Stellar Images Research Articles

Experimental confirmation of long-memory correlations in star-wander data.

In this Letter we have analyzed the temporal correlations of the angle-of-arrival fluctuations of stellar images. Experimentally measured data were carefully examined by implementing multifractal detrended fluctuation analysis. This algorithm is able to discriminate the presence of fractal and multifractal structures in recorded time sequences. We have confirmed that turbulence-degraded stellar wavefronts are compatible with a long-memory correlated monofractal process. This experimental result is quite significant for the accurate comprehension and modeling of the atmospheric turbulence effects on the stellar images. It can also be of great utility within the adaptive optics field.

CCD PHOTOMETRY OF BRIGHT STARS USING OBJECTIVE WIRE MESH

Obtaining accurate photometry of bright stars from the ground remains tricky because of the danger of overexposure of the target and/or lack of suitable nearby comparison star. The century-old method of the objective wire mesh used to produce multiple stellar images seems attractive for precision CCD photometry of such stars. Our tests on beta Cep and its comparison star differing by 5 magnitudes prove very encouraging. Using a CCD camera and a 20 cm telescope with objective covered with a plastic wire mesh, located in poor weather conditions we obtained differential photometry of precision 4.5 mmag per 2 min exposure. Our technique is flexible and may be tuned to cover as big magnitude range as 6 - 8 magnitudes. We discuss the possibility of installing a wire mesh directly in the filter wheel.

Defining a weak lensing experiment in space

This paper describes the definition of a typical next-generation space-based weak gravitational lensing experiment. We first adopt a set of top-level science requirements from the literature, based on the scale and depth of the galaxy sample, and the avoidance of systematic effects in the measurements which would bias the derived shear values. We then identify and categorize the contributing factors to the systematic effects, combining them with the correct weighting, in such a way as to fit within the top-level requirements. We present techniques which permit the performance to be evaluated and explore the limits at which the contributing factors can be managed. Besides the modelling biases resulting from the use of weighted moments, the main contributing factors are the reconstruction of the instrument point spread function, which is derived from the stellar images on the image, and the correction of the charge transfer inefficiency in the CCD detectors caused by radiation damage.

3D hydrodynamical simulations to interpret observations of stellar surfaces of red supergiant stars

As red supergiants are the largest and brightest stars, they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show the results obtained with the synergy between the radiative hydrodynamics code CO5BOLD and the post-processing radiative transfer code Optim3D. Such simulations support a proper and quantitative analysis of these observations, and the observations provide the theoretical work with strong constraints.

Atmospheric Dispersion Effects in Weak Lensing Measurements

ABSTRACTThe wavelength dependence of atmospheric refraction causes elongation of finite-bandwidth images along the elevation vector, which produces spurious signals in weak gravitational lensing shear measurements unless this atmospheric dispersion is calibrated and removed to high precision. Because astrometric solutions and point spread function (PSF) characteristics are typically calibrated from stellar images, differences between the reference stars’ spectra and the galaxies’ spectra will leave residual errors in both the astrometric positions () and in the second moment (width) of the wavelength-averaged PSF (ΔV) for galaxies. We estimate the level of ΔV that will induce spurious weak lensing signals in PSF-corrected galaxy shapes that exceed the statistical errors of the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST) cosmic-shear experiments. We also estimate the signals that will produce unacceptable spurious distortions after stacking of exposures taken at different air masses and hour angles. Using standard galaxy and stellar spectral templates we calculate the resultant errors in the griz bands and find that atmospheric dispersion shear systematics, left uncorrected, are up to 6 and 2 times larger in g and r bands, respectively, than the thresholds at which they become significant contributors to the DES error budget, but can be safely ignored in i and z bands. For the stricter LSST requirements, the factors are about 30, 10, and 3 in g, r, and i bands, respectively. These shear systematic errors scale with observed zenith angle z as 〈 tan2 z〉, for which we take a nominal value of unity—simulations of DES and LSST suggest 0.6–1.0. We find that a simple correction linear in galaxy color is accurate enough to reduce dispersion shear systematics to insignificant levels in the r band for DES and i band for LSST, but still as much 5× above the threshold of significance for LSST r-band observations. More complex approaches to correction of the atmospheric dispersion signal will likely be able to reduce the systematic cosmic shear errors below statistical errors for LSST r band. But g-band dispersion effects remain large enough that it seems likely that induced systematics will dominate the statistical errors of both surveys, and cosmic-shear measurements should rely on the redder bands.

A Modified Watershed Algorithm for Stellar Image

A modified gray scale watershed image segmentation algorithm suitable for low contrast image has been proposed. Digital images acquired from far away stellar objects (like stars, planets, galaxies, comets etc.) are prone to be severally affected by various types of noises and the contrast of these categories of images are generally found to be low. In present study, a preserving de noising method is presented by a contrast adjustment based on adaptive histogram equalization technique. The proposed method has been found to yield satisfactory segmentation of the stellar images. The entropy of the original and the segmented image is compared and the result confirms to the reality.

Imaging submilliarcsecond stellar features with intensity interferometry using air Cherenkov telescope arrays

Recent proposals have been advanced to apply imaging air Cherenkov telescope arrays to stellar intensity interferometry (SII). Of particular interest is the possibility of model-independent image recovery afforded by the good (u, v)-plane coverage of these arrays, as well as recent developments in phase retrieval techniques. The capabilities of these instruments used as SII receivers have already been explored for simple stellar objects, and here the focus is on reconstructing stellar images with non-uniform radiance distributions. We find that hot stars (T > 6000 K) containing hot and/or cool localized regions (T \sim 500 K) as small as \sim 0.1 mas can be imaged at short wavelengths ({\lambda} = 400 nm).

The impact of CCD radiation damage on Gaia astrometry - II. Effect of image location errors on the astrometric solution

Gaia, the next astrometric mission of the European Space Agency, will use a camera composed of 106 CCDs to collect multiple observations for one billion stars. The astrometric core solution of Gaia will use the estimated location of the stellar images on the CCDs to derive the astrometric parameters (position, parallax and proper motion) of the stars. The Gaia CCDs will suffer from charge transfer inefficiency (CTI) mainly caused by radiation damage. CTI is expected to significantly degrade the quality of the collected images which ultimately affects the astrometric accuracy of Gaia. This paper is the second and last in a study aiming at characterizing and quantifying the impact of CCD radiation damage on Gaia astrometry. Here we focus on the effect of the image location errors induced by CTI on the astrometric solution. We apply the Gaia Astrometric Global Iterative Solution (AGIS) to simulated Gaia-like observations for 1 million stars including CTI-induced errors as described in the first paper. We show that a magnitude-dependent image location bias is propagated in the astrometric solution, biasing the estimation of the astrometric parameters as well as decreasing its precision. We demonstrate how the Gaia scanning law dictates this propagation and the ultimate sky distribution of the CTI-induced errors. The possibility of using the residuals of the astrometric solution to improve the calibration of the CTI effects is investigated. We also estimate the astrometric errors caused by (faint) disturbing stars preceding the stellar measurements on the CCDs. Finally, we show that, for single stars, the overall astrometric accuracy of Gaia can be preserved to within 10 per cent of the CTI-free case for all magnitudes by appropriate modelling at the image location estimation level and using the solution residuals. (Less)

Self-adjusting data acquisition system for Low Layer SCIDAR

We present a real-time, fully automated method to remove instrumental signatures from stellar scintillation images. The method was developed in connection with the Low Layer SCIDAR (Scintillation Detection and Ranging) (LOLAS), an instrument dedicated to the measurement of atmospheric optical-turbulence profiles, ⁠, in the first kilometre of altitude, with high altitude-resolution. By examining the scintillation images and their correlation maps we evaluate, and then correct, image deficiencies typical of a portable telescope, such as defocus, image movement, field rotation and tracking errors, prior to processing according to the SCIDAR technique. In addition, detector spreading noise is corrected for, and flux variations caused by clouds or fog are monitored. The efficiency of the methods is demonstrated by comparing double-star scintillation autocorrelations obtained with and without the use of the self-adjusting procedures. Applying those stabilization methods, LOLAS was successfully exploited to characterize the vertical profiles along the lower layer on top of Mauna Kea Observatory.

Three-dimensional interferometric, spectrometric, and planetary views of Procyon

We used a new realistic 3D radiative-hydrodynamical model atmosphere of Procyon generated with the Stagger Code and synthetic spectra computed with the radiative transfer code Optim3D to re-analyze interferometric and spectroscopic data from the optical to the infrared of Procyon. We compute intensity maps in two optical filters centered at 500 and 800 nm (MARK III) and one infrared filter centered at 2200 nm (VINCI). We constructed stellar disk images accounting for the center-to-limb variations and used them to derive visibility amplitudes and closure phases. We provide 3D limb-darkening coefficients in the optical as well as in the infrared. We show that visibility curves and closure phases show clear deviations from circular symmetry from the 3rd lobe on. These deviations are detectable with current interferometers using closure phases. We derive new angular diameters at different wavelengths with two independent methods based on 3D simulations. We find a diameter_Vinci = 5.390 \pm 0.03 mas that this is confirmed by an independent asteroseismic estimation. The resulting Teff is 6591 K, which is consistent with the infrared flux method determinations. We find also a value of the surface gravity log g = 4.01 \pm 0.03 that is larger by 0.05 dex from literature values. Spectrophotometric comparisons with observations provide very good agreement with the spectral energy distribution and photometric colors, allowing us to conclude that the thermal gradient of the simulation matches fairly well Procyon. Finally, we show that the granulation pattern of a planet hosting Procyon-like star has a non-negligible impact on the detection of hot Jupiters in the infrared using interferometry closure phases. It is then crucial to have a comprehensive knowledge of the host star to directly detect and characterize hot Jupiters. In this respect, RHD simulations are very important to reach this aim.

The impact of CCD radiation damage on Gaia astrometry - I. Image location estimation in the presence of radiation damage

The Gaia mission has been designed to perform absolute astrometric measurements with unprecedented accuracy; the end-of-mission parallax standard error is required to be 30 micro-arcseconds for a G2V type star of magnitude 15. These requirements set a stringent constraint on the accuracy of the estimation of the location of the stellar image on the CCD for each observation: e.g., 0.3 milli-arseconds (mas) or 0.005 pixels for the same V=15 G2V star. However the Gaia CCDs will suffer from charge transfer inefficiency (CTI) caused by radiation damage that will degrade the stellar image quality and may degrade the astrometric performance of Gaia if not properly addressed. For the first time at this level of detail, the potential impact of radiation damage on the performance of Gaia is investigated. In this first paper we focus on the evaluation of the CTI impact on the image location accuracy. We show that CTI decreases the stellar image signal-to-noise ratio and irreversibly degrades the image location estimation precision. As a consequence the location estimation standard errors increase by up to 6% for a radiation damage level equivalent to the end-of-mission. In addition the CTI-induced image distortion introduces a systematic bias in the image location estimation (up to 0.05 pixels or 3 mas in the Gaia operating conditions). We present a novel approach to CTI mitigation that enables, without correction of the raw data, the unbiased estimation of the image location and flux from damaged observations. Its implementation reduces the maximum measured location bias for the faintest magnitude to 0.005 pixels (~4e-4 pixels at magnitude 15). In a second paper we will investigate how the CTI effects affect the final astrometric accuracy of Gaia by propagating residual errors through the astrometric solution.

Diffracto-Astrometry Measurements: Accuracy of the Measuring Algorithm

ABSTRACTWe present a theoretical analysis of the measuring algorithm we use when applying the diffracto-astrometry technique to Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) saturated stellar images. Theoretical point-spread functions (PSFs) were generated using the Tiny Tim software for the four CCDs in the WFPC2 and for some of the available filters. These images were then measured with our diffracto-astrometry measuring algorithm using only their diffraction pattern, and positions for the simulated PSFs on each generated CCD frame were obtained. The measuring algorithm recovers the original positions reasonably well (± 0.1 to ± 0.4 pixels). However, slight deviations from the original values are observed. These also vary with position over the entire surface of the CCD. We adjust the difference between the real and the measured positions with a quadratic function of the coordinates. The transformation coefficients also present a slight correlation with the filter effective wavelength. Application of these transformation coefficients allows us to determine the position of a stellar image with a precision of a few hundredths of a pixel.

Ground-Based Submillimagnitude CCD Photometry of Bright Stars Using Snapshot Observations

ABSTRACTWe demonstrate ground-based submillimagnitude (< 10-3) photometry of widely separated bright stars using snapshot CCD imaging. We routinely achieved this photometric precision by (1) choosing nearby comparison stars of a similar magnitude and spectral type, (2) defocusing the telescope to allow high signal (>107 e-) to be acquired in a single integration, (3) pointing the telescope so that all stellar images fall on the same detector pixels, and (4) using a region of the CCD detector that is free of nonlinear or aberrant pixels. We describe semiautomated observations with the Supernova Integrated Field Spectrograph (SNIFS) on the University of Hawaii 2.2 m telescope on Mauna Kea, with which we achieved photometric precision as good as 5.2 × 10-4(0.56 mmag) with a 5 minute cadence over a 2 hr interval. In one experiment, we monitored eight stars, each separated by several degrees, and achieved submillimagnitude precision with a cadence (per star) of ∼17 minutes. Our snapshot technique is suitable for automated searches for planetary transits among multiple bright stars.

A detailed dust energy balance study of the Sombrero galaxy

AbstractThe Sombrero galaxy (M104) is an interesting object for a dust energy balance study due to its very symmetric dust lane, its proximity and its (nearly edge-on) inclination of 84°. From a panchromatic radiative transfer analysis, including scattering, absorption and thermal dust re-emission, we construct a standard model for M104 accounting for observations in the optical wave bands (stellar SED, images and extinction profiles in the V and RC band). This standard model underestimates the observed dust emission at infrared wavelengths by a factor of ~ 3, similar to the discrepancy found in other energy balance studies of edge-on spirals. Supplementing this standard model with a young stellar component of low star formation activity in both the inner disk (SFR ~ 0.21 M⊙ yr−1) and dust ring (SFR ~ 0.05 M⊙ yr−1), we are capable of solving the discrepancy in the dust energy budget of the Sombrero galaxy at wavelengths shortwards of 100 μm. To account for the remaining discrepancy at longer wavelengths, we propose a secondary dust component distributed in quiescent clumps. This model with a clumpy dust structure predicts three-quarters of the total dust content to reside in compact dust clouds with no associated embedded sources.

基于离焦星点图的RC式望远镜装调技术

In order to realize the alignment of large aperture telescopes,the RC telescope alignment techniques were investigated based on the image processing and analysis of out-of-focus stellar images.Firstly,the relationship between the disalignment comas due to decentered or tilted errors and the centers of contour circle was analyzed according to transversal aberration.The decrease of ellipse eccentricity with the increase of major axis length caused by residual astigmatism were investigated after removing the coma.Then,the dependence of spherical aberration caused by longitudinal errors on the least circle radius of defocused stellar image was analyzed.Finally,the analysis above was verified by the emulation of Zemax modeling and how to use the out-of-focus stellar image to guide the alignment process and how to process the image were discussed.Experimental results indicate that both the lateral and longitudinal errors are well consistent with the theoretically analyzed results.The precision errors of coma and astigmatism adjusted respectively by tilt and eccentricity are both less than 5%.

Seeing and atmospheric extinction at Mt. Maidanak observatory from observations with the 1.5-m AZT-22 telescope

We have determined the realistic seeing of the 1.5-m AZT-22 telescope of the Mt. Maidanak Observatory (Astronomical Institute, Uzbek Academy of Sciences) using more than 20 000 CCD frames with stellar images in the UBV RI bands acquired in 1996–2005: e = 1.065″ in the V band. The characteristic seeing reduced to unit air mass, e med V (M(z) = 1), is 0.945″. We derived color equations for the CCD detectors used with the telescope. Atmospheric-extinction coefficients in different photometric bands were also determined. The mean V -band atmospheric extinction is 0.20 m ± 0.04 m . The time needed for the conditions to settle, in the free atmosphere as well as inside the telescope dome, is 2–2.5 hours after the end of astronomical twilight. For nights with e med V > 0.9″, we find a persistent difference between the seeing found at this telescope and measured simultaneously with a differential image motion monitor, amounting to ∼0.1 m .

Performance Improvement of Near Earth Space Survey (NESS) Wide-Field Telescope (NESS-2) Optics

We modified the optical system of 500 mm wide-field telescope of which point spread function showed an irregularity. The telescope has been operated for Near Earth Space Survey (NESS) located at Siding Spring Observatory (SSO) in Australia, and the optical system was brought back to Korea in January 2008. After performing a numerical simulation with the tested value of surface figure error of the primary mirror using optical design program, we found that the surface figure error of the mirror should be fabricated less than root mean square (RMS) <TEX>$\lambda$</TEX>/10 in order to obtain a stellar full width at half maximum (FWHM) below <TEX>$28\;{\mu}m$</TEX>. However, we started to figure the mirror for the target value of RMS <TEX>$\lambda$</TEX>/20, because system surface figure error would be increased by the error induced by the optical axis adjustment, mirror cell installation, and others. The radius of curvature of the primary mirror was 1,946 mm after the correction. Its measured surface figure error was less than RMS <TEX>$\lambda$</TEX>/20 on the table of polishing machine, and RMS <TEX>$\lambda$</TEX>/15 after installation in the primary mirror cell. A test observation performed at Daeduk Observatory at Korea Astronomy and Space Science Institute by utilizing the exiting mount, and resulted in <TEX>$39.8\;{\mu}m$</TEX> of stellar FWHM. It was larger than the value from numerical simulation, and showed wing-shaped stellar image. It turned out that the measured-curvature of the secondary mirror, 1,820 mm, was not the same as the designed one, 1,795.977 mm. We fabricated the secondary mirror to the designed value, and finally obtained a stellar FWHM of <TEX>$27\;{\mu}m$</TEX> after re-installation of the optical system into SSO NESS Observatory in Australia.

UCAC3 PIXEL PROCESSING

The third US Naval Observatory (USNO) CCD Astrograph Catalog, UCAC3 was released at the IAU General Assembly on 2009 August 10. It is a highly accurate, all-sky astrometric catalog of about 100 million stars in the R = 8 to 16 magnitude range. Recent epoch observations are based on over 270,000 CCD exposures, which have been re-processed for the UCAC3 release applying traditional and new techniques. Challenges in the data have been high dark current and asymmetric image profiles due to the poor charge transfer efficiency of the detector. Non-Gaussian image profile functions were explored and correlations are found for profile fit parameters with properties of the CCD frames. These were utilized to constrain the image profile fit models and adequately describe the observed point-spread function of stellar images with a minimum number of free parameters. Using an appropriate model function, blended images of double stars could be fit successfully. UCAC3 positions are derived from 2-dimensional image profile fits with a 5-parameter, symmetric Lorentz profile model. Internal precisions of about 5 mas per coordinate and single exposure are found, which are degraded by the atmosphere to about 10 mas. However, systematic errors exceeding 100 mas are present in the x,y-data which have been corrected in the astrometric reductions following the x,y-data reduction step described here.

PYRAMIR: Exploring the On-Sky Performance of the World’s First Near-Infrared Pyramid Wavefront Sensor

ABSTRACTThis paper presents the on-sky performance of the unmodulated infrared pyramid wavefront sensor PYRAMIR mounted on the ALFA adaptive optics system at the 3.5 m telescope of the Calar Alto Observatory. The performance of the system is compared with the performance of the Shack-Hartmann wavefront sensor of the ALFA system. We carried out a series of measurements to characterize the performance of PYRAMIR under open-loop tip-tilt compensation, and high-order closed-loop conditions, using stars of different magnitudes. We measured the tip-tilt jitter by following the centroid position of a stellar image on a fast series of frames. Additionally from the pyramid wavefront sensor data we could estimate the tip-tilt jitter in closed-loop. Under closed-loop conditions we also measured the long-exposure Strehl ratio. We compared the results of the wavefront sensor measurements with those of the Shack-Hartmann sensor on the same telescope, especially regarding the distribution of the error budged over the Karhunen-Loève modes, and the power spectral density. Our first finding is that we can indeed start up this nonmodulated pyramid system, even under bad seeing conditions. Under good conditions the Strehl ratio reaches ≥60% in K′ band. We found that the minimum signal-to-noise ratio (S/N) in each subaperture required to close the high-order loop is only 0.4. This is a surprisingly low number. To compare the performance to existing systems, we introduce the S/N per subaperture per loop cycle as a device-independent measure. Using this scheme, we find that the ratio between the low-order residuals and the high-order residuals in the case of PYRAMIR is lower than that of the Shack-Hartmann system, especially in the faint flux regime. This is an important finding because it means that the pyramid-based system removes the halo, i.e., light scattered by the atmosphere, around the target star better than a Shack-Hartmann sensor-based system. A comparison of the power spectral density of the PYRAMIR and the Shack-Hartmann sensor measurements, and a comparison of the noise propagation coefficients of PYRAMIR with theoretical predictions from the literature, confirm this superiority of the pyramid over the Shack-Hartmann sensor.

MASSCLEAN—MASSIVE CLUSTER EVOLUTION AND ANALYSIS PACKAGE: DESCRIPTION AND TESTS

We present MASSCLEAN, a new, sophisticated and robust stellar cluster image and photometry simulation package. This visualization tool is able to create color–magnitude diagrams (CMDs) and standard FITS images in any of the traditional optical and near-infrared bands based on cluster characteristics input by the user, including but not limited to distance, age, mass, radius, and extinction. At the limit of very distant, unresolved clusters, we have checked the integrated colors created in MASSCLEAN against those from other simple stellar population models with consistent results. We have also tested models that provide a reasonable estimate of the field star contamination in images and CMDs. We demonstrate the package by simulating images and CMDs of well-known massive Milky Way clusters and compare their appearance to real data. Because the algorithm populates the cluster with a discrete number of tenable stars, it can be used as part of a Monte Carlo Method to derive the probabilistic range of characteristics (integrated colors, for example) consistent with a given cluster mass and age. Our simulation package is available for download and will run on any standard desktop running UNIX/Linux. Full documentation on installation and its use is also available. Finally, a Web-based version of MASSCLEAN, which can be immediately used and is sufficiently adaptable for most applications, is available through a Web interface.