James H. Crocker

The Discovery of a Field Methane Dwarf from Sloan Digital Sky Survey Commissioning Data

We report the discovery of the coolest field dwarf yet known, selected as an unresolved object with extremely red colors from commissioning imaging data of the Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 μm is dominated by strong bands of H2O and CH4. Its spectrum and colors over this range are very similar to those of Gl 229B, the only other known example of a methane dwarf. It is roughly 1.2 mag fainter than Gl 229B, suggesting that it lies at a distance of ~10 pc. Such a cool object must have a mass well below the hydrogen-burning limit of 0.08 M☉ and therefore is a genuine brown dwarf, with a probable mass in the range 0.015-0.06 M☉ for an age range of 0.3-5 Gyr.

Colors of 2625 Quasars at 0 < z < 5 Measured in the Sloan Digital Sky Survey Photometric System*

We present an empirical investigation of the colors of quasars in the Sloan Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625 quasars with SDSS photometry: 1759 quasars found during SDSS spectroscopic commissioning and SDSS follow-up observations on other telescopes, 50 matches to FIRST quasars, 573 matches to quasars from the NASA Extragalactic Database, and 243 quasars from two or more of these sources. The quasars are distributed in a 25 wide stripe centered on the celestial equator covering ~529 deg2. Positions (accurate to 02) and SDSS magnitudes are given for the 898 quasars known prior to SDSS spectroscopic commissioning. New SDSS quasars, which range in brightness from i* = 15.39 to the photometric magnitude limit of the survey, represent an increase of over 200% in the number of known quasars in this area of the sky. The ensemble average of the observed colors of quasars in the SDSS passbands are well represented by a power-law continuum with αν = -0.5 (fν ∝ να) and are close to those predicted by previous simulations. However, the contributions of the small blue (or λ3000) bump and other strong emission lines have a significant effect upon the colors. The color-redshift relation exhibits considerable structure, which may be of use in determining photometric redshifts for quasars from their colors alone. The range of colors at a given redshift can generally be accounted for by a range in the optical spectral index with a distribution αν = -0.5 ± 0.65 (95% confidence), but there is a red tail in the distribution. This tail may be a sign of internal reddening, especially since fainter objects at a given redshift tend to exhibit redder colors than the average. Finally, we show that there is a continuum of properties between quasars and Seyfert galaxies, and we test the validity of the traditional dividing line (MB = -23) between the two classes of active galactic nuclei.

Advanced camera for the Hubble Space Telescope

The Advanced Camera for the Hubble Space Telescope has three cameras. The first, the Wide Field Camera, will be a high- throughput, wide field, 4096 X 4096 pixel CCD optical and I-band camera that is half-critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is a 1024 X 1024 pixel CCD camera that is critically sampled at 500 nm. The HRC has a 26 inch X 29 inch field of view and 29 percent throughput at 250 nm. The HRC optical path includes a coronagraph that will improve the HST contrast near bright objects by a factor of approximately 10 at 900 nm. The third camera, the solar-blind camera, is a far-UV, pulse-counting array that has a relatively high throughput over a 26 inch X 29 inch field of view. The advanced camera for surveys will increase HSTs capability for surveys and discovery by a factor of approximately 10 at 800 nm.

The First Hour of Extragalactic Data of the Sloan Digital Sky Survey Spectroscopic Commissioning: The Coma Cluster

On 1999 May 26, one of the Sloan Digital Sky Survey (SDSS) fiber-fed spectrographs saw astronomical first light. This was followed by the first spectroscopic commissioning run during the dark period of 1999 June. We present here the first hour of extragalactic spectroscopy taken during these early commissioning stages: an observation of the Coma cluster of galaxies. Our data samples the southern part of this cluster, out to a radius of 15 (1.8 h-1 Mpc, approximately to the virial radius) and thus fully covers the NGC 4839 group. We outline in this paper the main characteristics of the SDSS spectroscopic systems and provide redshifts and spectral classifications for 196 Coma galaxies, of which 45 redshifts are new. For the 151 galaxies in common with the literature, we find excellent agreement between our redshift determinations and the published values, e.g., for the largest homogeneous sample of galaxies in common (63 galaxies observed by Colless & Dunn) we find a mean offset of 3 km s -1 and an rms scatter of only 24 km s -1. As part of our analysis, we have investigated four different spectral classification algorithms: measurements of the spectral line strengths, a principal component decomposition, a wavelet analysis and the fitting of spectral synthesis models to the data. We find that these classification schemes are in broad agreement and can provide physical insight into the evolutionary histories of our cluster galaxies. We find that a significant fraction (25%) of our observed Coma galaxies show signs of recent star formation activity and that the velocity dispersion of these active galaxies (emission-line and poststarburst galaxies) is 30% larger than the absorption-line galaxies. We also find no active galaxies within the central (projected) 200 h-1 kpc of the cluster. The spatial distribution of our Coma active galaxies is consistent with that found at higher redshift for the CNOC1 cluster survey. Beyond the core region, the fraction of bright active galaxies appears to rise slowly out to the virial radius and are randomly distributed within the cluster with no apparent correlation with the potential merger or postmerger of the NGC 4839 group. We briefly discuss possible origins of this recent galaxy star formation.

Optical performance of the Corrective Optics Space Telescope Axial Replacement (COSTAR)

This paper presents the initial on-orbit performance of the corrective optics space telescope axial replacement or COSTAR. At this time the FOC and FOS channels are aligned and the data analyzed. The GHRS is aligned and initial results are presented. The installation and checkout of the COSTAR into the HST was flawless. The optical performance should enable the science lost to spherical aberration of the HSTs primary mirror to be fully recovered.

In-orbit performance of the COSTAR-corrected Faint Object Camera

The improvements due to the Corrective Optics Space Telescope Axial Replacement (COSTAR) on imaging with the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST) are presented. The encircled energy performance is dramatically improved, such that 85% of the total light in the Point Spread Function (PSF) is now enclosed within a circle of radius 0.1 sec at 486 nm wavelength, compared to 18% in the spherically aberrated PSF. This is equivalent to a sensitivity increase of 1.6 mag. The effective angular resolution is also improved from 66 to 43 mas at 486 nm. These improvements are slightly offset by a 20% lower total throughput at visible wavelengths. The plate scale is changed from 22.3 mas/pixel to 14.35 mas/pixel, resulting in a decrease in the field of view from 11 x 11/sq arcsec to 7.3 x 7.3/sq arcsec for the workhorse 512 x 512 format.

Advanced camera for surveys

The Advanced Camera for Surveys (ACS) is a third generation instrument for the Hubble Space Telescope (HST). It is currently planned for installation in HST during the fourth servicing mission in Summer 2001. The ACS will have three cameras.

POST: A Polar Stratospheric Telescope

The lower stratosphere in the polar regions offers conditions for observation in the near-infrared comparable to those obtained from space. We describe a concept for a 6-meter, diluted aperture, near-infrared telescope carried by a tethered aerostat flying at 12 km altitude, to serve as a testbed for future space astronomical observatories while producing frontier science.

On-orbit alignment of the spectrograph channels of the Corrective Optics Space Telescope Axial Replacement (COSTAR)

The corrective optics space telescope axial replacement (COSTAR), designed to restore the high image quality that was originally expected from the Hubble Space Telescope, was successfully installed in the observatory during the first HST servicing mission in December 1993. While verification testing in the lab has shown that the imaging performance of COSTAR is nearly ideal, realization of the expected high spatial resolution of the COSTAR-corrected images depends strongly on the ability to align the COSTAR mirrors with respect to the telescope and axial science instruments. We report on the methods used in the weeks following the servicing mission to optimize the HST-COSTAR performance for the Faint Object Spectrograph and Goddard High Resolution Spectrograph channels. The relatively low spatial resolution afforded by the spectrographs as well as defects in their imaging modes presented special challenges to rapid optimization of the COSTAR correction. Nevertheless, the alignment process quickly converged and has yielded images that are superior to the COSTAR specifications.

Status and optical performance of the corrective optics space telescope axial replacement (COSTAR)

The Corrective Optics Space Telescope Axial Replacement (COSTAR) is designed to restore the high image quality, lost to a primary mirror manufacturing defect, that was originally expected from the Hubble Space Telescope (HST). Though its rapid development is both a technical and programmatic challenge, COSTAR is nearing completion at Ball Aerospace and is scheduled for deployment during the first HST servicing shuttle mission late this year. This paper describes the extensive optical testing that has been planned to assure the high quality imaging performance of the COSTAR-corrected HST. Independent, redundant tests at both component and system levels are employed to verify that design tolerances are sufficient and manufacturing tolerances are achieved. We report the results of the critical tests which have been completed to date and their implications for the on-orbit performance.