HH 30 in Taurus has been imaged with the Hubble Space T elescope WFPC2. The images show in reÑected light a Ñared disk with a radius of about 250 AU that obscures the protostar. The disk resembles detailed accretion disk models that constrain its density distribution and show that its inclination is less than 10¡. There are bipolar emission-line jets perpendicular to the disk, a very clear demonstration of the standard paradigm for accretion disk and jet systems. However, asymmetries in the light distribution show that the disk has not completely settled into a quasi-equilibrium accretion state, or that some of the observed scattering is from an asymmetric envelope. The emission-line jet itself is resolved into a number of knots with typical lengths and separations of much smaller and more numerous than 0A .4, indicated by lower resolution ground-based studies. There are indications of still Ðner structures in the jet all the way to the resolution limit ofThe knots have proper motions ranging from 100 to 300 0A .1. km s~1 and are therefore generated at the surprisingly high rate of about 0.4 knots per jet per year. The jet appears to be collimated within a cone of opening angle 3¡ and can be seen to within 30 AU of the star.Both single-and multiple-scattering disk models have a range of possible solutions, but by requiring pressure support and temperature equilibrium, a self-consistent model emerges. There is evidence for pressure support because the disk appears to have a Gaussian height proÐle. The temperature at each point in the disk is determined by the disk geometry, which in turn Ðxes the temperature in a selfconsistent manner. The extinction to the protostar is unknown but constrained to be greater than 24 mag. The optical properties of the scattering grains in the disk are determined and found to imply a large scattering asymmetry, but they seem to follow the interstellar reddening law. The absolute magnitude and colors of the unseen protostar, which has a brightness in the I bandpass of about 0.16 times solar and is very red, are obtained. The disk mass is about 0.006 times solar and has an expected lifetime of about 105 yr.

We present an optical/near IR selected catalog of 79 clusters distributed over an area of 5.1 square degrees. The catalog was constructed from images obtained with the 4-Shooter CCD mosaic camera on the Hale 5m telescope operating in \scan" mode. The survey, hereafter known as the Palomar Distant Cluster Survey (PDCS), was conducted in two broad band lters that closely resemble V and I. The 4 limiting magnitudes for our 300 s exposures are 23.8 (V ) and 22.5 (I). A matched lter algorithm was developed and employed to identify the cluster candidates by using positional and photometric data simultaneously. The clusters cover the range 0:2 < z < 1:2, based on the redshift estimates derived in the cluster detection procedure. An accurate selection function is generated from extensive simulations. We nd that the cumulative surface density of clusters with richness class R 1 is about a factor of 5 higher than the extrapolated density of R 1 Abell clusters. The PDCS results are consistent with a constant comoving density of clusters to z < 0:6, albeit at the above high density level. Constraints on cluster abundances at z > 0:6 will be possible with the acquisition of spectroscopic redshifts for a large subset of these cluster candidates. We also present a supplemental catalog of 28 clusters that do not satisfy all our selection criteria but which include some of the most distant systems detected in the survey. Finding charts for all 107 cluster candidates are provided. (Astro-ph readers: due to their large size, the nding charts are not included in this version of the paper).Subject headings: galaxies: clustering; cosmology: observations { 3 { { 5 { associated with CCD readout; one is exposing essentially 100% of the time), 2) Data taken in TDI mode can be extremely well at-elded (see Schneider, Schmidt, & Gunn 1994 and x2.3 of this paper). This property is especially important for these survey images, as the Texas Instruments CCDs have very poor fringing properties at near-infrared wavelengths. 3) The con guration of the 4-Shooter focal plane (Figure 1) is well-suited for two-lter observations in TDI mode.The PDCS data were obtained by moving the telescope north along lines of constant Right Ascension (equinox 1950.0) at a rate of 0 00 : 893 s 1 ; this yields a CCD transit time of 300 seconds. Because of the aforementioned pyramid shadowing in the 4-Shooter, the e ective exposure time is 293 seconds. The data are acquired in two colors, nearly simultaneously, by making use of the fact that the 4-Shooter can be equipped with one set of lters for the leading two cameras (CCDs 1 and 4, see Figure 1) in the scan and another set for the trailing ones (CCDs 2 and 3).A survey eld consists of eight adjacent scans. Each scan requires approximately 80 minutes to complete and overlaps the adjacent scan by 30 00 . The total area imaged in each eld is usually slightly larger than a square degree (see Table 1).Imaging of the ve elds began in October 1986 and required approximately three years to complete. The data were all obtained...

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We present images of the Crab synchrotron nebula obtained with the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. These data are compared with ROSAT HRI images, and with 0':5 resolution Canada-France-Hawaii Telescope (CFHT) images previously published by van den Bergh & Pritchet (1989). These data strengthen the emerging picture of the Crab as a cylindrically symmetrical object with an axis running southeast to northwest, and inclined by ~20°-30° with respect to the plane of the sky. Identification of structure very near to the pulsar which shares this symmetry helps to better establish the link between the symmetry axis of the nebula and the spin axis of the pulsar. We report the discovery of a bright knot of visible emission located 0':65 to the southeast of the pulsar, along the axis of the system. This knot and a second knot 3':8 from the pulsar appear to be present but not well resolved in the 1988 CFHT image, indicating that they are persistent structures. The inner knot is interpreted as a shock in the pulsar wind ~ 1500 AU above the pole of the pulsar. No corresponding knots are seen to the northwest of the pulsar, which may indicate that the characteristics of the wind from the two poles are not symmetrical. The closest of the "wisps" to the northwest of the pulsar appear to close into a ringlike "halo" encircling the axis of the nebula. The wisps are resolved, with widths of ~ 0':2. This allows• calculation of their volumes and volume emissivities, and in turn their equipartition fields and pressures. Equipartition pressures calculated for the knots and wisps are typically 10 to as much as 80 times the equipartition pressure calculated for the nebula as a whole. The wisps show significant substructure which changed considerably between 1988 and 1994. Previous reports of relativistic motions of the wisps were probably due to changes in the unresolved substructure of these features. Comparison of the CFHT and WFPC2 images show remarkable changes in the inner nebula, but inferences about physical conditions based on this comparison are limited by the resolution of the CFHT data and the long 5 year baseline between the images. The structure of the nebula in 1994 may be inconsistent with the recent model by Gallant & Arons (1994). Very fine fibrous texture visible in the WFPC2 image follows the structure of the X-ray torus. A puzzling anticorrelation is seen between the X-ray and visible surface brightness through part of the torus. Long contiguous low contrast features with widths of ~ 1"-2" are seen to run throughout the volume of the nebula. These features are seen to move outward through the nebula at velocities in excess of homologous expansion. These features trace the magnetic structure of the nebula; they are probably due to differences in emissivity accompanying varying degrees of departure from equipartition at roughly constant total pressure. Visible fibers "drapt< over" and appear to expand away from an X-ray counterjet to the northwest of the pulsar, supporting th...

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.