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The WFPC2 was installed in the Hubble Space Telescope (HST) in 1993 December. Since then, the instrument has been providing high-quality images. A significant amount of calibration data has been collected to aid in the understanding of the on-orbit performance of the instrument. Generally, the behavior of the camera is similar to its performance during the system-level thermal vacuum test at JPL in 1993 May. Surprises were a significant charge-transfer-efficiency (CTE) problem and a significant growth rate in hot pixels at the original operating temperature of the CCDs (-76 °C). The operating temperature of the WFPC2 CCDs was changed to -88 °C on 1994 April 23, and significant improvements in CTE and hot pixels are seen at this temperature. In this paper we describe the on-orbit performance of the WFPC2. We discuss the optical and thermal history, the instrument throughput and stability, the PSF, the effects of undersampling on photometry, the properties of cosmic rays observed on-orbit, and the geometric distortion in the camera. We present the best techniques for the reduction of WFPC2 data, and describe the construction of calibration products including superbiases, superdarks, and flat fields.

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The protoÈplanetary nebula, CRL 2688, has been imaged through a wideband Ðlter centered at 606 nm (F606W) with the Wide Field Planetary Camera 2 onboard the Hubble Space T elescope. CRL 2688 is the prototypical bipolar reÑection nebula in which a star is surrounded by a dense, Ñattened cocoon of dust seen nearly edge-on and starlight escapes preferentially along the polar directions producing a pair of bright nebulosities, one above and one below the equatorial plane. We Ðnd a pair of radial "" searchlight beams ÏÏ emerging from within the dusty cocoon which intersect at the position expected for the central star when extrapolated inside the cocoon. The beams are crisscrossed by a large number of roughly round arcs with their center of curvature in the vicinity of the central star. The arcs are not systematically elongated along the polar axis of the nebula, as would be expected in the current model of CRL 2688 where the nebular density decreases with latitude.Our image directly shows the last D13,000 yr history of mass ejection from the central star while it was on the tip of the asymptotic giant branch (AGB). We Ðnd that the average surface brightness varies as r~3.7, implying that the average mass-loss rate or the scattering opacity of the dust grains varies as r~0.7, i.e., one or both of these parameters have steadily increased with time. The temporal resolution of B25 yr in our images has provided direct evidence for episodic increases in the mass-loss rate by factors of D2 or more, occurring every 150È450 yr, and lasting over periods of 75È200 yr. These irregularities in the mass-loss rate are roughly spherically symmetric. We have resolved the edges of the searchlight beams, as well as the peculiar structure of the inner region of the nebula Our data (1A .5 \ radius \ 6A). require a new model for CRL 2688 in which the beams result from starlight escaping through a pair of nonuniform annular holes in the dust cocoon which are coaxial with the polar axis of the nebula. The holes have probably been generated by a young (less than 200 yr) high-velocity outÑow which streams out through these holes and interacts with the surrounding dense AGB wind to produce the peculiar structure of the inner nebula. The cocoon contains dust grains of size about 0.6 km, signiÐcantly larger than those in the extended nebula.