaminations, since high temperatures would remove the UV flood, which in turn severely limited UV science capabilities. Some QE instability was also seen, particularly in the B band, due to changes in the UV flood. WFPC2 CCDs support multi-pinned phase (MPP) operation which eliminates quantum efficiency hysteresis.

• Charge Transfer Efficiency: WF/PC-1 devices suffered from significant charge transfer efficiency (CTE) errors at image intensities below ~200 electrons per pixel. This was overcome by preflashing virtually all science images. WFPC2 devices have very little CTE error, and hence no preflash is used. Low-level charge traps are present in the WFPC2 devices, and result in a loss of about 4% of the signal when a star image is clocked down through all rows of the CCD. In the presence of background, the effect is reduced. For most applications, CTE is negligible or calibratable and pre-flash exposures are not required.

• Detector MTF: The WFPC2 Loral devices do suffer from poorer CCD detector MTF than the WF/PC-1 CCDs, perhaps caused by scattering in the frontside electrode structure. The effect is to blur images and decrease the limiting magnitude by about 0.5 magnitudes.

• Flat field quality: WF/PC-1 CCDs were chemically thinned devices and therefore varied in thickness across the field-of-view causing large features in the flat fields. WFPC2 CCDs are un-thinned and the intrinsic response is uniform to ~3% across the field.

• DQE: The WFPC2 CCDs have intrinsically lower QE than WF/PC-1 CCDs above 4800Å, which is due to attenuation by frontside electrode structures.

• Gain switch: WF/PC-1 had only a single analog-to-digital converter gain setting of 8 e- DN-1 which saturated at about 30,000e-. Two gains are available with WFPC2: a 7 e- DN-1 channel which gives reasonable sampling of the 5e- read noise, and which saturates at about 27,000e-, and a 14 e- DN-1 channel which saturates at about 53,000e- and extends the useful dynamic range.

• Quantization: The systematic analog-to-digital converter errors that were present in the low order bits on WF/PC-1 have been largely eliminated, contributing to a lower effective read noise in WFPC2.

• Calibration Channel: WF/PC-1 contained a solar UV flood channel which was physically in the location of the present WFPC2 calibration channel. This transmitted solar UV light into the camera to provide a UV flood capability.

• Entry Port: The WF/PC-1 camera was sealed by an afocal MgF2 window immediately behind the shutter. The WFPC2 entry port is open.

• Chronographic Capability: WF/PC-1 contained a low reflectance spot on the pyramid (known as the Baum spot) which could be used to occult bright objects. This has been eliminated from WFPC2, since the spherical aberration severely reduces its utility.

• Contamination Control: Since launch, WF/PC-1 suffered from the accumulation of molecular contaminants on the cold (-87 degrees C) CCD windows. This molecular accumulation resulted in the loss of FUV (1150-2000Å) throughput and attenuation at wavelengths as long as 5000Å. Another feature of the contamination was the "measles" -- multiple isolated patches of low volatility contamination on the CCD window. Measles were present even after decontamination cycles, when most of the accumulated molecular contaminants were boiled off by warming the CCDs. In addition to preventing UV imaging, these molecular contamination layers scattered light and seriously impacted the calibration of the instrument. WFPC2 has far less contamination than WF/PC-1 owing to pre-launch cleaning and bake-out procedures, careful design of venting paths to protect the optical bench area, and inclusion of Zeolite molecular absorbers in the design. There is now a decrease in throughput of about 30% per month at 1700Å, but monthly decontamination procedures completely remove this material. This throughput drop is also highly predictable and can be calibrated out during photometric analyses.