Showing papers by "European Southern Observatory published in 1993"
Abstract: We have measured line-strength indices as a function of radius in several elliptical galaxies. All of them show strong radial gradients in Mg, but much weaker gradients in Fe and Hs. The isophotes and contours of constant line-strength have the same flattening. More luminous galaxies have shallower gradients, contrary to the prediction of models of dissipative collapse. Most of the galaxies observed show weak central emission which can partially fill the Balmer absorption lines.
339 citations
TL;DR: In this paper, long-slit spectra in the range 4500-6500 A have been obtained for a sample of 42 galaxies and correlations of Mg 2 gradients with various physical parameters are studied.
Abstract: Long-slit spectra in the range 4500-6500 A have been obtained for a sample of 42 galaxies. Rotation velocities and velocity dispersions, together with radial line strength gradients of Mg 2 , Mg 1 , Hβ, Na D, TiO 1 , TiO 2 and Fe 5270 , have been measured to, on average, half an effective radius. To a high level of significance, Mg 2 gradients positively correlate with those of Mg 2 and Na D, but no correlation is observed with the other indices. In addition, correlations of Mg 2 gradients with various physical parameters are studied. For galaxies smaller than about 10 11 M ○. , the Mg 2 gradient increases with increasing mass, but more massive objects show no correlation. In low-mass galaxies, the correlation of Mg 2 gradients with mass suggests dissipative collapse as the mechanism acting during the initial star formation episode
222 citations
TL;DR: In this paper, the authors presented optical spectra for a complete sample of radio galaxies and quasars comprising all sources from the Wall & Peacock 2.7 GHz sample with redshifts z<0.7 and declinations δ<+10°; this sample is complete down to a flux density of 2 Jy.
Abstract: We present optical spectra for a complete sample of radio galaxies and quasars comprising all sources from the Wall & Peacock 2.7-GHz sample with redshifts z<0.7 and declinations δ<+10°; this sample is complete down to a flux density of 2 Jy. Although not all of the 2-Jy sources have spectroscopic redshifts, we argue that most of the unidentified objects are at high redshifts and that our z<0.7 sample is largely complete. The optical data will be analysed in a future paper
210 citations
TL;DR: In this article, the authors present a set of optical observations of SN 1988Z covering a period of 3 years from the discovery, showing an unusual photometric behaviour, with broad light curves in the B, V and R bands resembling those of SN 1987F.
Abstract: We present a set of optical observations of SN 1988Z covering a period of 3 yr from the discovery. This relatively bright supernova shows an unusual photometric behaviour, with broad light curves in the B, V and R bands resembling those of SN 1987F. The colour evolution is unique, with a slow blueward evolution in the early months, after which it remains nearly constant. The spectrum is characterized at all epochs by strong Balmer emission, without the usual P Cygni absorption component which is typical of SNeII at early stages. Several He I emission lines are also visible. These lines have a complex structure, with several components evolving with time with respect to both width and intensity
169 citations
TL;DR: In this paper, a large observational project directed toward the detection of solar-like oscillations in an ensemble of open cluster stars was presented, where seven groups collaborated in 1992 January to observe twelve stars in M67 with 4 m class telescopes for a one week period.
Abstract: Results are presented from a large observational project directed toward the detection of solar-like oscillations in an ensemble of open cluster stars. Seven groups collaborated in 1992 January to observe twelve stars in M67 with 4 m class telescopes for a one week period. High quality time series were collected on 22 telescope nights for a total of 156 h. The technique of CCD ensemble photometry allowed precisions of about 250 μmag per minute to be reached in the best cases, and provided robust results in conditions that sometimes were far from photometric. The longitude-distributed network, coupled with generally low noise levels, provided a good window function and yielded detection thresholds of about 20 μmag (five times solar) for solar-like oscillations in the best ensemble stars
100 citations
TL;DR: In this paper, the 3D flow structure of wind accretion in a binary system is computed numerically using SPH, taking into account binary rotation as well as wind acceleration, and the flow structure is considerably more complicated than in the simple plane-parallel picture that is usually employed to describe these systems.
Abstract: The 3D flow structure of wind accretion in a binary system is computed numerically using SPH, taking into account binary rotation as well as wind acceleration. In the adiabatic γ=1.5 model, high temperatures are reached in a bow shock close to the accreting star. This region could be responsible for the UV emission observed in such systems. In the isothermal model, the bow shock is no longer detached. Moreover, an accretion disc forms around the star. The disc is inclined with respect to the orbital plane and is in differential rotation. The flow structure is considerably more complicated than in the simple plane-parallel picture that is usually employed to describe these systems
100 citations
TL;DR: In this paper, the optical polarization properties of radio galaxies with redshifts greater than 0.1 were studied and a strong tendency for the E-vector to be either perpendicular or parallel to the radio axis, and perpendicular to the axis in all the high-redshift galaxies.
Abstract: We review the optical polarization properties of radio galaxies with redshifts greater than 0.1. We find a strong tendency for the E-vector to be either perpendicular or parallel to the radio axis, and perpendicular to the axis in all the high-redshift galaxies. The observed degree of linear polarization is correlated with redshift and radio power, and is anticorrelated with the rest-frame wavelength of the measurement and the radio spectral index. No correlation is found with absolute magnitude. For the high-polarization objects (P≥3 per cent), the polarization also increases with the radio asymmetry parameter Q, which is thought to be related to the interstellar-medium density ratio
79 citations
TL;DR: In this article, the authors compared the observed Balmer discontinuity with the measured forbidden line ratio of [O III], T e ([O III]na] for 14 planetary nebulae and showed that on average, Te(Bal) tends to be lower than the Te(O III) for the same object.
Abstract: Electron temperatures derived from the observed Balmer discontinuity are presented for 14 planetary nebulae. Electron temperatures deduced from the Paschen discontinuity are also obtained for four of them. The new measurements, together with those published by others, are compared to temperatures deduced from the nebular-to-auroral forbidden line ratio of [O III], T e ([O III]na). It is shown that, on average, Te(Bal) tends to be lower than the Te([O III]na) for the same object, indicating the presence of large temperature fluctuations. Except for a few planetary nebulae with t 2 > 0.10, t 2 = 0.03 is a representative value. Possible mechanisms that could be responsible for such large temperature fluctuations are discussed
73 citations
TL;DR: Inbuilt corrective devices that can compensate for both types of degradations as observations are conducted are constructed for full use of the newly emerged class of 8-meter telescopes.
Abstract: Optical astronomy is crucial to our understanding of the universe, but the capabilities of ground-based telescopes are severely limited by the effects of telescope errors and of the atmosphere on the passage of light. Recently, it has become possible to construct inbuilt corrective devices that can compensate for both types of degradations as observations are conducted. For full use of the newly emerged class of 8-meter telescopes, such active corrective capabilities, known as active and adaptive optics, are essential. Some physical limitations in the adaptive optics field can be overcome by artificially created reference stars, called laser guide stars. These new technologies have lately been applied with success to some medium and very large telescopes.
69 citations
TL;DR: In this article, the first determination of the optical evolution and luminosity function of radio-loud quasars is obtained from the PG sample, the only sizeable complete sample of optically selected quasARS with full radio information.
Abstract: The first determination of the optical evolution and luminosity function of radio-loud quasars is obtained from the PG sample, the only sizeable complete sample of optically selected quasars with full radio information. The optical evolution of radio-loud quasars is found to be similar to (if the R definition of radio-loudness is applied), or possibly stronger than (if the luminosity definition is applied), the evolution of radio-quiet quasars. The shapes of the luminosity functions for the radio-loud and radio-quiet classes appear to be different in both cases. The relative number of radio-loud quasars is in fact a function of optical luminosity, ranging (locally) from ∼ 20-50 per cent at M B ≤ −24.5 to ∼ 7-8 per cent at fainter absolute magnitudes; in total, about 16-20 per cent of quasars brighter than ∼ − 21.5 are radio-loud
57 citations
TL;DR: An immersion grating with high refractive index n increases spectral resolution n-fold over that of a surface reflection grating of equal length.
Abstract: An immersion grating with high refractive index n increases spectral resolution n-fold over that of a surface reflection grating of equal length.
TL;DR: In this paper, the authors present new photometric and spectroscopic observations of three type II supernovae with plateau light curves: SNe 1988A, 1988H and 1989C.
Abstract: We present new photometric and spectroscopic observations of three type II supernovae with plateau light curves: SNe 1988A, 1988H and 1989C. From the new photometry of SN 1988A, we do not confirm the previous claim of a departure of the late-time light curve from 56 Co radioactive decay and thus have no indication of the emergence of an additional source of energy. The early spectral evolution of SN 1988A seems rather normal, but for the appearance of a blue emission-like feature on the Hα profile resembling a similar feature detected in SN 1987A. As in that supernova, this may be a manifestation of the appearance of radioactive 56 Co above the photosphere
TL;DR: In this paper, the contribution to the nuclear flux from the underlying galaxy is estimated by deconvolving the radial luminosity profiles into an unresolved nuclear component, and bulge and disc components.
Abstract: We present results from an optical four-colour (BVRI) CCD imaging study of a hard X-ray flux-limited sample of AGN, which are mainly Seyfert 1s. This work complements our near-infrared study of the same sample (Papers I and II). The contribution to the nuclear flux from the underlying galaxy is estimated by deconvolving the radial luminosity profiles into an unresolved nuclear component, and bulge and disc components. We find that starlight contributes a significant fraction of the nuclear light even in a small aperture for most of the Seyferts. Lower luminosity nuclei have a higher fraction of their nuclear emission from the underlying galaxy. This correlation is tighter than was found at the near-infrared wavelengths, probably a result of the higher accuracy of the deconvolution process at optical wavelengths compared to that achieved using the near-infrared images
TL;DR: In this article, it was shown that the global efficiency for reprocessing the number of ionizing photons into H-beta line photons is equal to or less than that given by recombination case B.
Abstract: Correlations of slope unity between the optical continuum luminosity and the Balmer line luminosity (Yee 1980, Shuder 1981) have been traditionally used to demonstrate that photoionization is the excitation mechanism of the emission lines in active galactic nuclei. This correlation can be expressed in terms of a constant and universal equivalent width of H-beta over five decades in optical-UV continuum luminosity. It is found that the equivalent width of EWH-beta of narrow line objects (Seyfert 2's, NLRG) is not systematically different from that of broad line dominated objects (QSOs, BLRG, Seyfert 1). Based on empirical evidence as well as on recent BLR models we argue that the global efficiency for reprocessing the number of ionizing photons into H-beta line photons is equal to or less than that given by recombination case B. Assuming a canonical cloud covering factor of Cf=0.1 of an isotropic UV source, for any reasonable value of the power law index alpha (Lv V^+alpha) we find a substantial deficit of ionizing photons in all AGN. For instance, an index of -1.4 or as flat as -0.6 leads to a deficit in ionizing photons of a factor of 25 and 4, respectively. A lower reprocessing efficiency as that characterizing the more recent BLR models brings the deficit in the range 8-15 (for alpha=-0.6). The solution envisaged to account for the apparent deficit and to explain how EWH-beta can be aspect independent from Seyfert to radio galaxies and quasars is that the observed optical continuum originates from or near the line emitting clouds (i.e., 'hazy' nucleus) and is distinct from the primary ionizing radiation directly impinging on the clouds. Evidence for this is provided by the detection of continuum emission associated with the spatially resolved emission line gas in radio galaxies and nearby Seyferts. The possible mechanisms envisaged to explain the spatial association of continuum emission and line emission are electron scattering, dust scattering and in situ generation of continuum by high velocity shocks, by thermal gas emission or by inverse self-compton scattering. The main observable consequences of this spatial association between the continuum and line emission are that (1) it can be explained in a natural way the observed narrow range of EWH-beta in AGN whether or not the primary ionizing source is anisotropic (2), it can result in similar temporal behavior of continuum and lien emission (i.e., short time lags of the BLR lines) as a result of inter-mixed continuum and line reverberation (3) and it explains why dust extinction of the nuclear continuum emission never exceeds that of the BLR lines (e.g., in Seyferts 1.8 and 9) since both emission processes being co-spatial are characterized by a similar path length to the observer.
TL;DR: In this article, the authors presented the first diffraction-limited image of a 0.12 binary star, made using optical interferometry, and resolved the confusion that has existed in the literature over its multiplicity.
Abstract: Although δ Sco is a bright and well-studied star, the details of its multiplicity have remained unclear. Here we present the first diffraction-limited image of this 0″.12 binary star, made using optical interferometry, and resolve the confusion that has existed in the literature over its multiplicity. Examining published speckle measurements, together with the present result, reveals a periodicity of 10.5 yr and allows calculation of the orbital parameters. The orbit has a high eccentricity (e = 0.82) and large inclination (i = 70°), making it a favorable target for radial velocity measurements during the next periastron (in the year 2000)
TL;DR: In this article, the authors interpret the close linear relation which Seykora found in terms of the common theory of atmospheric optics in the presence of turbulence in the case of turbulence.
Abstract: In a recent article Seykora (1993) compares measurements of scintillation in the solar irradiance with solar seeing measurements. In this article I interpret the close linear relation which he found in terms of the common theory of atmospheric optics in the presence of turbulence.
TL;DR: BV CCD photometry for the RR Lyrae variable V9 (Hv 810) in the globular cluster 47 Tuc is presented in this article, showing V9 is unusually bright and hot relative to the red horizontal branch, and has an abonormally long pulsation period for its amplitude.
Abstract: BV CCD photometry for the RR Lyrae variable V9 (Hv 810) in the globular cluster 47 Tuc is presented. Radial-velocity measurements indicate V9 is acluster member while two other candidates, V12 (HV 814) UX Tuc (HV 809) are not. A color-magnitude diagram is constructed from a subset of the 300 short-exposure CCD frames, showing V9 is unusually bright and hot relative to the red horizontal branch, and has an abonormally long pulsation period for its amplitude, both of which indicate it is highly evolved. There is also a suggestion of a "spike" in the light curve near phi~0.08, which could be caused by the emergence of the echo of the prior minimum-radius pressure wave bouncing off the helium core. The photometry will be utilized in a subsequent Baade-Wesselink analysis.
TL;DR: In this article, high-resolution UBVRI surface photometry of the nearby elliptical NGC 4486 (M87) is presented, where the major axis of the core isophotes is found to be aligned with the jet axis.
Abstract: High-resolution UBVRI surface photometry of the nearby elliptical NGC 4486 (M87) is presented. The subtraction of the contributions of the unresolved nuclear component and the jet structure allows an accurate study also of the inner galaxy regions. The major axis of the core isophotes is found to be aligned with the jet axis, while in the region >3 arcsec the isophotes are, on the contrary, nearly perpendicular to it. On the basis of photometry performed on the isolated jet structure, a change of the spectral index at the position of knot A onwards is found to be significant at the 3σ level. In addition, the radio-optical spectral index in the eastern radio lobe is shown to steepen in the surroundings of the hotspot. The available kinematic data are found to have insufficient resolution to be constrained by the photometric results to verify the presence of a supermassive black hole
TL;DR: In this article, the authors derived the distribution function f(ζ ≡ b/a) assuming a Gaussian distribution g(ξ ≡ c/a), where a > b > c are the intrinsic axes.
Abstract: There are several pieces of evidence suggesting that the discs of spiral and S0 galaxies are generally slightly triaxial. Starting from the observed flattenings, in this paper we derive the distribution function f(ζ ≡ b/a) assuming a Gaussian distribution g(ξ ≡ c/a), where a > b > c are the intrinsic axes. A volume-limited sample of 1064 objects, selected from the RC3 catalogue on the basis of the distances relative to the Virgo cluster published in the Kraan-Korteweg catalogue, is used. Distribution functions are derived respectively for S0 and spiral galaxies. The latter are also subdivided into two subsamples, «early- and late-type spirals»
TL;DR: In this article, the authors presented the first diffraction-limited image of delta Scorpii, made using optical interferometry, and resolved the confusion that has existed in the literature over its multiplicity.
Abstract: Although delta Scorpii is a bright and well-studied star, the details of its multiplicity have remained unclear. Here we present the first diffraction-limited image of this 0.12 arcsec binary star, made using optical interferometry, and resolve the confusion that has existed in the literature over its multiplicity.
Examining published speckle measurements, together with the present result, reveals a periodicity of 10.5 yr and allows calculation of the orbital parameters. The orbit has a high eccentricity (e=0.82) and large inclination (i=70 degrees), making it a favourable target for radial velocity measurements during the next periastron (in 2000).
TL;DR: In this article, a new homogeneous analysis of the available data confirms that the slope of the stellar mass function for Galactic globular clusters seems to depend on the position relative to the potential of the Galaxy.
Abstract: A new homogeneous analysis of the available data confirms that the slope of the stellar mass function for Galactic globular clusters seems to depend on the position relative to the potential of the Galaxy. It is shown that the observed dependence can be accounted for by developing the idea that all globular clusters are born with identical mass functions which then evolve through rapid interactions with the Galactic disc (disc-shocking). The limits of our semi-analytical approach based on the impulse approximation are tested by N-body simulations
TL;DR: In this paper, a detailed quantitative study of Bowen fluorescence mechanis-m and the charge transfer reaction in planetary nebulae is presented, and the Bowen efficiency drops substantially when the nebular expansion velocity 2V exp (O2+) > 55 km/sec.
Abstract: Deep, medium resolution, long-slit spectrophotometric data have been taken for a number of high-excitation planetary nebulae, covering a wavelength range from 3100 A to 7200 A with some selected regions observed at higher resolution. For about half the objects, the whole optical region has been observed, from 3100 A to 11600 A. Accurate flux calibration is achieved over this whole wavelength range. These data allow a detailed quantitative study of the Bowen fluorescence mechanis-m and the charge transfer reaction \({{O}^{{3 + }}}{{(}^{2}}{{P}_{{3/2,1/2}}}) + {{H}^{0}} - {{O}^{{2 + }}}\left( {^{{2{{{s'}}_{a}} + }}{{{L'}}_{{a {{{j'}}_{a}}}}}} \right) + {{H}^{ + }} \) in planetary nebulae, the primary goal of this program. In this paper measurements of O2+ Bowen fluorescence and charge transfer lines are presented. We show that LS coupling fails for the O2+ permitted transitions studied in this program and intermediate coupling may be a better assumption. Efficiencies of the Bowen fluorescence mechanism are derived for 15 objects, and a wide range of possible values is apparent. There is a remarkable linear positive correlation between the Bowen efficiency and the fractional abundance of oxygen in the ionization stage of O2+ and the fractional abundance of helium in the form of He+. Evidence that the Bowen efficiency is anticorrelated with the electron temperature, as first noted by Likkel and Aller, is established. The Bowen efficiency drops substantially when the nebular expansion velocity 2V exp (O2+) > 55 km/sec. For lower expansion velocities there is no detectable correlation between these two quantities. There are no observable differences in Bowen efficiency among objects of different morphological type nor between objects excited by stars of different spectral types as suggested by Likkel and Aller.
TL;DR: In this paper, the power spectrum of the distribution of clusters of galaxies in the northern and southern galactic hemispheres has been evaluated and the effects of incompleteness of data and observational errors have been investigated.
Abstract: The power spectrum of the distribution of clusters of galaxies in the northern and southern galactic hemispheres has been evaluated. Corrections have been applied for the smoothing effect, and for the Poisson noise. The effects of incompleteness of data and observational errors have been investigated. The cluster spectrum has been transformed to galaxy and matter power spectra. Data suggest that the power spectrum has an index − 2 ≤ n ≤ − 1 on intermediate scales; on very large scales the spectrum is consistent with the Harrison-Zeldovich index n = 1. The transition from the Harrison-Zeldovich index to a lower index occurs at the scale λ t ≃ 150 ± 50 h −1 Mpc
TL;DR: In this paper, the authors have discovered three very faint carbon stars (m V > 19) in the course of a multicolour survey for high-redshift quasars.
Abstract: We have discovered three very faint carbon stars (m V >19) in the course of a multicolour survey for high-redshift quasars. Proper motion measurements have been made of these stars to determine their luminosity class. On this basis, the star 0045-259 is confirmed as a halo dwarf, joining the four dwarf carbon stars already known. These five dwarfs appear to have similar absolute magnitudes and tangential velocities. A second star, 0041 - 295, is also a dwarf, yet either its absolute magnitude or its tangential velocity differs from those of the five other dwarfs; it may be the first disc dwarf carbon star found. The time baseline for the proper motion measurement for the third star, 2048-48, is too small to allow the luminosity class to be determined
TL;DR: In this paper, the azimuthal shape of spiral arms is estimated through numerical models simulating a two-armed spiral perturbation in galactic disks, and significant changes as a function of perturbations amplitude and location relative to resonances are found.
Abstract: The variation of the azimuthal shape of spiral arms is estimated through numerical models simulating a two-armed spiral perturbation in galactic disks. Significant changes as a function of perturbation amplitude and location relative to resonances are found. Models with radial force perturbations larger than 5% show significant nonlinear effects. These results are compared with observations of NGC 5247 in the K' filter, which indicates that its spiral perturbation is nonlinear.
TL;DR: In this article, a silicon immersion grating with a high refractive index, n, was developed and tested; initial results are reported here, showing that n increases the spectral resolution by a factor n over that of a reflective surface grating of equal length.
Abstract: An immersion grating with a high refractive index, n, increases the spectral resolution by a factor n over that of a reflective surface grating of equal length. A silicon immersion grating has been developed and tested; initial results are reported here.
TL;DR: In this article, the authors used extensive seeing and low-atmosphere meteorlogical data obtained at the European Southern Observatory's La Silla complex in Chile, and at the proposed site of the Very Large Telescope at Cerro Paranal.
Abstract: We use extensive seeing and low-atmosphere meteorlogical data obtained at the European Southern Observatory's La Silla complex in Chile, and at the proposed site of the Very Large Telescope at Cerro Paranal. The differences in seeing between these two locations are studied. A prototype system is set up for predicting seeing values, given knowledge of the meteorlogical environment. A novel pattern recognition methodology is developed in order to do this. We assess the quality, and quantify the limitations, of such predictions. Broadly speaking, we can carry out predictions in about 80% of cases; and our good seeing nowcasts are about 70% reliable.
TL;DR: The experience so far with restoration methodology applied to HST imagery is summarized and a first attempt is made at drawing some general conclusions about its usefulness.
Abstract: Images and spectra obtained with the Hubble Space Telescope (HST) in its current configuration suffer from severe spherical aberration and are therefore routinely restored for visualization and scientific evaluation. Among the non-linear, iterative restoration methods investigated so far, the Richardson-Lucy (RL) algorithm stands out for the quality of the restorations it delivers at modest computational cost. In this contribution we summarize our experience so far with restoration methodology applied to HST imagery and make a first attempt at drawing some general conclusions about its usefulness.
TL;DR: In this article, the authors detect non-equilibrium motions in the cores of elliptical galaxies by separating radial velocity profiles into their odd and even components, the latter being zero in equilibrium Newtonian systems.
Abstract: We have attempted to detect non-equilibrium motions in the cores of elliptical galaxies by separating radial velocity profiles into their odd and even components, the latter being zero in equilibrium Newtonian systems. Our sample of 24 objects has been assembled by considering published radial velocity profiles with error bars smaller than 10 km s −1 . We detect such motions for 13 objects, but, surprisingly, in 12 of these cases the core is redshifted with respect to the outer parts. Although most individual cases have only a limited statistical significance, the significance for the complete sample is 99.9 per cent. We suggest that this systematic redshift of cores is a result of gravitational redshift
TL;DR: The Masked APerture-Plane Interference Telescope (MAPPIT) as discussed by the authors uses a mask with several small holes over the aperture of the telescope and recording a succession of short-exposure interferograms.
Abstract: Turbulence in the Earth's atmosphere usually prevents large optical telescopes from achieving their full resolving power. Instead, the image of a star is blurred to a seeing disk about one arcsecond in diameter. Several techniques are available which use interferometry to overcome atmospheric seeing and restore diffraction-limited performance. This thesis describes the design, construction and use of an optical interferometer located at the coude focus of the 3.9m Anglo-Australian Telescope. The instrument, known as MAPPIT (Masked APerture-Plane Interference Telescope), uses the technique of non-redundant masking. This involves placing a mask with several small holes over the aperture of the telescope and recording a succession of short-exposure interferograms. These interferograms are analysed to determine the power spectrum and closure phases of the object, which are used to reconstruct a diffraction-limited image. One advantage of using a non-redundant aperture mask is that, at least for bright objects, it increases the signal-to-noise ratios of the power spectrum and closure phase measurements relative to observations with an unobstructed aperture. This is despite the fact that much of the light is blocked by the mask. Another advantage is that it improves the accuracy with which one can correct for variations in atmospheric seeing, something which is often the limiting factor in high-resolution imaging. The main drawback of non-redundant masking is a less efficient coverage of spatial frequencies. However, for simple objects such as multiple and barely resolved stars, adequate spatial frequency coverage can be obtained by combining observations made with different masks and with the masks rotated to several different position angles on the sky. An important feature of MAPPIT is the use of a prism to disperse the interference pattern in wavelength. This overcomes a restriction common to all forms of interferometry, namely the requirement that one observe over a narrow band of wavelengths. By using an aperture mask with a linear array of holes, one obtains a one-dimensional fringe pattern which can then be dispersed in the direction parallel to the fringes. In MAPPIT, a prism and a cylindrical lens produce wavelength-dispersed fringes using a novel combination of image-plane and pupil-plane imaging. Results presented in this thesis include observations of several close double stars. One of these is delta Sco, and I provide the first image of this star and the first determination of its orbit. Another bright star, sigma Sgr, is found to a barely-resolved double with a separation of just 12 milliarcsec. Observations of two resolved single stars, alpha Sco and beta Gru, are also reported. The measured angular diameter of aSco agrees well with published values. For the M5 III giant beta Gru, which has not previously been resolved, I find a uniform-disk diameter of (27±3) milliarcsec.