The Hubble Space Telescope (HST) was placed into orbit by the Discovery shuttlecraft on April 25, 1990, and first light images were obtained on May 20, 1990 ( Feinberg, 1990a; Palga, 1990). On June 26, NASA announced that the telescope suffered from spherical aberration and that, instead of focusing 70 percent of a star's light in an area 0.2 arcsec in diameter, the light was spread over an area with a diameter of 1.4 arcsec. The resulting point-spread function (PSF) contained a 0.2 arc sec central spike that contained 15% of the light and, because the problem concerned two excellent yet mismatched mirrors, the resulting apron of the PSF could be represented by a predictable diffraction pattern, indicating that part of the resolution of the two cameras could be recovered by image deconvolution.
As a part of the effort to assess the magnitude of the problem, the Wide Field/Planetary Camera (WFPC-1) team obtained broadband blue, green and red images of Saturn on August 26, 1990 ( Westphal et al., 1991). Using PSFs derived from a star for the 547 nm filter and theoretical PSFs for the 439 nm and 718 nm filters, the WFPC-1 team used Lucy's algorithm (1974) to deconvolve the data and produced a composite three-color image. This effort showed that a narrow gap, the Encke Division, in the outer ring could be resolved and produced an effective resolution of the deconvolved images that was better than the best groundbased results. Even with the focusing problems, this effective resolution---combined with the fact that HST suffers less from scattered light, has UV sensitivity and yields the same resolution from observation to observation---recommended it for planetary studies involving spatial and temporal variability. These initial observations revealed that Saturn's north polar hexagon, seen by Voyager ( Allison et al., 1990), was visible, implying that it is not a transient feature. The latitudinal locations of belt-zone interfaces differed from those recorded in 1981 Voyager images, indicating that seasonal or undetected sporadic convective activity had altered the reflective properties of the atmosphere.
With the problem of deconvolution partially resolved, the planetary community moved on to planned observations. Due to the fact that Mars was near a favorable opposition, observations were begun before Orbit Verification Activities had been completed. This campaign ( James et al., 1994) involved multicolor images with the Planetary Camera mode of the WFPC-1 and pole-to-pole scans with the Faint Object Spectrograph (FOS). The goal was to acquire climatic Martian data for the lifetime (approx. 15 years) of HST. This required that the system be capable of tracking a target that moved relative to the background stars. Because the planned software was not yet available, these observations required considerable manpower to implement. Although progress has been made on these problems, all planned capabilities have not been implemented. Track 51, capable of linear tracking in both right ascension and declination, has been implemented. There is no plan to develop track 48, capable of nonlinear tracking, in the near future; however, a procedure that allows guide star handoff has been implemented. Throughout the 30-50 minute observing window in a given orbit the tracking can be broken into segments, where the shutter is closed and the object reacquired ( Hart, 1994). Guidelines for planning to observe moving targets are available in Downes (1994).