Mariner 9 ultraviolet spectrometeer experiment: 1971 Mars' dust storm

The Mariner 9 ultraviolet spectrometer observed the brightness of a region on the south polar cap centered at approximately ?87°S, 10°W. Measurements taken at various incidence and emission angles (i and ?) show that the brightness increased with decreasing air mass, ≈(sec i + sec ?). The observed intensity consists primarily of a component reflected from the cap and twice-attenuated by the atmosphere and a component diffusely reflected from the atmosphere. The diffusely reflected component was determined from nearby observations of non-polar regions at the same incidence and emission angles and was substrated from the total intensity. Inversion of the intensity difference using a formula analogous to the Bouger-Langley law yielded the optical thickness of the atmosphere. The dust cloud over the polar cap was moderately thick between November 26 and December 2, 1971. At this time the optical thickness was near unity, and it decreased approximately linearly with time, reaching a value close to that of a Rayleigh atmosphere by mid-February. The optical thickness showed little dependence on the wavelength during the early orbital observations. As the dust storm cleared, the atmospheric optical thickness exhibited increasingly strong inverse wavelength dependence. Particles large compared with the wavelength dominated the Martian dust storm. These particles are estimated to have a mean radius of about 2 μm.     

On the instrumental and atmospheric stray-light for solar observations

From aureole measurements made with a 40 cm-Vacuum-Telescope at Izaña (Tenerife) in the wavelength region 417 nm < λ < 785 nm and from a comparison with other aureole measurements we conclude the following: (a) Within one solar radius from the limb, the aureole is mostly of instrumental origin, (b) Beyond that distance, the contribution of atmospheric stray-light becomes noticeable, (c) The atmospheric contribution to the aureole intensity is, under good conditions at mountain stations, a very slowly decreasing function, and amounts to some 10^?5 of the solar disk intensity. A procedure is given to separate the variable atmospheric component of the stray-light from the constant instrumental one.     

Limb Darkening Measurements Free of Instrumental Stray Light

We have used the 40 cm coronagraph with an inverse occulting disk to observe the limb darkening free of instrumental stray light. Our results agree remarkably well with those of Petro et al. (1984, Astrophys. J. 283, 426) who used the McMath – Pierce tower telescope but employed a different correction procedure for instrumental stray light than Neckel and Labs (1994, Solar Phys. 153, 91), whose results for λ=445.1 nm deviate systematically, apparently overcorrecting for stray light. Future continuation of such highly reproducible measurements offers an interesting independent diagnostic of possible slow trends in effective temperature (T _eff) and in total solar irradiance.     

Solar and Heliospheric Observatory/Large Angle Spectrometric Coronagraph Polarimetric Calibration

We present a polarimetric characterization and correction for the Solar and Heliospheric Observatory/Large Angle Spectrometric Coronagraph (SOHO/LASCO) C2 and C3 white light coronagraphs. By measuring the uncorrected polarization angles in solar minimum C2 coronal images, we have determined that the coronagraph acts as an optical phase retarder which converts a small fraction of the incoming radiation polarization from linear to circular. In addition, from the measurements of polarization angle in C3 coronal images we have determined that a component of the instrumentally scattered light in that instrument is polarized. We infer the retardation angle for C2 and compute the corresponding Mueller matrix, and determine the polarized stray light spatial profile in C3. The C2 Mueller matrix and C3 polarized stray light profiles are used to correct for instrumental effects in solar minimum coronal observations to obtain polarized brightness between two and thirty-two solar radii, which show deep polar coronal holes extending to the limit of the field of view.     

云南1m太阳塔真空封窗减反射膜的设计

云南1m太阳塔(YNST)[1]是一个真空结构的塔式太阳望远镜,依靠4块平板封窗玻璃把整个光学系统密封在真空系统中。封窗玻璃的引入,不仅会对太阳光有一定的吸收和反射,使来自太阳辐射的能量有一定的损失,并会形成杂散光,影响成像质量。本文讨论了这些影响。根据云南1m红外太阳塔科学指导委员会确定的工作波段的要求,设计了一种膜系设计方法,可以利用最少的膜系层数来达到需要的结果。并用一组材料进行了模拟设计实验,获得了较为理想的优化结果,在关键波长处显著减小了反射率,减少了杂散光的形成。这种设计方法很适合象太阳塔这种很宽的波段里面对一组波长有很高要求的设计任务,将大大提高光能利用率,具有重要的价值。     

The Depiction of Coronal Structure in White-Light Images

The very steep decrease in density with heliocentric distance makes imaging of coronal density structures out to a few solar radii challenging. The radial gradient in brightness can be reduced using numerous image processing techniques, thus quantitative data are manipulated to provide qualitative images. We introduce a new normalizing-radial-graded filter (NRGF): a simple filter for removing the radial gradient to reveal coronal structure. Applied to polarized brightness observations of the corona, the NRGF produces images which are striking in their detail. Total-brightness, white-light images include contributions from the F corona, stray light, and other instrumental contributions which need to be removed as effectively as possible to properly reveal the electron corona structure. A new procedure for subtracting this background from LASCO C2 white-light, total-brightness images is introduced. The background is created from the unpolarized component of total-brightness images and is found to be remarkably time-invariant, remaining virtually unchanged over the solar cycle. By direct comparison with polarized-brightness data, we show that the new background-subtracting procedure is superior in depicting coronal structure accurately, particularly when used in conjunction with the NRGF. The effectiveness of the procedures is demonstrated on a series of LASCO C2 observations of a coronal mass ejection (CME).     

The performance of a large echelle grating in a solar spectrometer

A number of tests of a large diffraction grating are described. It is shown that useful information about the performance of the grating in a spectrometer can be obtained from an analysis of the diffracted wavefront by the method of Stroke. Comparison of the instrumental profile at three wavelengths strongly suggests that the influence of changes in the angle of diffraction on the profile cannot be neglected. A resolving power of 6.9 × 10⁵ is attained at 6328 Å. At the same wavelength the first order Rowland ghosts in the ninth order of diffraction have an intensity of only 8 × 10^-4 of the parent line. The influence of stray light on the central intensities of deep Fraunhofer lines is shown to be small but by no means negligible.     

Martian atmospheric and indigenous components of xenon and nitrogen in the Shergotty,Nakhla, and Chassigny group meteorites

Abstract— In a study of the isotopic signatures of trapped Xe in shock-produced glass of shergottites and in ALH 84001, we observe three components: (1) modern Martian atmospheric Xe that is isotopically mass fractionated relative to solar Xe, favoring the heavy isotopes, (2) solar-like Xe, as previously observed in Chassigny, and (3) an isotopically fractionated (possibly ancient) component with little or no radiogenic ¹²⁹Xe_rad. In situ-produced fission and spallation components are observed predominantly in the high-temperature steps. Heavy N signatures in ALH 84001, EET 79001 and Zagami reveal Martian atmospheric components. The low-temperature release of ALH 84001 shows evidence for the presence of a light N component (δ¹⁵N ≤ -21%), which is consistent with the component observed in the other Shergotty, Nakhla and Chassigny (SNC) group meteorites. The highest observed ¹²⁹Xe/¹³⁰Xe ratio of 15.60 in Zagami and EET 79001 is used here to represent the present Martian atmospheric component, and the isotopic composition of this component is compared with other solar system Xe signatures. The ¹²⁹Xe/¹³⁰Xe ratios in ALH 84001 are lower but appear to reflect varying mixing ratios with other components. The consistently high ¹²⁹Xe/¹³⁰Xe ratios in rocks of different radiometric ages suggest that Martian atmospheric Xe evolved early on. As already concluded in earlier work, only a small fission component is observed in the Martian atmospheric component. Assuming that a chondritic ²⁴⁴Pu/¹²⁹I initial ratio applies to Mars, this implies that either Pu-derived fission Xe is retained in the solid planet (in fact, in situ-produced fission Xe is observed in ALH 84001) or may reflect a very particular degassing history of the planet.     

Solar rotation measurements at Mount Wilson

This paper describes a thorough reevaluation of the procedures for reducing the data acquired at the Mt. Wilson Observatory synoptic program of solar observations at the 150-foot tower. We also describe a new program of acquiring as many scans per day as possible of the solar magnetic and velocity fields. We give a new fitting formula which removes the background velocity field from each scan. An important new feature of our reduction algorithm is our treatment of the limb shift which permits time variation in this function. We identify the difference between the limb shift along the north-south axis and the east-west axis as potentially being a result of meridional circulation. Our analysis interprets the time variation in the east-west limb shift as being the result of changes in a vertical component of the meridional circulation.The performance of the system improved in 1982 as a result of the installation of a new exit slit assembly. The amplitude of the limb shift variations found prior to 1982 is larger than is easily explained with simple ideas of meridional circulation. However, we have not been able to firmly identify instrumental causes for the variations although small changes in the band-pass of the exit slit assembly could have contributed.We have established a correlation between the observed stray light in the system and a component of the velocity field which is antisymmetric with respect to the solar central meridian. We remove this stray light effect by adding an additional term to the fitting function.Finally, we show that the inclusion of the above improvements allows us to study the torsional oscillations at high latitude using a procedure which can retain the longitude dependent information about the velocity pattern.     

Relative umbral intensity of a large sunspot

Simultaneous observations of relative umbral intensities in four wavelength regions are presented. In the visual wavelength region the umbral intensities show lower values than given by most authors. By observing the same spot during different seeing conditions the method of correction for stray light is found to be consistent within the accuracy of the method. In addition, a new simple correction method is suggested.     

The Galway astronomical Stokes polarimeter: an all-Stokes optical polarimeter with ultra-high time resolution

Many astronomical objects emit polarised light, which can give information both about their source mechanisms, and about (scattering) geometry in their source regions. To date (mostly) only the linearly polarised components of the emission have been observed in stellar sources. Observations have been constrained because of instrumental considerations to periods of excellent observing conditions, and to steady, slowly or periodically-varying sources. This leaves a whole range of interesting objects beyond the range of observation at present. The Galway Astronomical Stokes Polarimeter (GASP) has been developed to enable us to make observations on these very sources. GASP measures the four components of the Stokes Vector simultaneously over a broad wavelength range 400–800 nm., with a time resolution of order microseconds given suitable detectors and a bright source - this is possible because the optical design contains no moving or modulating components. The initial design of GASP is presented and we include some preliminary observational results demonstrating that components of the Stokes vector can be measured to $<1$ % in conditions of poor atmospheric stability. Issues of efficiency and stability are addressed. An analysis of suitable astronomical targets, demanding the unique properties of GASP, is also presented.     

A Model for the Stray-Light Contamination of the UVCS Instrument on SOHO

We present a detailed model of stray-light suppression in the spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. The control of diffracted and scattered stray light from the bright solar disk is one of the most important tasks of a coronagraph. We compute the fractions of light that diffract past the UVCS external occulter and non-specularly pass into the spectrometer slit. The diffracted component of the stray light depends on the finite aperture of the primary mirror and on its figure. The amount of non-specular scattering depends mainly on the micro-roughness of the mirror. For reasonable choices of these quantities, the modeled stray-light fraction agrees well with measurements of stray light made both in the laboratory and during the UVCS mission. The models were constructed for the bright H i Lyα emission line, but they are applicable to other spectral lines as well.     

The Near-Contact Binary FU Ara： New Observations, a Photometric Study and Preliminary Elements

A new CCD (V) light curve is presented for'the semi-detached binary system FU Ara. The light curve, obtained in 2007, is the first one since the last 50 years. With our data we were able to determine six new times of minimum light and refined the period of the system to 0.8645049 days. A Wilson-Devinney analysis leads to a solution of a semi-detached configuration, composed of a main-sequence primary component of spectral type F5, fractionally smaller than its Roche lobe, and an evolved secondary component of spectral type K I which fills its Roche lobe, and which is overluminous and oversized as compared with the main-sequence. The two components of FU Ara differ considerably in effective temperature. It is classified as an FO Virginis type of near-contact binary system. Assuming a reasonable value for the mass of the primary component, an estimate of the absolute elements of FU Ara has been made, on the assumption that the primary has a mass corresponding to its spectral type according to Svechnikov & Taidakova.     

Correction of solar observations for stray light by numerical integration,with application to Mercury's drop

A numerical method for correction of stray light in solar observations has been developed. In particular a regular sunspot, where the circular contours of penumbra and umbra are projected as ellipses, has been studied. When a specified set of values for the stray light parameters is given, and also tentative values for the relative intensities of penumbra and umbra, the integration of stray light can be performed in any point. The result will be the observable intensity if the conditions were as given by these initial values.By means of limb observations the stray light parameters may be improved, and finally a variation of the penumbra- and umbra intensities in the computation, enables a determination of these quantities by comparison with observations.The method is tested on observations of the transit of Mercury, May 9, 1970. Calculation of isophotes with Mercury close to the limb shows the black drop phenomenon; which thus may be explained as an effect of stray light only.It is also shown that the Wilson effect on a sunspot cannot be produced by stray light alone.     

Influence Of Stray Light in the Instrument on Measurements of Longitudinal Magnetic Signals and Magnetic Field Azimuths

Magnetic saturation is investigated by numerical simulations and observations. It is found that this effect is not a main disturbance factor in measurements of longitudinal field signals. To examine the stray light in the instrument in magnetograph observations, we determined the point spread function of the Video Vector Magnetograph mounted on the Solar Magnetic Field Telescope (SMFT) installed at the Huairou Solar Observing Station (HSOS). Then we obtained the distribution of large-spread-angle (LSA) stray light intensity as a function of distance from disk center. A new way to correct LSA stray light is proposed. Also, we investigate stray light influence on measuring magnetic field azimuths.     

The diurnal variation of polarization characteristics of the Earth treated as an exoplanet

The diurnal polarization variation of the Earth, treated as an exoplanet with an unresolved disk but resolvable from its host star, is presented in three wavelength bands centered at 490 nm, 670 nm and865 nm respectively according to French satellite-borne PARASOL data. We aim to estimate disk-integrated polarization of the Earth with a phase angle of 55?. It is shown that:(1) the linear polarization signal and its variation are ascribed to the combination of surface feature distribution and atmospheric conditions acting as a variable polarimetric modulator; and(2) the polarimetric wavelength dependence is strong due to the atmospheric wavelength sensitivity. During the period when the PARASOL data were acquired, the cloud coverage ranged from 44.0% to 57.7%, and the polarimetric diurnal variation amplitude was within 1.8%in the 490 nm band, 1.3% in the 670 nm band and 1.5% in the 865 nm band.     

Computation of the atmosphere-less light intensity curve during a total solar eclipse by using Lunar Reconnaissance Orbiter topography data and the DE430 astronomical ephemeris

Observations of the sky irradiation intensity in the visible wavelengths during a solar eclipse permit to model the Sun diameter,a key number to constrain the internal structure of our star.In this paper,we present an algorithm that takes advantage of the precise Moon topography from Lunar Reconnaissance Orbiter to compute,with a high resolution in time,the geometrical part(i.e.top-of-atmosphere,and for a given wavelength)of the sky irradiation at any given location on the Earth during these events.The algorithm is also able to model the Baily’s beads.We give as an application the theoretical computation of the light curve corresponding to the solar eclipse observed at Lakeland(Queensland,North Australia)on 2012 November 13.The application to real data,with the introduction of atmospheric and instrumental passbands,will be considered in a forthcoming paper.     

Neural networks and the separation of cosmic microwave background and astrophysical signals in sky maps

We implement an independent component analysis (ICA) algorithm to separate signals of different origin in sky maps at several frequencies. Owing to its self-organizing capability, it works without prior assumptions on either the frequency dependence or the angular power spectrum of the various signals; rather, it learns directly from the input data how to identify the statistically independent components, on the assumption that all but, at most, one of the components have non-Gaussian distributions.
We have applied the ICA algorithm to simulated patches of the sky at the four frequencies (30, 44, 70 and 100 GHz) used by the Low Frequency Instrument of the European Space Agency's Planck satellite. Simulations include the cosmic microwave background (CMB), the synchrotron and thermal dust emissions, and extragalactic radio sources. The effects of the angular response functions of the detectors and of instrumental noise have been ignored in this first exploratory study. The ICA algorithm reconstructs the spatial distribution of each component with rms errors of about 1 per cent for the CMB, and 10 per cent for the much weaker Galactic components. Radio sources are almost completely recovered down to a flux limit corresponding to ≃0.7 σ _CMB, where σ _CMB is the rms level of the CMB fluctuations. The signal recovered has equal quality on all scales larger than the pixel size. In addition, we show that for the strongest components (CMB and radio sources) the frequency scaling is recovered with per cent precision. Thus, algorithms of the type presented here appear to be very promising tools for component separation. On the other hand, we have been dealing here with a highly idealized situation. Work to include instrumental noise, the effect of different resolving powers at different frequencies and a more complete and realistic characterization of astrophysical foregrounds is in progress.     

Test of a differential photometer for extinction gradient correction in full-disk helioseismology

The ground based full disk velocity Doppler measurements used in helioseismology suffer from an atmospheric noise component when the sky transparency is not perfect. It is due to the non uniform integration of the line of sight component of the solar rotation produced by the differential atmospheric extinction across the direction of the solar equator. A simple two-channel differential photometer is proposed for measuring this differential extinction. The first laboratory tests of this instrument show that it has the capability of performing the required correction without adding a significant level of new instrumental noise contribution.