High-resolution Telescope Research Articles

Constraining the magnetic vector in the quiet solar photosphere and the impact of instrumental degradation

Context. With the advent of next generation high resolution telescopes, our understanding of how the magnetic field is organized in the internetwork (IN) photosphere is likely to advance significantly. Aims. We aim to evaluate the extent to which we can retrieve accurate information about the magnetic vector in the IN photosphere using inversion techniques. Methods. We use a snapshot produced from high resolution three-dimensional magnetohydrodynamic (MHD) simulations and employ the Stokes Inversions based on Response functions (SIR) code to produce synthetic observables in the same near infrared spectral window as observed by the GREGOR Infrared Spectrograph (GRIS), which contains the highly magnetically sensitive photospheric Fe I line pair at 15 648.52 Å and 15 652.87 Å. We then use a parallelized wrapper to SIR to perform nearly 14 million inversions of the synthetic spectra to test how well the ‘true’ MHD atmospheric parameters can be constrained statistically. Finally, we degrade the synthetic Stokes vector spectrally and spatially to GREGOR resolutions and examine how this influences real observations, considering the impact of stray light, spatial resolution and signal-to-noise (S/N) in particular. Results. We find that the depth-averaged parameters can be recovered by the inversions of the undegraded profiles, and by adding simple gradients to magnetic field strength, inclination, and line of sight velocity we show that an improvement in the χ2 value is achieved. We also evaluate the extent to which we can constrain these parameters at various optical depths, with the kinematic and thermodynamic parameters sensitive deeper in the atmosphere than the magnetic parameters. We find the S/N and spatial resolution both play a significant role in determining how the degraded atmosphere appears. At the same time, we find that the magnetic and kinematic parameters are invariant upon inclusion of an unpolarized stray light. We compare our results to recent IN observations obtained by GREGOR. We studied a linear polarization feature which resembles those recently observed by GRIS in terms of appearing as ‘loop-like’ structures and exhibiting very similar magnetic flux density. Thus, we demonstrate that realistic MHD simulations are capable of showing close agreement with real observations, and the symbiosis between them and observations continues to prove essential. We finally discuss the considerations that must be made for DKIST-era observations.

Coronal Microjets in Quiet-Sun Regions Observed with the Extreme Ultraviolet Imager on Board the Solar Orbiter

Abstract We report the smallest coronal jets ever observed in the quiet Sun with recent high-resolution observations from the High Resolution Telescopes (HRIEUV and HRILyα ) of the Extreme Ultraviolet Imager on board the Solar Orbiter (SO). In the HRIEUV (174 Å) images, these microjets usually appear as nearly collimated structures with brightenings at their footpoints. Their average lifetime, projected speed, width, and maximum length are 4.6 minutes, 62 km s−1, 1.0 Mm, and 7.7 Mm, respectively. Inverted-Y shaped structures and moving blobs can be identified in some events. A subset of these events also reveal signatures in the HRILyα (H i Lyα at 1216 Å) images and the extreme ultraviolet images taken by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Our differential emission-measure (EM) analysis suggests a multithermal nature and an average density of ∼1.4 × 109 cm−3 for these microjets. Their thermal and kinetic energies were estimated to be ∼3.9 × 1024 erg and ∼2.9 × 1023 erg, respectively, which are of the same order of the released energy predicted by the nanoflare theory. Most events appear to be located at the edges of network lanes and magnetic flux concentrations, suggesting that these coronal microjets are likely generated by magnetic reconnection between small-scale magnetic loops and the adjacent network field.

The Polarimetric and Helioseismic Imager on Solar Orbiter

Aims.This paper describes the Polarimetric and Helioseismic Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and helioseismology instrument to observe the Sun from outside the Sun-Earth line. It is the key instrument meant to address the top-level science question: How does the solar dynamo work and drive connections between the Sun and the heliosphere? SO/PHI will also play an important role in answering the other top-level science questions of Solar Orbiter, while hosting the potential of a rich return in further science.Methods.SO/PHI measures the Zeeman effect and the Doppler shift in the Fe I617.3 nm spectral line. To this end, the instrument carries out narrow-band imaging spectro-polarimetry using a tunable LiNbO3Fabry-Perot etalon, while the polarisation modulation is done with liquid crystal variable retarders. The line and the nearby continuum are sampled at six wavelength points and the data are recorded by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data are reduced on board, including being inverted under the assumption of a Milne-Eddington atmosphere, although simpler reduction methods are also available on board. SO/PHI is composed of two telescopes; one, the Full Disc Telescope, covers the full solar disc at all phases of the orbit, while the other, the High Resolution Telescope, can resolve structures as small as 200 km on the Sun at closest perihelion. The high heat load generated through proximity to the Sun is greatly reduced by the multilayer-coated entrance windows to the two telescopes that allow less than 4% of the total sunlight to enter the instrument, most of it in a narrow wavelength band around the chosen spectral line.Results.SO/PHI was designed and built by a consortium having partners in Germany, Spain, and France. The flight model was delivered to Airbus Defence and Space, Stevenage, and successfully integrated into the Solar Orbiter spacecraft. A number of innovations were introduced compared with earlier space-based spectropolarimeters, thus allowing SO/PHI to fit into the tight mass, volume, power and telemetry budgets provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal) challenges posed by the mission’s highly elliptical orbit.

Simulating gas kinematic studies of high-redshift galaxies with the HARMONI integral field spectrograph

ABSTRACT We present simulated observations of gas kinematics in a galaxy formed in a 10 pc resolution cosmological simulation with the hydrodynamical + N-body code ramses, using the new ramses2hsim pipeline with the simulated observing pipeline (hsim) for the Extremely Large Telescope High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI IFS). We post-process the galaxy's gas kinematics and Hα line emission for each simulation cell, and integrate the emission to produce an extinction-corrected input cube. We then simulate observations of the input cube with HARMONI, for a range of exposure times, spatial sampling, and spectral resolution. We analyse the mock observations to recover galaxy properties such as its kinematics and compare with the known simulation values. We investigate the cause of biases between the ‘real’ and ‘observed’ kinematic values, demonstrating the sensitivity of the inferred rotation curve to knowledge of the instrument’s point spread function.

Determination of Cloud-top Height through Three-dimensional Cloud Reconstruction using DIWATA-1 Data

Cloud-top height is a useful parameter with which to elucidate cloud vertical growth, which often indicates severe weather such as torrential rainfall and thunderstorms; it is widely used in meteorological research. However, general cloud-top height estimation methods are hindered by observational and analytical constraints. This study used data from DIWATA-1, the Philippines’ first microsatellite, to overcome these limitations and successfully produce sophisticated three-dimensional cloud models via stereo-photogrammetry. High-temporal snapshot 200-ms-interval imaging of clouds over Iloilo, Philippines, is performed. Two types of telescopes were used to capture 30 stereoscopic cloud images at ~60- and ~3-m ground sampling resolutions; these were used to construct three-dimensional cloud models with 40- and 2-m vertical resolutions, respectively. The imaged clouds have heights of 2.0 to 4.8 km, which is below freezing level for the Philippines and typical of stratocumulus and cumulus clouds. The results are validated using cloud-edge heights determined by measuring the distance from the clouds to their ground shadows. An RMSE of 0.32 km and a maximum difference of 0.03 km are found for the low- and high-resolution telescopes, respectively. For further validation, the results are compared with cloud-top heights estimated from HIMAWARI-8 images captured on the same day, yielding an average vertical difference of 0.15 km and a maximum difference of 1.7 km.

Possible Production of Solar Spicules by Microfilament Eruptions

Abstract We examine Big Bear Solar Observatory (BBSO) Goode Solar Telescope (GST) high spatial resolution (0.″06), high-cadence (3.45 s), Hα-0.8 Å images of central-disk solar spicules, using data of Samanta et al. We compare with coronal-jet chromospheric-component observations of Sterling et al. Morphologically, bursts of spicules, referred to as “enhanced spicular activities” by Samanta et al., appear as scaled-down versions of the jet’s chromospheric component. Both the jet and the enhanced spicular activities appear as chromospheric-material strands, undergoing twisting-type motions of ∼20–50 km s−1 in the jet and ∼20–30 km s−1 in the enhanced spicular activities. Presumably, the jet resulted from a minifilament-carrying magnetic eruption. For two enhanced spicular activities that we examine in detail, we find tentative candidates for corresponding erupting microfilaments, but not the expected corresponding base brightenings. Nonetheless, the enhanced-spicular-activities’ interacting mixed-polarity base fields, frequent-apparent-twisting motions, and morphological similarities to the coronal jet’s chromospheric-temperature component, suggest that erupting microfilaments might drive the enhanced spicular activities but be hard to detect, perhaps due to Hα opacity. Degrading the BBSO/GST-image resolution with a 1.″0-FWHM smoothing function yields enhanced spicular activities resembling the “classical spicules” described by, e.g., Beckers. Thus, a microfilament eruption might be the fundamental driver of many spicules, just as a minifilament eruption is the fundamental driver of many coronal jets. Similarly, a 0.″5-FWHM smoothing renders some enhanced spicular activities to resemble previously reported “twinned” spicules, while the full-resolution features might account for spicules sometimes appearing as 2D-sheet-like structures.

Keck Observations Confirm a Super-Jupiter Planet Orbiting M Dwarf OGLE-2005-BLG-071L

Abstract We present adaptive optics imaging from the NIRC2 instrument on the Keck II telescope that resolves the exoplanet host (and lens) star as it separates from the brighter source star. These observations yield the K-band brightness of the lens and planetary host star, as well as the lens-source relative proper motion, , in the heliocentric reference frame. The measurement allows for the determination of the microlensing parallax vector, , which had only a single component determined by the microlensing light curve. The combined measurements of and K L provide the masses of the host star, , and planet, m p = 3.27 ± 0.32M Jupiter with a projected separation of 3.4 ± 0.5 au. This confirms the tentative conclusion of a previous paper that this super-Jupiter mass planet, OGLE-2005-BLG-071Lb, orbits an M dwarf. Such planets are predicted to be rare by the core accretion theory and have been difficult to find with other methods, but there are two such planets with firm mass measurements from microlensing, and an additional 11 planetary microlens events with host mass estimates and planet mass estimates >2 Jupiter masses that could be confirmed by high angular follow-up observations. We also point out that OGLE-2005-BLG-071L has separated far enough from its host star that it should be possible to measure the host-star metallicity with spectra from a high angular resolution telescope such as Keck, the Very Large Telescope, the Hubble Space Telescope, or the James Webb Space Telescope.

The SDSS-HET Survey of Kepler Eclipsing Binaries. Description of the Survey and First Results

Abstract The Kepler mission has provided a treasure trove of eclipsing binaries (EBs), observed at extremely high photometric precision, nearly continuously for several years. We are carrying out a survey of ∼100 of these EBs to derive dynamical masses and radii with precisions of 3% or better. We use multiplexed near-infrared H-band spectroscopy from the Sloan Digital Sky Survey-III and -IV APOGEE instrument and optical spectroscopy from the Hobby–Eberly Telescope High-resolution Spectrograph to derive double-lined spectroscopic orbits and dynamical mass ratios (q) for the EB sample, two of which we showcase in this paper. This orbital information is combined with Kepler photometry to derive orbital inclination, dynamical masses of the system components, radii, and temperatures. These measurements are directly applicable for benchmarking stellar models that are integrating the next generation of improvements, such as the magnetic suppression of convection efficiency, updated opacity tables, and fine-tuned equations of state. We selected our EB sample to include systems with low-mass ( ) primary or secondary components, as well as many EBs expected to populate the relatively sparse parameter space below ∼0.5 M ⊙. In this paper, we describe our EB sample and the analytical techniques we are utilizing, and also present masses and radii for two systems that inhabit particularly underpopulated regions of mass–radius–period space: KIC 2445134 and KIC 3003991. Our joint spectroscopic and photometric analysis of KIC 2445134 ( ) yields masses and radii of , , , , and a temperature ratio of our analysis of KIC 3003991 ( ) yields , , , , and a temperature ratio of .

The GMRT High-resolution Southern Sky Survey for Pulsars and Transients. II. New Discoveries, Timing, and Polarization Properties

Abstract We have been conducting the Giant Metrewave Radio Telescope High Resolution Southern Sky (GHRSS) survey for the last four years and have discovered 18 pulsars to date. The GHRSS survey is an off-Galactic-plane survey at 322 MHz in a region of the sky (decl. range −40° to −54°) complementary to other ongoing low-frequency surveys. In this paper we report the discovery of three pulsars (PSRs), J1239−48, J1516−43, and J1726−52. We also present timing solutions for three pulsars previously discovered with the GHRSS survey: PSR J2144−5237, a millisecond pulsar with a period P = 5 ms in a 10 days orbit around a ≤0.18 M⊙ companion; PSR J1516−43, a mildly recycled P = 36 ms pulsar in a 228 days orbit with a companion of mass ∼0.4 M ⊙; and the P = 320 ms PSR J0514−4408, which we show is a source of pulsed γ-ray emission. We also report radio polarimetric observations of three of the GHRSS discoveries, PSRs J0418−4154, J0514−4408, and J2144−5237.

2.5 m wide-field and high-resolution telescope

We propose to construct a 2.5 m wide-field and high-resolution telescope (WeHiT).It will be the first facility in the world with a special innovative design and can conduct bothhigh-resolution solar observations and the wide-field night sky survey. Its scientific objectivescover solar physics, time-domain astronomy, galactic astrophysics, and exoplanet science, all ofwhich are recent hot topics in astronomy. The telescope can provide top-ranked data for Chineseastronomers. In particular, as the largest on-axis solar telescope in the world with wide-field thanall large solar telescopes operating at present, it can provide unprecedented high-resolution solardata, which can help us obtain breakthrough achievements in the study of solar active regionsand solar eruptions, such as solar flares and coronal mass ejections (CMEs). The data are also veryuseful for the study of space weather. As a large telescope in the China time-zone, the telescopeis very unique in continuously monitoring short-time transient events by filling the gap in thetime-zone. Furthermore, it can provide valuable high-quality data of the narrow-band extragalacticsurvey, nearby galaxy study and exoplanet survey. Moreover, the telescope is a top-level one whichcan make essential contribution to training the graduate and undergraduate students majoring inobservational astronomy.

The MEV project: Design and testing of a new high-resolution telescope for muography of Etna Volcano

The MEV project aims at developing a muon telescope expressly designed for the muography of Etna Volcano. In particular, one of the active craters in the summit area of the volcano would be a suitable target for this experiment. A muon tracking telescope with high imaging resolution was built and tested during 2017. The telescope is a tracker based on extruded scintillating bars with WLS fibers and featuring an innovative read-out architecture. It is composed of 3×1m2 XY planes; the angular resolution does not exceeds 0.4msr and the total angular aperture is about ±45∘. A special effort concerned the design of mechanics and electronics in order to meet the requirements of a detector capable to work in a hostile environment such as the top of a tall volcano, at a far distance from any facility. The test phase started in January 2017 and ended successfully at the end of July 2017. An extinct volcanic crater (the Monti Rossi, in the village of Nicolosi, about 15 km from Catania) was the target of the measurement. The detector acquired data for about 120 days and the preliminary results are reported in this work.

Design, fabrication and space suitability tests of wide field of view, ultra-compact, and high resolution telescope for space application.

We report on the design, manufacture, and testing of an ultra-compact telescope for 16 unit (16U) CubeSats for Earth and space observation. This telescope provides 1 arcsec resolution at a 2.9 degree field of view. Dimensions are optimized to 230 × 230 × 330mm3 with a mass of less than 6kg including support structure. Our catadioptric 5-element design consists of a full-aperture corrector, a Mangin primary mirror (PM), a secondary mirror (SM), and a 2-lens field corrector. The focal length is 745mm, and squared-circular aperture has an equivalent diameter of 241mm. The designed modulation transfer function (MTF) is 0.275 for the entire unit including baffles at a Nyquist frequency of 161 cycles/mm for the 450-800nm band. As one of the distinguishing features of our state-of-the-art design, all optical surfaces are spherical to simplify adjustment. For the best thermal stability, all optical elements are produced from fused silica. We describe the details of design, adjustment, and laboratory performance tests for space environments in accordance with the requirements for in-orbit operation onboard Earth-observation micro-satellites to be launched in 2018.

SALT HRS Discovery of the Binary Nucleus of the Etched Hourglass Nebula MyCn 18

AbstractThe shaping of various morphological features of planetary nebulae is increasingly linked to the role of binary central stars. Identifying a binary within a planetary nebula offers a powerful tool with which to directly investigate the formation mechanisms behind these features. The Etched Hourglass Nebula, MyCn 18, is the archetype for several binary-linked morphological features, yet it has no identified binary nucleus. It has the fastest jets seen in a planetary nebula of 630 km s−1, a central star position offset from the nebula centre, and a bipolar nebula with a very narrow waist. Here we report on the Southern African Large Telescope High Resolution Spectrograph detection of radial velocity variability in the nucleus of MyCn 18 with an orbital period of 18.15 ± 0.04 d and a semi-amplitude of 11.0 ± 0.3 km s−1. Adopting an orbital inclination of 38 ± 5° and a primary mass of 0.6 ± 0.1 M⊙ yields a secondary mass of 0.19 ± 0.05 M⊙ corresponding to an M5V companion. The detached nature of the binary rules out a classical nova as the origin of the jets and the offset central star as hypothesised in the literature. Furthermore, scenarios that produce the offset central star during the AGB and that form narrow waist bipolar nebulae result in orbital separations 80–800 times larger than observed in MyCn 18. The inner hourglass and jets may have formed from part of the common envelope ejecta that remained bound to the binary system in a circumbinary disk, whereas the offset central star position may best be explained by proper motion. Detailed simulations of MyCn 18 are encouraged that are compatible with the binary nucleus to further investigate its complex formation history.

Galaxies as High-resolution Telescopes

Abstract Recent observations show a population of active galaxies with milliarcsecond offsets between optical and radio emission. Such offsets can be an indication of extreme phenomena associated with supermassive black holes including relativistic jets, binary supermassive black holes, or even recoiling supermassive black holes. However, the multi-wavelength structure of active galaxies at a few milliarcseconds cannot be resolved with direct observations. We propose using strong gravitational lensing to elucidate the multi-wavelength structure of sources. When sources are located close to the caustic of a lensing galaxy, even a small offset in the position of the sources results in a drastic difference in the position and magnification of mirage images. We show that the angular offset in the position of the sources can be amplified more than 50 times in the observed position of mirage images. We find that at least 8% of the observed gravitationally lensed quasars will be in the caustic configuration. The synergy between SKA and Euclid will provide an ideal set of observations for thousands of gravitationally lensed sources in the caustic configuration, which will allow us to resolve the multi-wavelength structure for a large ensemble of sources and to study the physical origin of radio emissions, their connection to supermassive black holes, and their cosmic evolution.

The First ALMA Observation of a Solar Plasmoid Ejection from an X-Ray Bright Point

Abstract Eruptive phenomena such as plasmoid ejections or jets are important features of solar activity and have the potential to improve our understanding of the dynamics of the solar atmosphere. Such ejections are often thought to be signatures of the outflows expected in regions of fast magnetic reconnection. The 304 Å EUV line of helium, formed at around 105 K, is found to be a reliable tracer of such phenomena, but the determination of physical parameters from such observations is not straightforward. We have observed a plasmoid ejection from an X-ray bright point simultaneously at millimeter wavelengths with ALMA, at EUV wavelengths with SDO/AIA, and in soft X-rays with Hinode/XRT. This paper reports the physical parameters of the plasmoid obtained by combining the radio, EUV, and X-ray data. As a result, we conclude that the plasmoid can consist either of (approximately) isothermal ∼105 K plasma that is optically thin at 100 GHz, or a ∼104 K core with a hot envelope. The analysis demonstrates the value of the additional temperature and density constraints that ALMA provides, and future science observations with ALMA will be able to match the spatial resolution of space-borne and other high-resolution telescopes.

Gravitational microlensing in Verlinde's emergent gravity

We propose gravitational microlensing as a way of testing the emergent gravity theory recently proposed by Eric Verlinde [1]. We consider two limiting cases: the dark mass of maximally anisotropic pressures (Case I) and of isotropic pressures (Case II). Our analysis of perihelion advancement of a planet shows that only Case I yields a viable theory. In this case the metric outside a star of mass M⁎ can be modeled by that of a point-like global monopole whose mass is M⁎ and a deficit angle Δ=(2GH0M⁎)/(3c3), where H0 is the Hubble rate and G the Newton constant. This deficit angle can be used to test the theory since light exhibits additional bending around stars given by, αD≈−πΔ/2. This angle is independent on the distance from the star and it affects equally light and massive particles. The effect is too small to be measurable today, but should be within reach of the next generation of high resolution telescopes. Finally we note that the advancement of periastron of a planet orbiting around a star or black hole, which equals πΔ per period, can be also used to test the theory.

Definitive test of the Rh = ct universe using redshift drift

Abstract The redshift drift of objects moving in the Hubble flow has been proposed as a powerful model-independent probe of the underlying cosmology. A measurement of the first- and second-order redshift derivatives appears to be well within the reach of upcoming surveys using as the Extremely Large Telescope high resolution spectrometer (ELT-HIRES) and the Square Kilometer Phase 2 Array (SKA). Here we show that an unambiguous prediction of the Rh = ct cosmology is zero drift at all redshifts, contrasting sharply with all other models in which the expansion rate is variable. For example, multiyear monitoring of sources at redshift z = 5 with the ELT-HIRES is expected to show a velocity shift Δv = −15 cm s−1 yr−1 due to the redshift drift in Planck ΛCDM, while Δv = 0 cm s−1 yr−1 in Rh = ct. With an anticipated ELT-HIRES measurement error of ±5 cm s−1 yr−1 after 5 yr, these upcoming redshift drift measurements might therefore be able to differentiate between Rh = ct and Planck ΛCDM at ∼3σ, assuming that any possible source evolution is well understood. Such a result would provide the strongest evidence yet in favour of the Rh = ct cosmology. With a 20-yr baseline, these observations could favour one of these models over the other at better than 5σ.

Radiation length imaging with high-resolution telescopes

The construction of low mass vertex detectors with a high level of system integration is of great interest for next generation collider experiments. Radiation length images with a sufficient spatial resolution can be used to measure and disentangle complex radiation length X/X0 profiles and contribute to the understanding of vertex detector systems. Test beam experiments with multi GeV particle beams and high-resolution tracking telescopes provide an opportunity to obtain precise 2D images of the radiation length of thin planar objects. At the heart of the X/X0 imaging is a spatially resolved measurement of the scattering angles of particles traversing the object under study. The main challenges are the alignment of the reference telescope and the calibration of its angular resolution. In order to demonstrate the capabilities of X/X0 imaging, a test beam experiment has been conducted. The devices under test were two mechanical prototype modules of the Belle II vertex detector. A data sample of 100 million tracks at 4GeV has been collected, which is sufficient to resolve complex material profiles on the 30μm scale.

Ultraviolet emission lines of Si ii in cool star and solar spectra

Recent atomic physics calculations for Si II are employed within the Cloudy modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra of three cool stars, Beta-Geminorum, Alpha-Centauri A and B, as well as previously published HST/GHRS observations of Alpha-Tau, plus solar quiet Sun data from the High Resolution Telescope and Spectrograph. Discrepancies found previously between theory and observation for line intensity ratios involving the 3s$^{2}$3p $^{2}$P$_{J}$--3s3p$^{2}$ $^{4}$P$_{J^{\prime}}$ intercombination multiplet of Si II at 2335 Angs are significantly reduced, as are those for ratios containing the 3s$^{2}$3p $^{2}$P$_{J}$--3s3p$^{2}$ $^{2}$D$_{J^{\prime}}$ transitions at 1816 Angs. This is primarily due to the effect of the new Si II transition probabilities. However, these atomic data are not only very different from previous calculations, but also show large disagreements with measurements, specifically those of Calamai et. al. (1993) for the intercombination lines. New measurements of transition probabilities for Si II are hence urgently required to confirm (or otherwise) the accuracy of the recently calculated values. If the new calculations are confirmed, then a long-standing discrepancy between theory and observation will have finally been resolved. However, if the older measurements are found to be correct, then the agreement between theory and observation is simply a coincidence and the existing discrepancies remain.

Ultraviolet spectroscopy of V Sagittae in high, intermediate and low states from HST and IUE satellites

We present the first phase resolved ultraviolet spectroscopic study of V Sge in high, intermediate and low states observed with the Hubble Space Telescope High Resolution Spectrograph (HST HRS) and International Ultraviolet Explorer (IUE) during the period 1978–1996 to diagnose the ultraviolet fluxes of C IV 1550 A and He II 1640 A emission lines originating in the accretion disk during different orbital phases. Different spectra showing the variations in line fluxes at different orbital phases are presented. The reddening of V Sge is determined from the 2200 A feature. We concentrated on calculating the line fluxes of C IV & He II emission lines. From HST and IUE data, we derived an accretion luminosity and an accretion rate for V Sge. The average temperature of the outer rim of the accretion disk ${\sim}10000~\mbox{K}$ . Our results show that there are variations in line fluxes, accretion luminosities and accretion rates with time for V Sge. These variations are attributed to the variations of both density and temperature as a result of a changing rate of mass transfer from the secondary star to the white dwarf. These results from the HST and IUE observations are consistent with the binary model consisting of a white dwarf, a disk around the white dwarf, and a lobe-filling main-sequence companion (Hachisu & Kato, Astrophys. J. 598:527H, 2003).