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A public release of slitless spectra, obtained with ACS/WFC and the G800L grism, is presented. Spectra were automatically extracted in a uniform way from 153 archival fields (or "associations") distributed across the two Galactic caps, covering all observations to 2008. The ACS G800L grism provides a wavelength range of 0.55-1.00 \mu$m, with a dispersion of $40 \ \AA / pixel$ and a resolution of $\sim 80\ \AA$ for point-like sources. The ACS G800L images and matched direct images were reduced with an automatic pipeline that handles all steps from archive retrieval, alignment and astrometric calibration, direct image combination, catalogue generation, spectral extraction and collection of metadata. The large number of extracted spectra (73,581) demanded automatic methods for quality control and an automated classification algorithm was trained on the visual inspection of several thousand spectra. The final sample of quality controlled spectra includes 47,919 datasets (65% of the total number of extracted spectra) for $32,149$ unique objects, with a median $i_{\rm AB}$-band magnitude of 23.7, reaching 26.5 AB for the faintest objects. Each released dataset contains science-ready 1D and 2D spectra, as well as multi-band image cutouts of corresponding sources and a useful preview page summarising the direct and slitless data, astrometric and photometric parameters. In order to characterize the slitless spectra, emission-line flux and equivalent width sensitivity of the ACS data were compared with public ground-based spectra in the GOODS-South field. An example list of emission line galaxies with two or more identified lines is also included, covering the redshift range $0.2-4.6$., Comment: Accepted for publication in Astronomy and Astrophysics; 29 pages, 16 Figures, 4 Tables in text and 3Tables in Appendix

We present our image processing system for the reduction of optical imaging data from multi-chip cameras. In the framework of the Garching Bonn Deep Survey (GaBoDS; Schirmer et al. 2003) consisting of about 20 square degrees of high-quality data from WFI@MPG/ESO 2.2m, our group developed an imaging pipeline for the homogeneous and efficient processing of this large data set. Having weak gravitational lensing as the main science driver, our algorithms are optimised to produce deep co-added mosaics from individual exposures obtained from empty field observations. However, the modular design of our pipeline allows an easy adaption to different scientific applications. Our system has already been ported to a large variety of optical instruments and its products have been used in various scientific contexts. In this paper we give a thorough description of the algorithms used and a careful evaluation of the accuracies reached. This concerns the removal of the instrumental signature, the astrometric alignment, photometric calibration and the characterisation of final co-added mosaics. In addition we give a more general overview on the image reduction process and comment on observing strategies where they have significant influence on the data quality., Comment: 34 pages, 33 figures; submitted to A&A main journal

The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z=0.570 cluster RX J0848+4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red R-K color and strong, narrow emission lines. Analysis of HST WFPC2 imaging and Keck optical and infrared spectroscopy shows that the arc is an \hii galaxy magnified by a factor of ~10 by a complex cluster environment. The high intrinsic luminosity, the emission line spectrum, the absorption components seen in Ly\alpha and C IV, and the restframe ultraviolet continuum are all consistent with a simple HII region model containing ~ 10^6 hot O stars. The best fit parameters for this model imply a very hot ionizing continuum (T_BB ~ 80,000 K), high ionization parameter (log U ~ -1), and low nebular metallicity (Z / Z_\odot ~ 0.05). The narrowness of the emission lines requires a low mass-to-light ratio for the ionizing stars, suggestive of an extremely low metallicity stellar cluster. The apparent overabundance of silicon in the nebula could indicate enrichment by past pair instability supernov\ae, requiring stars more massive than ~140 M_\odot., Comment: 37 pages, 9 color figures; ApJ in press (v596 n2 October 20, 2003)

We present deep optical and near-infrared imaging of the rich cluster A2218 at z=0.17. Our optical imaging comes from new multicolour Hubble Space Telescope WFPC2 observations in the F450W (B), F606W (V) and F814W (I) passbands. These observations are complemented by deep near-infrared, Ks-band, imaging from the new INGRID imager on the 4.2-m William Herschel Telescope. This combination provides unique high-precision multicolour optical-infrared photometry and morphological information for a large sample of galaxies in the core of this rich cluster at a lookback time of ~3Gyrs. We analyse the (B-I), (V-I) and (I-Ks) colours of galaxies spanning a range of a factor of 100 in K-band luminosity in this region and compare these with grids of stellar population models. We find that the locus of the colours of the stellar populations in the luminous (>0.5L*) early-type galaxies, both ellipticals and S0s, traces a sequence of varying metallicity at a single age. At fainter luminosities (<0.1L*), this sequence is extended to lower metallicities by the morphologically-classified ellipticals. However, the faintest S0s exhibit very different behaviour, showing a wide range in colours, including a large fraction (30%) with relatively blue colours which appear to have younger luminosity-weighted ages for their stellar populations, 2-5Gyrs. We show that the proportion of these young S0s in the cluster population is consistent with the observed decrease in the S0 population seen in distant clusters when interpreted within the framework of a two-step spectroscopic and morphological transformation of accreted spiral field galaxies into cluster S0s., Comment: 11 pages, 1 jpeg figure, Submitted to MN

We present the imaging observations made with the Space Telescope Imaging Spectrograph of the Hubble Deep Field - South. The field was imaged in 4 bandpasses: a clear CCD bandpass for 156 ksec, a long-pass filter for 22-25 ksec per pixel typical exposure, a near-UV bandpass for 23 ksec, and a far-UV bandpass for 52 ksec. The clear visible image is the deepest observation ever made in the UV-optical wavelength region, reaching a 10 sigma AB magnitude of 29.4 for an object of area 0.2 square arcseconds. The field contains QSO J2233-606, the target of the STIS spectroscopy, and extends 50"x50" for the visible images, and 25"x25" for the ultraviolet images. We present the images, catalog of objects, and galaxy counts obtained in the field., Comment: 44 pages, 10 figures, accepted for publication in the Astronomical Journal, February 2000. The resolution of the images has been reduced; preprints with full resolution images are available at http://hires.gsfc.nasa.gov/~gardner/hdfs/stispaper/index.html

We have developed a method for the linear reconstruction of an image from undersampled, dithered data. The algorithm, known as Variable-Pixel Linear Reconstruction, or informally as Drizzle, preserves photometry and resolution, can weight input images according to the statistical significance of each pixel, and removes the effects of geometric distortion both on image shape and photometry. This paper presents the method and its implementation. The photometric and astrometric accuracy and image fidelity of the algorithm as well as the noise characteristics of output images are discussed. In addition, we describe the use of drizzling to combine dithered images in the presence of cosmic rays., Comment: 19 pages, 5 figures, accepted for publication in February 2002 PASP. Major changes and much additional material since original submission in 1998

As a prerequisite for interpreting new observations of the most distant radio galaxies, we make an optical study of the closest powerful radio source, Cygnus A. Using Keck imaging- and spectro-polarimetry in conjunction with HST, WFPC2 broad and narrow band imaging, we are able to identify specific geometrical structures in the galaxy with optical continuum components distinguished by both colour and polarimetric properties. A 4 kpc diameter dusty ring of young stars forms the equator of a double ionization cone which is co-axial with the radio jets., Comment: 11 pages in PDF format. Last page is a colour plate. To be published in the KNAW colloquium on: "The most distant radio galaxies", Amsterdam, 15-17 October 1997, Roettgering, Best and Lehnert, eds, Reidel

Images of the Pluto-Charon system were obtained with the Faint Object Camera (FOC) of the Hubble Space Telescope (HST) after the refurbishment of the telescope. The images are of superb quality, allowing the determination of radii, fluxes, and albedos. Attempts were made to improve the resolution of the already diffraction limited images by image restoration. These yielded indications of surface albedo distributions qualitatively consistent with models derived from observations of Pluto-Charon mutual eclipses.