Photometric Model Research Articles

Toward a CIE supplementary system of photometry: brightness at any level including mesopic vision

Photometry for brightness at any level including mesopic vision is described on the basis of the work conducted towards the development of a Commission Internationale de l'Eclairage (CIE) supplementary system of photometry in CIE technical committees. Several critical items in developing such a system are discussed: (1) how to scale brightness using the concept of equivalent luminance; (2) the basic vision model of brightness perception; (3) a description of the chromatic contribution to brightness at the photopic as well as at the mesopic level; (4) formulation of an adaptation coefficient for rod-cone interaction. A possible photometric model for brightness is described based on these considerations, together with a quantitative evaluation of the model using experimental data.

Photometric analysis of 1 Ceres and surface mapping from HST observations

The highest resolution (pixel scale 30 km) images of Ceres to date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at 535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band [Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23–29] in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be 0.087 ± 0.003 , 0.056 ± 0.002 , and 0.039 ± 0.003 , respectively. V-band geometric albedo is calculated to be 0.090 ± 0.003 , consistent with earlier observations [Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 1090–1138]. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be 0.070 ± 0.002 , 0.046 ± 0.002 , and 0.032 ± 0.003 , respectively. The photometric roughness of Ceres' surface is found to be about 44 ° ± 5 ° from photometric modeling using Hapke's theory, consistent with earlier radar observations [Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. F., Shapiro, I.I., Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113–133]. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40–350 km. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites.

The Astrometric-Spectroscopic Binary System HIP 50796: An Overmassive Companion

We report spectroscopic observations of the star HIP 50796, previously considered (but later rejected) as a candidate member of the TW Hya association. Our measurements reveal it to be a single-lined binary with an orbital period of 570 days and an eccentricity of e = 0.61. The astrometric signature of this orbit was previously detected by Hipparcos in the form of curvature in the proper-motion components, although the period was unknown at the time. By combining our radial velocity measurements with the Hipparcos intermediate data (abscissa residuals) we are able to derive the full three-dimensional orbit and determine the dynamical mass of the unseen companion, as well as a revised trigonometric parallax that accounts for the orbital motion. Given our primary mass estimate of 0.73 M⊙ (mid-K dwarf), the companion mass is determined to be 0.89 M⊙, or ~20% larger than the primary. The likely explanation for the larger mass without any apparent contribution to the light is that the companion is itself a closer binary composed of M dwarfs. The near-infrared excess and X-ray emission displayed by HIP 50796 support this. Our photometric modeling of the excess leads to a lower limit to the mass ratio of the close binary of q ~ 0.8 and individual masses of 0.44-0.48 and 0.41-0.44 M⊙. The new parallax (π = 20.6 ± 1.9 mas) is significantly smaller than the original Hipparcos value and more precise.

Broad-band colours and overall photometric properties of template galaxy models from stellar population synthesis

We present here a new set of evolutionary population synthesis models for template galaxies along the Hubble morphological sequence. The models, which account for the individual evolution of the bulge, disc, and halo components, provide basic morphological features, along with bolometric luminosity and colour evolution (including Johnson/Cousins, Gunn g, r, i, and Washington C, M, T1, T2 photometric systems) between 1 and 15 Gyr. The luminosity contribution from residual gas is also evaluated, both in terms of nebular continuum and Balmer-line enhancement. Our theoretical framework relies on the observed colours of present-day galaxies, coupled with a minimal set of physical assumptions related to simple stellar population (SSP) evolution theory, to constrain the overall distinctive properties of galaxies at earlier epochs. A comparison with more elaborate photometric models, and with empirical sets of reference spectral energy distributions (SEDs) for early- and late-type galaxies is accomplished, in order to test output reliability and investigate the internal uncertainty of the models. The match with observed colours of present-day galaxies tightly constrain the stellar birth rate, b, which smoothly increases from E to Im types. The comparison with the observed supernova (SN) rate in low-redshift galaxies shows, as well, a pretty good agreement, and allows us to tune up the inferred star formation activity and the SN and hypernova rates among the different galaxy morphological types. Among others, these results could find useful application also in cosmological studies, given for instance the claimed relationship between hypernova events and gamma-ray bursts. One outstanding feature of the back-in-time evolution model is the prevailing luminosity contribution of the bulge at early epochs. As a consequence, the current morphological look of galaxies might drastically change when moving to larger distances, and we discuss here how sensibly this bias could affect the observation (and the interpretation) of high-redshift surveys. In addition to broad-band colours, the modelling of Balmer line emission in disc-dominated systems shows that striking emission lines, like Hα, can very effectively track stellar birth rate in a galaxy. For these features to be useful age tracers as well, however, one should first assess the real change of b versus time on the basis of supplementary (and physically independent) arguments.

The Oxford-Dartmouth Thirty Degree Survey - II. Clustering of bright Lyman break galaxies: strong luminosity-dependent bias atz= 4

We present measurements of the clustering properties of bright ($L>L_{*}$) z$\sim$4 Lyman Break Galaxies (LBGs) selected from the Oxford-Dartmouth Thirty Degree Survey (ODT). We describe techniques used to select and evaluate our candidates and calculate the angular correlation function which we find best fitted by a power law, $\omega(\theta)=A_{w}\theta^{-\beta}$ with $A_{w}=15.4$ (with $\theta$ in arcseconds), using a constrained slope of $\beta=0.8$. Using a redshift distribution consistent with photometric models, we deproject this correlation function and find a comoving $r_{0}=11.4_{-1.9}^{+1.7}$ h$_{100}^{-1}$ Mpc in a $\Omega_m=0.3$ flat $\Lambda$ cosmology for $i_{AB}\leq24.5$. This corresponds to a linear bias value of $b=8.1_{-2.6}^{+2.0}$ (assuming $\sigma_{8}=0.9$). These data show a significantly larger $r_{0}$ and $b$ than previous studies at $z\sim4$. We interpret this as evidence that the brightest LBGs have a larger bias than fainter ones, indicating a strong luminosity dependence for the measured bias of an LBG sample. Comparing this against recent results in the literature at fainter (sub-$L_{*}$) limiting magnitudes, and with simple models describing the relationship between LBGs and dark matter haloes, we discuss the implications on the implied environments and nature of LBGs. It seems that the brightest LBGs (in contrast with the majority sub-$L_{*}$ population), have clustering properties, and host dark matter halo masses, that are consistent with them being progenitors of the most massive galaxies today.

Adaptively merging large-scale range data with reflectance properties

In this paper, we tackle the problem of geometric and photometric modeling of large intricately shaped objects. Typical target objects we consider are cultural heritage objects. When constructing models of such objects, we are faced with several important issues that have not been addressed in the past-issues that mainly arise due to the large amount of data that has to be handled. We propose two novel approaches to efficiently handle such large amounts of data: A highly adaptive algorithm for merging range images and an adaptive nearest-neighbor search to be used with the algorithm. We construct an integrated mesh model of the target object in adaptive resolution, taking into account the geometric and/or photometric attributes associated with the range images. We use surface curvature for the geometric attributes and (laser) reflectance values for the photometric attributes. This adaptive merging framework leads to a significant reduction in the necessary amount of computational resources. Furthermore, the resulting adaptive mesh models can be of great use for applications such as texture mapping, as we will briefly demonstrate. Additionally, we propose an additional test for the k-d tree nearest-neighbor search algorithm. Our approach successfully omits back-tracking, which is controlled adaptively depending on the distance to the nearest neighbor. Since the main consumption of computational cost lies in the nearest-neighbor search, the proposed algorithm leads to a significant speed-up of the whole merging process. In this paper, we present the theories and algorithms of our approaches with pseudo code and apply them to several real objects, including large-scale cultural assets.

Mapping textures on 3D geometric model using reflectance image

Texture mapping on scanned objects, that is, the method of mapping current color images on a 3D geometric model measured by a range sensor, is a key technique of photometric modeling for virtual reality. Usually range and color images are obtained from different viewing positions, through two independent range and color sensors. Thus, in order to map those color images on the geometric model, it is necessary to determine relative relations between these two viewpoints. This paper focuses on the reflectance image of the laser scanner which is given as side products of range images for most of the range sensors, and proposes a new calibration method for the texture mapping using reflectance images and the iterative pose estimation using robust M-estimator. © 2005 Wiley Periodicals, Inc. Syst Comp Jpn, 36(13): 92–101, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.10392

Analysis and synthesis of textured motion: particles and waves

Natural scenes contain a wide range of textured motion phenomena which are characterized by the movement of a large amount of particle and wave elements, such as falling snow, wavy water, and dancing grass. In this paper, we present a generative model for representing these motion patterns and study a Markov chain Monte Carlo algorithm for inferring the generative representation from observed video sequences. Our generative model consists of three components. The first is a photometric model which represents an image as a linear superposition of image bases selected from a generic and overcomplete dictionary. The dictionary contains Gabor and LoG bases for point/particle elements and Fourier bases for wave elements. These bases compete to explain the input images and transfer them to a token (base) representation with an O(10(2))-fold dimension reduction. The second component is a geometric model which groups spatially adjacent tokens (bases) and their motion trajectories into a number of moving elements--called "motons." A moton is a deformable template in time-space representing a moving element, such as a falling snowflake or a flying bird. The third component is a dynamic model which characterizes the motion of particles, waves, and their interactions. For example, the motion of particle objects floating in a river, such as leaves and balls, should be coupled with the motion of waves. The trajectories of these moving elements are represented by coupled Markov chains. The dynamic model also includes probabilistic representations for the birth/death (source/sink) of the motons. We adopt a stochastic gradient algorithm for learning and inference. Given an input video sequence, the algorithm iterates two steps: 1) computing the motons and their trajectories by a number of reversible Markov chain jumps, and 2) learning the parameters that govern the geometric deformations and motion dynamics. Novel video sequences are synthesized from the learned models and, by editing the model parameters, we demonstrate the controllability of the generative model.

Rotational Brightness Modulation and Starspots on the RS CVn-type Stars IN Com, IL Com, UX Ari, and V711 Tau

UBVRI photometric observations and models of spotting are presented for four noneclipsing RS CVn systems: IN Com (G5III/IV), IL Com (F8V+F8V), UX Ari (K01V+G5V), and V711 Tau (K1IV+G5V). A low amplitude variability caused by cold (ΔT=1700K)spots which can occupy up to 19% of the star's surface is confirmed for the little-studied star IL Com. Long-term light curves are constructed and the stellar magnitudes and color indices of the unspotted photospheres are estimated for IN com, UX Ari, and V711 Tau. It is shown that UX Ari becomes bluer with decreasing brightness, so its variability cannot be fully explained in terms of cold spots. Models of spotting on In Com and V711 Tau are constructed from the full set of available photometric observations. The spots on both of these variables lie at middle latitudes and occupy up to 22% (In Com) and 33% (V711 Tau) of the stars' surfaces. Both stars manifest a tendency for the width of the spots to decrease as their area increases. This is a crude analog of the Maunder butterflies. These stars experience cyclical spot activity and have a differential rotation of the type found on the sun.

Photometric and polarimetric observations and model simulations of (216) Kleopatra

We performed photometric and polarimetric observations, on November 8 and 9, 1999, of an M-type main belt asteroid, (216) Kleopatra by using the HBS spectropolarimeter installed at Dodaira observatory, National Astronomical Observatory of Japan (NAOJ). Photometric amplitude of lightcurve in the V band was 0.12 mag, and the averaged degree of polarization was -1.01±0.1%. It seems that the polarimetric data might also show a slight change in the degree of polarization (∼0.2%) at the second minimum of the photometric lightcurve, but we could not confirm that the feature was real because of the large errors of data. With the assumption that the surface is uniform, we have carried out lightcurve simulations based on shape models by Ostro et al. (2000), Tanga et al. (2001) and Roche binary (Cellino et al., 1985). The results of simulations were compared to the configurations of lightcurves which had been obtained at different 4 geometric positions (1980, 1982, 1987 and 1999). The model by Cellino et al. (1985) reproduced almost all the data points without the 1987 observations within ∼0.05 mag., which is the best result among the 3 models. The model by Tanga et al. (2001) well reproduced the lightcurves, but failed in reproducing the 1982 amplitude (difference Δdiff ∼ 0.2 mag.). We also confirmed that the model by Ostro et al. (2000) could not explain the observed lightcurves.

Photometric analysis of Eros from NEAR data

A photometric model of (433) Eros at wavelengths from 450 to 1050 nm is constructed using the combination of the images from the multispectral imager (MSI) obtained during the one-year long orbital phase of the NEAR mission, ground-based lightcurves from earlier observations, and our theoretical forward modeling simulations coupled with the NEAR shape model. The single scattering albedo is found to be 0.33 ± 0.03 at 550 nm, which is smaller than past findings by 30%. The amplitude and width of the opposition effect are 1.4 ± 0.1 and 0.010 ± 0.004 from ground based lightcurves. It is confirmed that the asymmetry factor of the single-particle phase function and the surface roughness parameter do not depend on wavelength from 450 to 1050 nm, and their values are estimated to be − 0.25 ± 0.02 and 28 ° ± 3 ° , respectively, comparable with the earlier measurements from the NEAR NIS data. The geometric albedo and the Bond albedo at 550 nm are calculated to be 0.23 and 0.093, respectively, which make Eros less reflective than previous models, but still slightly more reflective than average S-type asteroids. The lower albedos of Eros are more consistent with our forward modeling simulations, as well as with its spectrum. Eros is a typical S-type asteroid like (951) Gaspra and (243) Ida, and has similar surface regolith properties. Combining the single-scattering albedo with the olivine composition of ordinary chondrites, taking into account space weathering darkening, we constrain the grain size of the regolith particles on Eros to a range of 50 to 100 μm.

A PC Based Model for Prediction of Visibility and Legibility for a Human Factors Engineer's Tool Box

Target detection and legibility prediction problems are generally considered complex as they require a good understanding of knowledge in photometry, geometry, human visual functions and visual performance measurement. Therefore, only simple geometric or photometric models are generally provided in human factors text books in the forms such as equations, graphs, or nomograms. This paper presents an easy to use but a more comprehensive model developed for a human factors engineering class to understand the variables, photometric measurements and use of human visual threshold data. The model can be easily down-loaded and exercised by a PC user to quickly estimate visibility and legibility in a number of situations. The model serves as a great tool to analyze a number of “what if” scenarios and learn the basics of human visual performance. The model will be an excellent candidate for including in a practicing human factors engineer's tool box.

Estimation of the luminance map for a Lambertian photometric model: application to the study of road surface roughness

The study of rough textured surfaces such as road coverings, is generally made on gray-level images. This supposes that the variations of gray levels are representative of the local variations of the relief. This assumption is justified, in the case of surfaces that are uniformly colored, but finds its limit in the case when these surfaces present variations of color or aspect. The corresponding image then presents variations of gray levels that can be related to the color variations or to the relief variations or both. It becomes difficult in this case to work out the criteria of roughness based on image analysis. It is then necessary to develop, before any study of roughness, an estimation of the luminance map linked to color variations. To do that, we have linked the gray-level value to the height variation of the road-covering surface. Thus, we express the luminance distribution according to these height variations, obtained through a laser sensor, and to the image gray level. This method enables us to compute the distribution of the luminance map. We characterize this distribution by considering statistical parameters of its histogram. Then we test the effectiveness of our approach by comparing the evolution of the criterion of roughness on road surfaces, first without considering the luminance distribution and then by taking it into account. The results obtained show that this approach leads to a good discrimination by the criterion of roughness in the case of colored surfaces.

Towards a complete dense geometric and photometric reconstruction under varying pose and illumination

This paper proposes a novel framework to construct a geometric and photometric model of a viewed object that can be used for visualisation in arbitrary pose and illumination. The method is solely based on images and does not require any specialised equipment. We assume that the object has a piece-wise smooth surface and that its reflectance can be modelled using a parametric bidirectional reflectance distribution function. Without assuming any prior knowledge on the object, geometry and reflectance have to be estimated simultaneously and occlusion and shadows have to be treated consistently. We exploit the geometric and photometric consistency using the fact that surface orientation and reflectance are local invariants. In a first implementation, we demonstrate the method using a Lambertian object placed on a turn-table and illuminated by a number of unknown point light-sources. A discrete voxel model is initialised to the visual hull and voxels identified as inconsistent with the invariants are removed iteratively. The resulting model is used to render images in novel pose and illumination.

The opposition surge of Enceladus: HST observations 338–1022 nm

Hubble Space Telescope (HST) Wide-Field Planetary Camera (WFPC2) observations at phase angles in the range α=0.26°–6.4° obtained at every opposition and near quadrature between October 1996 and December 2002 reveal the opposition effect of Enceladus. We present a photometric analysis of nearly 200 images obtained through the five broadband UVBRI filters (F336W, F439W, F555W, F675W, and F814W) and the F785LP and F1042M filters from which we generate mutually consistent solar and rotational phase curves. Our solar phase curves reveal a dramatic, sharp increase in the albedo (from 0.11 mag in the F675W filter to 0.17 mag in the F785LP filter) as phase angles decrease from 2° to 0.26°. A slight opposition effect is evident in data from the F1042M filter ( λ eff=1022 nm); however, the smallest phase angle currently available for observations from this filter is α=0.58°. With the addition of data from the F255W filter we demonstrate the wavelength dependence of the albedo of the trailing hemisphere from 275 to 1022 nm. Our rotation curves show that the trailing hemisphere is ∼0.06 mag brighter than the leading when observed at wavelengths between 338 and 868 nm and 0.11 mag brighter than the leading at 1022 nm. We have supplemented the phase curve from the F439W filter ( λ eff=434 nm) with Voyager clear filter ( λ eff=480 nm) observations made at larger phase angles ( α=13°–43°) to produce a phase curve with the most extensive phase angle coverage possible to date. This newly expanded range of phase angles enhances the ability of the Hapke photometric model (Hapke B., 2002, Icarus 157, 523–534) to relate physical characteristics of the surface of Enceladus to the manner in which incident light is reflected from it. We present Hapke 2002 model fits to solar phase curves from each UVBRI filter as well as from the F785LP and F1042M filters. Geometric albedos derived from these model fits range from p=0.92±0.01 at 1022 nm to p=1.41±0.03 at 549 nm, necessitating an increase of about 20% from previously derived values. Our Hapke fits demonstrate that the opposition surge of Enceladus is best described by a model which combines both moderate shadow-hiding and narrow coherent backscattering components.

Photometric modeling of Saturn's rings. II. Azimuthal asymmetry in reflected and transmitted light

Dynamical N-body simulations (Salo, 992, Nature 359, 619) suggest the formation of trailing density enhancements in the outer portions of Saturn's rings, due to local gravitational instabilities. These Julian–Toomre type wakes, having a pitch angle of about 20°–25° with respect to the local tangential direction, seem to provide a plausible explanation for the observed quadrupole brightness variation in Saturn's A ring (Salo and Karjalainen, 1999, Bull. Am. Astron. Soc. 31, 1160; French et al., 2000, Bull. Am. Astron. Soc. 32, 806; Porco et al., 2001, Bull. Am. Astron. Soc. 33, 1091). We have carried out systematic photometric modeling of gravitational wake structures seen in dynamical simulations, performed for the parameter values of the A ring, using the Monte Carlo radiative transfer code described in Salo and Karjalainen (2003, Icarus 164, 428). Comparisons to the observed asymmetry in various cases are presented (asymmetry in reflected and transmitted light, ring longitude and opening angle dependence), in all cases confirming the applicability of the wake model. Typically, minimum brightness corresponds to viewing/illumination along the long axis of wakes; however, the sense of modeled asymmetry reverses at small tilt angles in diffuse transmission. Implications of wakes on the occultation optical depth profiles and the A ring overall brightness behavior are also discussed: it is shown that the wake structure needs to be taken into account when the Cassini occultation profiles for the A ring are interpreted in terms of variations in surface density. Also, the presence of wakes offers a plausible explanation for the inverse tilt effect seen in the mid A-ring.

Photometric modeling of Saturn's rings. II. Azimuthal asymmetry in reflected and transmitted light

Dynamical N-body simulations (Salo, 992, Nature 359, 619) suggest the formation of trailing density enhancements in the outer portions of Saturn's rings, due to local gravitational instabilities. These Julian–Toomre type wakes, having a pitch angle of about 20°–25° with respect to the local tangential direction, seem to provide a plausible explanation for the observed quadrupole brightness variation in Saturn's A ring (Salo and Karjalainen, 1999, Bull. Am. Astron. Soc. 31, 1160; French et al., 2000, Bull. Am. Astron. Soc. 32, 806; Porco et al., 2001, Bull. Am. Astron. Soc. 33, 1091). We have carried out systematic photometric modeling of gravitational wake structures seen in dynamical simulations, performed for the parameter values of the A ring, using the Monte Carlo radiative transfer code described in Salo and Karjalainen (2003, Icarus 164, 428). Comparisons to the observed asymmetry in various cases are presented (asymmetry in reflected and transmitted light, ring longitude and opening angle dependence), in all cases confirming the applicability of the wake model. Typically, minimum brightness corresponds to viewing/illumination along the long axis of wakes; however, the sense of modeled asymmetry reverses at small tilt angles in diffuse transmission. Implications of wakes on the occultation optical depth profiles and the A ring overall brightness behavior are also discussed: it is shown that the wake structure needs to be taken into account when the Cassini occultation profiles for the A ring are interpreted in terms of variations in surface density. Also, the presence of wakes offers a plausible explanation for the inverse tilt effect seen in the mid A-ring.

The jovian rings: new results derived from Cassini, Galileo, Voyager, and Earth-based observations

Cassini's Imaging Science Subsystem (ISS) instrument took nearly 1200 images of the Jupiter ring system during the spacecraft's 6-month encounter with Jupiter (Porco et al., 2003, Science 299, 1541–1547). These observations constitute the most complete data set of the ring taken by a single instrument, both in phase angle (0.5°–120° at seven angles) and wavelength (0.45–0.93 μm through eight filters). The main ring was detected in all targeted exposures; the halo and gossamer rings were too faint to be detected above the planet's stray light. The optical depth and radial profile of the main ring are consistent with previous observations. No broad asymmetries within the ring were seen; we did identify possible hints of 1000 km-scale azimuthal clumps within the ring. Cassini observations taken within 0.02° of the ring plane place an upper limit on the ring's full thickness of 80 km at a phase angle of 64°. We have combined the Cassini ISS and VIMS (Visible and Infrared Mapping Spectrometer) observations with those from Voyager, HST (Hubble Space Telescope), Keck, Galileo, Palomar, and IRTF (Infrared Telescope Facility). We have fit the entire suite of data using a photometric model that includes microscopic silicate dust grains as well as larger, long-lived ‘parent bodies’ that engender this dust. Our best-fit model to all the data indicates an optical depth of small particles of τ s =4.7×10 −6 and large bodies τ l =1.3×10 −6. The dust's cross-sectional area peaks near 15 μm. The data are fit significantly better using non-spherical rather than spherical dust grains. The parent bodies themselves must be very red from 0.4–2.5 μm, and may have absorption features near 0.8 and 2.2 μm.

SDSS J115517.35+634622.0: A Newly Discovered Gravitationally Lensed Quasar

We report the discovery of SDSSJ115517.35+634622.0, a previously unknown gravitationally lensed quasar. The lens system exhibits two images of a $z = 2.89$ quasar, with an image separation of $1{\farcs}832 \pm 0.007$ . Near-IR imaging of the system reveals the presence of the lensing galaxy between the two quasar images. Based on absorption features seen in the Sloan Digital Sky Survey (SDSS) spectrum, we determine a lens galaxy redshift of $z = 0.1756$. The lens is rather unusual in that one of the quasar images is only $0{\farcs}22\pm0{\farcs}07$ ($\sim 0.1 R_{\rm eff}$) from the center of the lens galaxy and photometric modeling indicates that this image is significantly brighter than predicted by a SIS model. This system was discovered in the course of an ongoing search for strongly lensed quasars in the dataset from the SDSS.

ULTRACAM photometry of the eclipsing cataclysmic variable OU Vir

We present high-speed, three-colour photometry of the faint eclipsing cataclysmic variable OU Vir. For the first time in OU Vir, separate eclipses of the white dwarf and the bright spot have been observed. We use timings of these eclipses to derive a purely photometric model of the system, obtaining a mass ratio of q = 0.175 ± 0.025, an inclination of i = 79°.2 ± 0°.7 and a disc radius of Rd/a = 0.2315 ± 0.0150. We separate the white dwarf eclipse from the light curve and, by fitting a blackbody spectrum to its flux in each passband, obtain a white dwarf temperature of T = 13 900 ± 600 K and a distance of D = 51 ± 17 pc. Assuming that the primary obeys the Nauenberg mass–radius relation for white dwarfs and allowing for temperature effects, we also find a primary mass Mw/M⊙ = 0.89 ± 0.20, a primary radius Rw/R⊙ = 0.0097 ± 0.0031 and an orbital separation a/R⊙ = 0.74 ± 0.05.