We describe ultraviolet and optical imaging and spectroscopy within the central few arcseconds of the Seyfert galaxy NGC 4151, obtained with the Faint Object Camera on the Hubble Space Telescope. A narrowband image including [O III] lambda 5007 shows a bright nucleus centered on a complex biconical structure having apparent opening angle similar to 65 degrees and axis at position angle along 65 degrees-245 degrees; images in bands including Lyman-alpha and CIV lambda 1550 and in the optical continuum near 5500 Angstrom, show only the bright nucleus. In an off-nuclear optical long-slit spectrum we find a high and a low radial velocity component within the narrow emission lines. We identify the low-velocity component with the bright, extended, knotty structure within the cones, and the high-velocity component with more confined diffuse emission. Also present are strong continuum emission and broad Balmer emission line components, which we attribute to the extended point spread function arising from the intense nuclear emission.Adopting the geometry pointed out by Pedlar et al. (1993) to explain the observed misalignment of the radio jets and the main optical structure we model an ionizing radiation bicone, originating within a galactic disk, with apex at the active nucleus and axis centered on the extended radio jets. We confirm that through density bounding the gross spatial structure of the emission line region can be reproduced with a wide opening angle that includes the line of sight, consistent with the presence of a simple opaque torus allowing direct view of the nucleus. In particular, our modelling reproduces the observed decrease in position angle with distance from the nucleus, progressing initially from the direction of the extended radio jet, through our optical structure, and on to the extended narrow-line region.We explore the kinematics of the narrow-line low- and high-velocity components on the basis of our spectroscopy and adopted model structure. For the low-velocity system both Keplerian rotation and isotropic outflow (or outflow confined to the ionizing cone) give plausible correspondence with our data. If interpreted as rotation we show consistency with earlier determinations indicating a central mass concentration of about 10(9) M.. The high-velocity system kinematically conforms to radial outflow within the galaxy disk, although this does not well reproduce the observed intensity structure.

We have observed the blue dwarf galaxy IZw 36 with the f/96 relay of the Faint Object Camera and have for the first time resolved massive stars, using the broad band filters F175W, F342W, F430W and F480LP. We have measured the fluxes of 143 of these objects and studied their characteristics in color-magnitude diagrams. A few stars may be red supergiants but their contribution to the integrated light is less than 5% in the F430W filter. The F175W-F430W color of the integrated stellar population is redder than expected from the current burst of star formation, suggesting therefore the presence of an older and unresolved underlying population. The ultraviolet measurements combined with synthetic photometry calculations allow us to place the massive stars in a bolometric magnitude vs. temperature diagram. In this diagram, the stars are compared to evolutionary tracks for different stellar masses. The current burst probably has an age less than 12 Myr. We infer an Initial Mass Function, with a power-law slope in the range -1.7 to -2.6 for masses M greater than or similar to 20 M.. This is consistent with most of the values reported for sites of star formation in the Galaxy and the Magellanic Clouds and does not support the view of an IMF flattening at low metallicity.

The core of the nearby and very concentrated globular cluster NGC 6397 has been imaged through the f/96-F140W, f/48-F140W, f/96-F210M, and f/48-F220W ultraviolet filters of the Faint Object Camera on board the Hubble Space Telescope down to an ultraviolet limiting magnitude of approximately 19. The most interesting objects in the field of view are six very bright, centrally concentrated, blue stragglers observed for the first time in the UV. Using these and other data from ground-based observations, we have been able to deduce from a comparison with Kurucz's atmosphere models, temperatures of approximately 10 000 K and masses of approximately 1.6 M. for the four brightest ones, which is remarkably close to twice the turn-off mass of NGC 6397. This finding supports the idea that two-star mechanisms (collisions, mergers) are at the origin of the blue stragglers in the core of NGC 6397. Since the central density is very high, collisions between main sequence stars are frequent, therefore providing the best formation mechanism. We have computed the number of such collisions in the core of NGC 6397 and found it to be of the same order as the number of bright blue stragglers observed there. Thanks to the HST resolution, we have also been able to resolve one object, previously classified as a yellow straggler, into a blend of a blue straggler and three redder stars.

Surface photometry data obtained with the Faint Object Camera of the Hubble Space Telescope in the cores of ten galaxies is presented. The major results are: (i) none of the galaxies show truly ''isothermal'' cores, (ii) galaxies with nuclear activity show very similar light profiles, (iii) all objects show central mass densities > 10(3) M . pc-3, (iv) four of the galaxies (M87, NGC 3862, NGC 4594, NGC 6251) show evidence for exceptional nuclear mass concentrations.

Observations with the Faint Object Camera on board the Hubble Space Telescope have revealed a new optical jet in the core of the elliptical galaxy NGC 3862 (3C 264). Morphologically, this jet is similar to the synchrotron jets seen in other galaxies, as it shows knots and bifurcations. The optical spectral index [-d log I(nu)/d log nu = alpha = 1.4] is also similar to that found in other jets. Thus, the nucleus of NGC 3862 appears to contain the fifth known example of an optical synchrotron jet. Since NGC 3862 is a typical radio-loud elliptical galaxy, it seems likely that many nonthermal jets found in the radio continuum may also have optical counterparts.

We have used the Faint Object Camera on board of the Hubble Space Telescope to obtain a high spatial resolution ultraviolet image of the central region of NGC 6240. This image resolves the well known double nucleus of the galaxy into several compact knots with sizes 0.1″−0.2″ (about 45-90 pc for H 0 = 75 km s −1 Mpc −1 ) and extended low surface brightness emission. The northern (fainter) component B of the double nucleus of the galaxy contains only one bright compact source while the southern (brighter) component A consists of a string of bright knots plus more conspicuous extended emission than B. Evidence is presented that an appreciable fraction (if not all) of the compact emission is continuum light. The distance between the compact sources in A and B exceeds the distance between the corresponding 2 cm radio sources by ∼0.5″

We present the first results of an exploratory objective prism survey of the far-ultraviolet (lambdalambda1200-1800) spectra of high-redshift quasars obtained with the Faint Object Camera on-board the Hubble Space Telescope. The prime objective of this survey is to identify one or more candidates among the known quasars at z(em) > 3 with sufficient flux at far-ultraviolet wavelengths to enable detailed follow-up observations of redshifted intergalactic He II lambda304 absorption using the HST grating spectrographs. FOC prism observations of 12 prime candidates selected for redshift (z(em) congruent-to 3.03-3.66), brightness (V less than or similar 18.5) and the appearance of their optical absorption spectra are presented and discussed. As anticipated beforehand, cumulative neutral hydrogen Lyman continuum absorption from the various classes of intervening absorption systems presents a serious obstacle for observing high redshift quasars at extreme ultraviolet rest energies. At our limiting sensitivity of F(lambda) congruent-to 2-4 x 10(-16) ergs s-1 cm-2 angstrom-1, 10 of the 12 objects observed show no far-UV flux shortward of lambda1800 observed wavelength. Two objects, Q0114-089 (UM 670; z(em) = 3.16) and Q1442+102 (OQ 172; z(em) = 3.53) are detected, but only down to lambda congruent-to 330 angstrom and lambda congruent-to 345 angstrom rest wavelength. No object is reliably detected at emitted He II lambda304. The severity of the cumulative Lyman continuum absorption is, however, such that these preliminary findings are still statistically consistent with the hypothesis that luminous quasars are intrinsically bright at extreme ultraviolet energies at flux levels comparable to those suggested by extrapolation of the power law continua seen at longer wavelengths. A larger number of high-redshift quasars need to be observed in the far-UV before the prospects for observing intergalactic He II lambda304 absorption with HST can be fully appraised.

We present a discussion of the various sources of FOC Point Spread Functions (PSFs), and the problems associated with their use. In particular, we highlight the time variability of the PSF halo structure and indicate some of the causes. We examine the usefulness of the PSF modelling software, TinyTim, and show that although this software creates visually similar images, these similarities are to a large extent superficial. We conclude by showing that the theoretical PSFs produced by TinyTim are inadequate for the restoration of high S/N FOC point source images. We also conclude that, because of 'breathing', there is a strong likelihood that empirical PSFs, whether pre-existing or specifically obtained, may not be sufficient either.

We have detected, in images taken with the HST FOC, the UV and optical counterpart of the X-ray source 4U 1820-30 in the globular cluster NGC 6624. Astrometric measurements place this object 2 sigma from the X-ray position of 4U 1820-30. The source dominates a far-UV FOC image and has the same flux at 1400 angstrom as was seen through the large IUE aperture by Rich et al. (1993). It has a B magnitude of 18.7 but is not detected in V. It is 0.66'' from the center of NGC 6624, a fact that may change the interpretation of the P of the 11 minute binary orbit. The flux drops between 1400 and 4300 angstrom at a rate that is nearly as steep as that of a Rayleigh-Jeans curve. The flux is far too large to come from the neutron star directly but could accord with radiation from a heated accretion disk and/or the heated side of the companion star.

The surface of Jupiter's Galilean satellite Io has been resolved for the first time in the near ultraviolet at 2850 A by the Faint Object Camera (FOC) on the Hubble Space Telescope (HST). The restored images reveal significant surface structure down to the resolution limit of the optical system corresponding to approximately 250 km at the sub-earth point.

Ultraviolet and optical images of the central region and jet of the nearby elliptical galaxy M87 have been obtained with approximately 0.1 arcsec resolution in several spectral bands with the Faint Object Camera (FOC) on the Hubble Space Telescope, including polarisation images. Deconvolution enhances the contrast of the complex structure and filamentary patterns in the jet already evident in the aberrated images. Morphologically there is close similarity between the FOC images of the extended jet and the best 2cm radio maps obtained at similar resolution, and the magnetic field vectors from the ultraviolet and radio polarimetric data also correspond well. We observe structure in the inner jet within a few tenths arcsec of the nucleus which also has been well studied at radio wavelengths. Our ultraviolet and optical photometry of regions along the jet shows little variation in spectral index from the value 1.0 between markedly different regions and no trend to a steepening spectrum with distance along the jet. The new results strongly support the model for the jet in which there is no in situ particle acceleration localized at strong shocks but electrons accelerated at the nucleus propagate with low dissipation along the inside of the jet and the observed synchrotron emission occurs primarily in a boundary layer between the jet and its external medium. We observe no evidence for a central star cluster. In the nucleus, our data show the presence of an unresolved bright continuum source of optical to ultraviolet spectral index approximately 1.4 and size less-than-or-equal-to 0.6 pc core radius (at distance 16 Mpc), and an [OIII]-emitting region of approximately 1.6 pc core radius.

A 5161 s exposure was taken with the FOC on the central 44" of M31, through a filter centered at 1750 angstrom. Much of the light is redleak from visible wavelengths, but nearly half of it is genuine UV. The image shows the same central peak found earlier by Stratoscope, with a somewhat steeper dropoff outside that peak. More than 100 individual objects are seen, some pointlike and some slightly extended. We identify them as post-asymptotic giant branch stars, some of them surrounded by a contribution from their accompanying planetary nebulae. These objects contribute almost a fifth of the total UV light, but fall far short of accounting for all of it. We suggest that the remainder may result from the cor-responding evolutionary tracks in a population more metal-rich than solar.

We report on the first observations of a solar system target with the Faint Object Camera of the HST. Observations of the Pluto-Charon system were obtained in f/96 and f/288 mode. Pluto and Charon were clearly resolved, and the observed separation and diameters are in accordance with expectations. The f/96 data were astrometrically and photometrically analyzed; preliminary results are presented.

R136 is the luminous central object of the giant H II region 30 Doradus in the Large Magellanic Cloud. We report on the first high-resolution observations of R136 with the Faint Object Camera on board the Hubble Space Telescope. The physical nature of the brightest component R136a has been a matter of some controversy over the last few years. The UV images obtained show that R136a is a very compact star cluster consisting of more than eight stars within 0".7 diameter. From these high-resolution images a mass upper limit can be derived for the most luminous stars observed in R136.

The Faint Object Camera on the Hubble Space Telescope was used to observe the radio galaxy PKS 0521-36 which hosts a prominent radio jet. Images of the jet show spatial structure comparable to VLA data and significantly better than optical ground-based observations. The jet structure is resolved at FOC resolution. In addition to the radio knot, well resolved by the FOC, an extension of the jet toward the nucleus is apparent. The rest of the jet does not show much clumpiness, implying that the synchrotron electrons must be accelerated all along the jet to account for the extent in the optical region.

The Faint Object Camera on the Hubble Space Telescope was used to observe the inner regions of the symbiotic Mira variable R Aquarii through two filters centered on the [O III] 5007 angstrom and the [O II] 3727 angstrom nebular emission lines. Both images show bright arcs, knots, and filaments superposed on fainter, diffuse nebulosity extending in a general SW-NE direction from the Mira variable to the edge of the field at 10" distance. The observed nebulosity is brighter and more complex in its northern extension where it attains a surface brightness of order 10(-12) ergs cm-2 s-1 arcsec-2 in the [O III] line. Although saturated due to the strong signal, the core is resolved in both lines into at least two bright knots of emission whose positions and structure are aligned with PA = 50-degrees. The central knots appear to be the source of a continuous, well-collimated, stream of material extending out to 3"-4" in the northern sector corresponding to a linear distance of approximately 1000 AU. The northern stream seems to bend around an opaque obstacle and form a spiral before breaking up into discrete wisps and knots deviating toward the north. The southern stream is composed of smaller, discrete parcels of emitting gas curving to the SE. A preliminary estimate of the [O III]-to-[O II] line ratio in the filaments shows that it is significantly higher than 1 indicating either that the features are density-bounded or that the ionizing radiation field has a nonthermal power-law form.