A versatile birefringent filter

The theory of the olc birefringent filter is complicated and the general formulae describing its optical properties do not invite to any simple physical picture. Due to these formal difficulties, many of its inherent possibilities have not been fully appreciated. In this paper we point out some new possibilities for filters based on this general design.The detailed shape of the transmission profile is a function of the angles of the crystal plate optic axes. It is shown how unwanted transmission sidelobes can be suppressed to any desired level by altering the distribution of plate angles. By the same means, the transmission band can be split into two symmetrically placed replicas, and the distance between the two bands can be varied. In this way the filter can easily be tuned over half the free spectral range.Some of the error sources that are important to filter performance are discussed. An expression is given for the amount of parasitic light introduced by errors in the orientation of the plate optic axes and it is found that errors as large as 0°.5 can be tolerated. A laboratory experiment with 16 birefringent plates has shown that accuracies an order of magnitude better than this figure can easily be achieved. Manufacturing errors in the thickness of the plates can be compensated for by assembling the pile of plates in a particular way. The very strict tolerances usually quoted can therefore be considerably relaxed.     

Universal birefringent filter with a new double passband mode

This article describes the universal birefringent filter (UBF) which will be mounted at Taiwan. The UBF permits observations of solar images, vector magnetic fields and/or line-of-sight velocity fields in any Fraunhofer line in the wavelength region from 4000 to 7000 with half width from 0.05 to 0.14 . We have tested it at ten Fraunhofer lines under a spectral telescope in Huairou Station of Beijing Astronomical Observatory and obtained the passband profiles. The principles of magnetic field and velocity field measurement with the UBF are also described.     

The intensity distribution of the D3 helium line near the solar limb

Cinematographic observations of the slitless flash spectrum near the D₃ helium line were performed in Yurgamysh at the total solar eclipse of September 22, 1968. The intensity distribution of the D₃ line was obtained with a height resolution of 44 km within the height interval between –3400 and +1700 km above the limb. The absorption line D₃ on the disk near the limb was discovered.     

High-resolution measurements of photosphere and sunspot velocity and magnetic fields using a narrow-band birefringent filter

Modifications to a Zeiss 1/4 Å filter are described which allow high spatial resolution observations of the line-of-sight velocities and magnetic fields in the photosphere and in sunspots. First results show: (1) the granular velocity field to be very strong; differences in upward motions in the granules and downward motions in between are as much as 6 km/sec; (2) the Evershed effect in sunspots to originate primarily in the dark regions between bright penumbral filaments.     

Transmission profile of the Dutch Open Telescope Hα Lyot filter

Accurate knowledge of the spectral transmission profile of a Lyot filter is important, in particular in comparing observations with simulated data. The paper summarizes available facts about the transmission profile of the Dutch Open Telescope (DOT) Hα Lyot filter pointing to a discrepancy between sidelobe‐free Gaussian‐like profile measured spectroscopically and signatures of possible leakage of parasitic continuum light in DOT Hα images. We compute wing‐to‐center intensity ratios resulting from convolutions of Gaussian and square of the sinc function with the Hα atlas profile and compare them with the ratios derived from observations of the quiet Sun chromosphere at disk center. We interpret discrepancies between the anticipated and observed ratios and the sharp limb visible in the DOT Hα image as an indication of possible leakage of parasitic continuum light. A method suggested here can be applied also to indirect testing of transmission profiles of other Lyot filters. We suggest two theoretical transmission profiles of the DOT Hα Lyot filter which should be considered as the best available approximations. Conclusive answer can only be given by spectroscopic re‐measurement of the filter. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The brightness of the helium D3 line in the undisturbed chromosphere from eclipse observations

Slitless spectra of the chromosphere, observed cinematographically at the total solar eclipse of 10 July 1972, were reduced. The surface brightness distribution of the helium D₃ line in the undisturbed chromosphere was obtained in agreement with results by other observers. The available eclipse data on the D₃ absolute brightness was analysed by means of theoretical curves of growth. Intensity data by some observers were found to be certainly too high. A trend was found that the D₃ absolute brightness in the quiet chromosphere decreases with the increasing solar activity (sunspot number). This perhaps indicates a variation of the spicule number over the solar surface during a sunspot cycle.     

Search for secular changes in the 3D profile of the synchrotron radiation around Jupiter

We present a summary of Jupiter data taken over an eighteen year span (1981-1998) by the Very Large Array at ∼21.0 cm. At this wavelength the emission is dominated by synchrotron radiation, which is roughly proportional to the product of the electron number density and magnetic field strength (N_eB). At each epoch 8-12 hours of data were taken, which allowed us to examine Jupiter during an entire rotation period. We mapped the longitudinal structure of the synchrotron radiation by using a 3D reconstruction technique developed by Sault et al. [Astron. Astrophys. 324 (1997) 1190] which enabled us to produce plots of the latitude, radial distance, and peak intensity vs. jovian longitude (System III). The results show the shape of the synchrotron radiation has remained stable (except, of course, during the period of comet Shoemaker-Levy 9 impacts). Specifically, the latitudinal structure has remained nearly constant. Furthermore, the general dependence of the radial intensity profile has remained the same throughout the years, though radial distance has slightly, though significantly, changed. This constancy implies that the spatial structure of both the particle distribution and magnetic field have varied little over the eighteen year span. The primary changes in the synchrotron radiation have been seen in the intensity of emission as a function of time. There are certain epochs (e.g., 1987) which show more emissivity than others (e.g., 1981, 1995) at all longitudes. When each epoch is longitudinally averaged, there may be an anti-correlation between the radial distance and corresponding peak intensities of the synchrotron radiation, as one might expect if radial diffusion is important. We examine these trends by comparing the data to plots of the total intensity at 13 cm (by Klein et al., in: Rucker, H.O., et al., Planetary Radio Emissions V. Austrian Acad. Sci. Press, Vienna, p. 221). Overall, variations in our 21-cm data are similar to those measured at 13 cm, but there appears to be a change in spectral index and perhaps in the spatial brightness distribution in 1992. We attribute this to a change in both the spatial and energy distribution of the relativistic electrons.     

A new tunable filter with a very narrow pass-band

A new tunable optical filter incorporated in a solar magnetograph consists of an interference filter followed by three automatically controlled Fabry-Pérot interferometers. It has a spectral resolving power of 10⁵, an angular resolution of 5″ arc and a field angle of 30′ arc for this spectral resolving power. By using auxiliary optics the angular resolution of the object may be improved at the expense of the angular field. Thus $$\frac{{angular field}}{{angular resolution}} = 6 are \min (are \sec )^{ - 1} $$ . It is operational over the range 400–650 nm and may be tuned through ± 0.2 nm about the selected transmission band.     

The determination of vector magnetic fields in prominences from the observations of the Stokes profiles in the D3 line of helium

A method is presented to measure the magnetic field vector in prominences by means of the polarimetric observations in the D₃ line of He obtained with the High Altitude Observatory Stokes polarimeter. The characteristics of the observed Stokes profiles are discussed. The theory of the Hanle effect is reformulated in the representation of the irreducible tensors of the density matrix, and is generalized to derive the circular polarization profiles across the spectral line in terms of the intensity and direction of the prominence magnetic field. The circular polarization profile so deduced can be employed to obtain useful information which adds to that carried by the linear polarization observations. A non-linear least-squares algorithm is proposed to derive the measurement of the magnetic field from the observations, and a consistency check is suggested to test the adequacy of the theoretical model to describe the physics of the He I atomic excitation in prominences.On leave from: Astrophysical Observatory of Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A high-contrast imaging polarimeter with a stepped-transmission filter based coronagraph

The light reflected from planets is polarized mainly due to Rayleigh scattering, but starlight is normally unpolarized. Thus it provides an approach to enhance the imaging contrast by inducing the imaging polarimetry technique. In this paper, we propose a high-contrast imaging polarimeter that is optimized for the direct imaging of exoplanets, combined with our recently developed stepped-transmission filter based coronagraph. Here we present the design and calibration method of the polarimetry system and the associated test of its high-contrast performance. In this polarimetry system, two liquid crystal variable retarders(LCVRs) act as a polarization modulator, which can extract the polarized signal. We show that our polarimeter can achieve a measurement accuracy of about 0.2% at a visible wavelength(632.8 nm)with linearly polarized light. Finally, the whole system demonstrates that a contrast of 10~(-9) at 5λ/D is achievable, which can be used for direct imaging of Jupiter-like planets with a space telescope.     

Spectroscopic investigation of a birefringent Lyot-filter for Hα

The instrumental profile of a Lyot-filter with 0.5 Å pass-band for H has been determined from photographic spectra obtained with a large grating spectrograph and a continuous light source.After fine adjustment of the filter, the transmission profile in parallel light agrees closely with the theoretical profile as given by crystal optics. The peak transmission has been found to be 12% of the incident unpolarized radiation.Convolving the measured instrumental profile with the average solar spectral distribution near H, it can be shown that the parasitic light (transmission outside the main pass-band) contributes 10% to the total light flux with the filter centered on the line. At wavelength shifts of 0.5 and 0.8Å, the parasitic light contribution is 25% and 40%, respectively.The deformation of the line-profile of various solar phenomena (faculae, filaments, flares) by the instrumental profile is discussed quantitatively.Mitteilungen aus dem Fraunhofer Institut Nr. 82.     

A statistical analysis of NaI D1 profile fluctuations at the center of the solar disk

Three radial-velocity fluctuation arrays V(Δλ, Y) and line-formation fluctuation arrays L(Δλ, Y),where Δλ is wavelength displacement from the center of Nai D₁ and Y is displacement on the Sun's surface along the spectrograph slit, were obtained from Sacramento Peak Observatory spectrograms. The variations of these line profile fluctuations are qualitatively described. The RMS_υ's, coherences, and power spectra shapes for V(Δλ, Y) fluctuations are examined at different Δλ with the corresponding effective heights of formation calculated with Mein weighting functions. Results include: (a) possible anticorrelation between continuum fluctuations and those near line center; (b) RMS _υ ^(cr) 's, which are root-mean-square values of the radial velocity corrected for instrumental and atmospheric blurring, are large (1.5 to 4.0 km s^?1) primarily due to large corrections for atmospheric blurring; (c) RMS _υ ^(cr) minima at effective heights of formation above 350 km suggest penetration of granulation velocities into the upper photosphere; (d) very rough determinations of RMS _υ ^(cr) 's, which are additionally corrected for line-of-sight averaging, range from around 5 km s^?1 in the low chromosphere to a sharp minimum ≤ 0.5 km s^?1 located in the upper photosphere; (e) power spectra shapes reflect decreasing average fluctuation scales above the temperature minimum (possibly high-frequency oscillations) and in the low and middle photosphere (possibly penetration of granulation); and (f) RMS _υ ^(cr) 's and average fluctuation scales suggest changes in the resolvable velocity field occurring near the temperature minimum.     

The evolution of a rapidly accreting helium white dwarf to become a low-luminosity helium star

We have examined the evolution of merged low-mass double white dwarfs which become low-luminosity (or high-gravity) extreme helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a helium white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. As the helium-burning shell moves inwards with repeating shell flashes, the effective temperature gradually increases as the star evolves towards the helium main sequence. When the mass interior to the helium‐burning shell is approximately 0.25 M_⊙, the star enters a regime where it is pulsationally unstable. We have obtained radial pulsation periods for these models.
These models have properties very similar to those of the pulsating helium star V652 Her. We have compared the rate of period change of the theoretical models with that observed in V652 Her, as well as with its position on the Hertzsprung–Russell diagram. We conclude that the merger between two helium white dwarfs can produce a star with properties remarkably similar to those observed in at least one extreme helium star, and is a viable model for their evolutionary origin. Such helium stars will evolve to become hot subdwarfs close to the helium main sequence. We also discuss the number of low-luminosity helium stars in the Galaxy expected for our evolution scenario.     

A polytropic model for the helium shell flash

A model consisting of two polytropes is constructed, to represent a helium core of a star during the helium shell flash occurring at the onset of helium burning in a degenerate core. The maximum temperature reached during the flash can be predicted as a function of core mass and mass inside the helium burning shell. This temperature will generally be too low for the production of neutrons out of¹⁴N. Some additional results on the helium shell flash in a star of 1.3M are also presented.Supported in part by the National Science Foundation [GP-9433] and the Office of Naval Research [Nonr-220(47)].On leave of absence from the Max Planck Institut für Physik und Astrophysik, München.     

Time-averaged observations of the sun with a 3840 Å filter

Observations of the Sun with a 3840 Å interference filter of 12 Å full-width at half-maximum are compared with broad band filtergrams at 5300 Å and 6103 Å magnetograms. The limit for detecting faculae by the three techniques is compared. An improvement in detecting faculae at 3840 Å can be obtained by time-averaged photographs with the time-averaging done either at the telescope or in the darkroom. The former method has certain disadvantages. The 3840 Å filter can show the presence of faculae near the limb much better than white-light photographs or magnetograms. This feature makes 3840 Å filtergrams useful in conjunction with solar oblateness studies.This research was supported by company funds of The Aerospace Corporation.     

First measurement of helium on Mars: implications for the problem of radiogenic gases on the terrestrial planets

108 +/- 11 photons of the martian He 584-angstroms airglow detected by the Extreme Ultraviolet Explorer satellite during a 2-day exposure (January 22-23, 1993) correspond to the effective disk average intensity of 43 +/- 10 Rayleigh. Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 +/- 20 K, result in a He mixing ratio of 1.1 +/- 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He+ by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4 x 10(5), 3 x 10(4), and 7 x 10(3) cm-2 sec-1, respectively. The lifetime of helium on Mars is 5 x 10(4) years. The He outgassing rate, coupled with the 40Ar atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which is applied to Mars and then to Venus. A similar model with known abundances of K, U, and Th is applied to Earth. The models for Earth and Mars presume loss of all argon accumulated in the atmospheres during the first billion years by large-scale meteorite and planetesimal impacts. The models show that the degassing coefficients for all three planets may be approximated by function delta = delta (0)(t(0)/t)1/2 with delta (0) = 0/1, 0.04, and 0.0125 Byr-1 for Earth, Venus, and Mars, respectively. After a R2 correction this means that outgassing processes on Venus and Mars are weaker than on Earth by factors of 3 and 30, respectively. Mass ratios of U and Th are almost the same for all three planets, while potassium is depleted by a factor of 2 in Venus and Mars. Mass ratios of helium and argon are close to 5 x 10(-9) and 2 x 10(-8) g/g in the interiors of all three planets. The implications of these results are discussed.     

一个土墩日珥的D3线光谱分析Ⅰ-D3线特征及其计算

198 4年 5月 5日太阳东边缘土墩日珥的Hα单色像和D3线被观测到了。D3发射线由两部分组成 :主成分和致宽成分。本文利用双层模型方法对该土墩日珥的 1 7条D3发射线成功地进行了计算 ,给出了D3线的计算结果。计算结果表明 :日珥的D3发射线主要由热的多普勒致宽和微观湍流致宽所致 ,其它致宽机制的作用可以忽略