High-Resolution Infrared Imaging of Neptune from the Keck Telescope

We present results of infrared observations of Neptune from the 10-m W. M. Keck I Telescope, using both high-resolution (0.04 arcsecond) broadband speckle imaging and conventional imaging with narrowband filters (0.6 arcsec resolution). The speckle data enable us to track the size and shape of infrared-bright features (“storms”) as they move across the disk and to determine rotation periods for latitudes −30 and −45°. The narrowband data are input to a model that allows us to make estimates of Neptune's stratospheric haze abundance and the size of storm features. We find a haze column density of ∼10⁶ cm⁻² for a haze layer located in the stratosphere, and a lower limit of 10⁷ cm⁻² and an upper limit of 10⁹ cm⁻² for a layer of 0.2 μm particles in the troposphere. We also calculate a lower limit of 7×10⁶ km² for the size of a “storm” feature observed on 13 October 1997.     

High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io

Io, the innermost Galilean satellite of Jupiter, is a fascinating world. Data taken by Voyager and Galileo instruments have established that it is by far the most volcanic body in the Solar System and suggest that the nature of this volcanism could radically differ from volcanism on Earth. We report on near-IR observations taken in February 2001 from the Earth-based 10-m W. M. Keck II telescope using its adaptive optics system. After application of an appropriate deconvolution technique (MISTRAL), the resolution, ∼100 km on Io's disk, compares well with the best Galileo/NIMS resolution for global imaging and allows us for the first time to investigate the very nature of individual eruptions. On 19 February, we detected two volcanoes, Amirani and Tvashtar, with temperatures differing from the Galileo observations. On 20 February, we noticed a slight brightening near the Surt volcano. Two days later it had turned into an extremely bright volcanic outburst. The hot spot temperatures (>1400 K) are consistent with a basaltic eruption and, being lower limits, do not exclude an ultramafic eruption. These outburst data have been fitted with a silicate-cooling model, which indicates that this is a highly vigorous eruption with a highly dynamic emplacement mechanism, akin to fire-fountaining. Its integrated thermal output was close to the total estimated output of Io, making this the largest ionian thermal outburst yet witnessed.     

NVST多通道高分辨观测系统软件设计

1 m太阳望远镜多通道高分辨成像系统是望远镜的重要终端设备之一,目前由Hα通道(线心656.283 nm)和Ti O通道(705.8 nm)构成。主要介绍了多通道高分辨观测系统软件的设计。观测系统在功能上主要实现了Hα通道多波长点扫描观测模式,Ti O通道多时间分辨率观测模式,以及为满足多通道发展的需求,如常规观测通道的增加以及探测器的更换等,在系统架构上采用了松耦合的分布式分层结构。     

High-Resolution Imaging of Mercury Using Earth-based Facilities

With a view to the further development of the short exposure method with a CCD detector, new observations of the planet Mercury were carried out at the Abastumany Astrophysical Observatory, Republic of Georgia, from October 30 to November 8, 2001. Comparison with the previous data, as well as the results of data processing based on newly developed algorithms, points to considerable progress achieved in the technique for observing Mercury. In some cases, under very favorable atmospheric conditions, the resolution attained is close to the diffraction limit of the astronomic instrument used. For the first time, topographic features on Mercury's surface were reliably resolved. Features with linear sizes as small as 120 km are successfully identified in the disk center.     

Very High-Resolution Solar X-Ray Imaging Using Diffractive Optics

This paper describes the development of X-ray diffractive optics for imaging solar flares with better than 0.1 arcsec angular resolution. X-ray images with this resolution of the ???10?MK plasma in solar active regions and solar flares would allow the cross-sectional area of magnetic loops to be resolved and the coronal flare energy release region itself to be probed. The objective of this work is to obtain X-ray images in the iron-line complex at 6.7?keV observed during solar flares with an angular resolution as fine as 0.1 arcsec ?C over an order of magnitude finer than is now possible. This line emission is from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma at temperatures in excess of ???10 MK. It provides information on the flare morphology, the iron abundance, and the distribution of the hot plasma. Studying how this plasma is heated to such high temperatures in such short times during solar flares is of critical importance in understanding these powerful transient events, one of the major objectives of solar physics. We describe the design, fabrication, and testing of phase zone plate X-ray lenses with focal lengths of ???100 m at these energies that would be capable of achieving these objectives. We show how such lenses could be included on a two-spacecraft formation-flying mission with the lenses on the spacecraft closest to the Sun and an X-ray imaging array on the second spacecraft in the focal plane ???100 m away. High-resolution X-ray images could be obtained when the two spacecraft are aligned with the region of interest on the Sun. Requirements and constraints for the control of the two spacecraft are discussed together with the overall feasibility of such a formation-flying mission.     

EMCCD附加噪声对天文高分辨成像的影响

讨论了电子倍增电荷耦合器件(Electron-Multiplying Charge-Coupled Device,EMCCD)附加噪声的产生机理及特性,通过模拟详细分析了EMCCD附加噪声对天文图像高分辨统计重建技术——斑点干涉术传递函数信噪比的影响,并和实验进行了对比,结果表明,在星体暗弱时,由附加噪声引入的偏差将使斑点干涉术重建的信噪比下降,严重影响重建的结果,必须加以改正.由实测数据重建结果可以看出,噪声偏差改正模型基本解决了EMCCD附加噪声对斑点干涉术在重建目标自相关或模时的影响.     

Structure of the Keck Telescope — An overview

The structure of the Keck Telescope is briefly described. The design required an innovative approach made necessary by the revolutionary nature of the segmented primary mirror, by the very stringent weight and cost limitations, and by observational and operational needs. Analysis of a progressively more detailed computer model predicts that all design objectives will be met, as shown in a summary of performance characteristics. The paper is illustrated with a number of drawings.Paper presented at the Symposium on the JNLT and Related Engineering Developments, Tokyo, November 29–December 2 1988.     

Keck Adaptive Optics Images of Uranus and Its Rings

We present adaptive optic images of Uranus obtained with the 10-m W. M. Keck II telescope in June 2000, at wavelengths between 1 and 2.4 μm. The angular resolution of the images is ∼0.06-0.09″. We identified eight small cloud features on Uranus's disk, four of which were in the northern hemisphere. The latter features are ∼1000-2000 km in extent and located in the upper troposphere, above the methane cloud, at pressures between 0.5 and 1 bar. Our data have been combined with HST data by Hammel et al. (2001, Icarus153, 229-235); the combination of Keck and HST data allowed derivation of an accurate wind velocity profile. Our images further show Uranus's entire ring system: the asymmetric ? ring, as well as the three groups of inner rings (outward from Uranus): the rings 6+5+4, α+β, and the η+γ+δ rings. We derived the equivalent I/F width and ring particle reflectivity for each group of rings. Typical particle albedos are ∼0.04-0.05, in good agreement with HST data at 0.9 μm.     

Keck observations of near-Earth asteroids in the thermal infrared

We present the results of thermal-infrared observations of 20 near-Earth asteroids (NEAs) obtained in the period March 2000-February 2002 with the 10-m Keck-I telescope on Mauna Kea, Hawaii. The measured fluxes have been fitted with thermal-model emission continua to determine sizes and albedos. This work increases the number of NEAs having measured albedos by 35%. The spread of albedos derived is very large (p_v=0.02−0.55); the mean value is 0.25, which is much higher than that of observed main-belt asteroids. In most cases the albedos are in the ranges expected for the spectral types, although some exceptions are evident. Our results are consistent with a trend of increasing albedo with decreasing size for S-type asteroids with diameters below 20 km. A number of objects are found to have unexpectedly low apparent color temperatures, which may reflect unusual thermal properties. However, the results from our limited sample suggest that high thermal-inertia, regolith-free objects may be uncommon, even amongst NEAs with diameters of less than 1 km. We discuss the significance of our results in the light of information on these NEAs taken from the literature and the uncertainties inherent in applying thermal models to near-Earth asteroids.     

Keck AO observations of Io in and out of eclipse

We present adaptive optics (AO) observations of Io taken with the W.M. Keck II telescope on 18 December 2001 (UT) before the satellite went into eclipse, and while it was in Jupiter's shadow. Making these kind of Io-in-eclipse observations, as well as the associated data reduction and analysis are challenging; hence one focus of the paper is to explain the methods and tools used for these data sets. For the sunlit images Io itself was used as the wavefront reference source, while nearby Ganymede was used as reference ‘star’ when Io was in eclipse. Observations were obtained in K′-, L′-, and M-bands. The sunlit images have been deconvolved using MISTRAL. The Io-in-eclipse data were deconvolved with IDAC and MISTRAL. The former gives better results, both in absolute photometry and in matching the original images. We determined the flux densities of the hot spots from the original Io-in-eclipse data with StarFinder, as well as from the deconvolved images by integrating the intensity over the relevant areas. We determined the highly anisoplanatic PSF via a FFT method from the original data, and used this in StarFinder and as a starting PSF for IDAC and MISTRAL. We derived temperatures and areal coverage of all 19 spots detected in both K′- and L′-band images of Io-in-eclipse. We also determined temperatures and areal coverage of the hot spots visible on the L′- and M-band images of sunlit Io. Most volcanoes contain a compact hot ‘core’ (?10 km² at 600-800 K) within a larger area at lower temperatures (e.g., ∼10²-10⁴ km² at 300-500 K). The total heat flow contributed by these active volcanoes is 0.2 W m⁻², ∼8% of the average global heat flow measured at 5-20 μm by Veeder et al. [J. Geophys. Res. 99 (1994) 17095].     

Keck adaptive optics imaging of near-Earth Asteroid 2004 XP14

We present resolved near-infrared images of near-Earth Asteroid 2004 XP14, obtained with the adaptive optics system on Keck II during July 2006. This is the first time a near-Earth asteroid has been directly imaged from the ground. Our result illustrates the capabilities of adaptive optics and complements radar and spectral observations.     

Speckle imaging of volcanic hotspots on Io with the Keck telescope

Using speckle imaging techniques on the 10-m W.M. Keck I telescope, we observed near-infrared emission at 2.2 μm from volcanic hotspots on Io in July-August 1998. Using several hundreds of short-exposure images we reconstructed diffraction-limited images of Io on each of three nights. We measured the positions of individual hotspots to ±0.004″ or better, corresponding to a relative positional error of ∼20 km on Io's surface. The sensitivity of normal ground-based images of Io is limited by confusion between overlapping sources; by resolving these multiple points we detected up to 17 distinct hotspots, the largest number ever seen in a single image.During the month-long span of our 1998 observations, several events occurred. Loki was at the end of a long brightening, and we observed it to fade in flux by a factor of 2.8 over the course of one month. At the 3-sigma level we see evidence that Loki's position shifts by ∼100 km. This suggests that the brightening may not have been located at the “primary” Loki emission center but at a different source within the Loki caldera. We also see a bright transient source near Loki. Among many other sources we detect a dim source on the limb of Io at the latitude of Pele; this source is consistent with 2.7% of the thermal emission from the Pele volcano complex being scattered by the Pele plume, which would be the first detection of a plume through scattered infrared hotspot emission.     

Keck II spectroscopy of mHz quasi-periodic oscillations in Hercules X-1

We present Keck II spectroscopy of optical mHz quasi-periodic oscillations (QPOs) in the light curve of the X-ray pulsar binary Hercules X-1. In the power spectrum it appears as 'peaked noise', with a coherency ∼2, a central frequency of 35 mHz and a peak-to-peak amplitude of 5 per cent. However, the dynamic power spectrum shows it to be an intermittent QPO, with a lifetime of ∼100 s, as expected if the lifetime of the orbiting material is equal to the thermal time-scale of the inner disc. We have decomposed the spectral time series into constant and variable components and used blackbody fits to the resulting spectra to characterize the spectrum of the QPO variability and constrain possible production sites. We find that the spectrum of the QPO is best fitted by a small hot region, possibly the inner regions of the accretion disc, where the ballistic accretion stream impacts on to the disc. The lack of any excess power around the QPO frequency in the X-ray power spectrum, created using simultaneous light curves from RXTE , implies that the QPO is not simply reprocessed X-ray variability.     

Keck observations of the 2002-2003 jovian ring plane crossing

We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼5000 km extension on the inside. The normal optical depth is 8×¹⁰⁻⁶, 15% of which is provided by bodies with radii a?5 cm. These bodies are as red as Metis. Half the optical depth, τ≈4×¹⁰⁻⁶, is attributed to micron-sized dust, and the remaining τ≈3×¹⁰⁻⁶ to grains tens to hundreds of μm in size. The inward extension consists of micron-sized (a?10 μm) dust, which probably migrates inward under Poynting-Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r=122,400 km), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000-20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼5×¹⁰⁻⁷, up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller.     

Keck Observations of Solar System Objects: Perspectives for Extremely Large Telescopes

From differential tracking techniques, required for appulse observations of KBOs with Laser Guide Star Adaptive Optics (LGSAO), to developing methods for collecting spectra at the precise moment of a predicted impact, each Solar System observation conducted on a large telescope presents a unique set of challenges. We present operational details and some key science results from our science program, adaptive optics observations of main belt asteroids and near earth objects; as well as the technical and operational details of several Keck Solar System observations conducted by other teams: the impact of Shoemaker-Levy 9 on Jupiter, volcanoes on Io, the Deep Impact mission to Comet 9P/Tempel 1, and recent observations of Pluto’s moons Nix and Hydra. For each of these observations, we draw from our Keck experience to predict what challenges may lie ahead when similar observations are conducted on next generation telescopes.