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.     

Scattered light - a comparison between theory and experiments during the 1973 transit of Mercury

We check the formalism used to derive stray light corrections from measured aureole intensities and correct an error in the pertinent literature. We solve the alledged problem of appropriately normalizing the spread function by treating blurring and scattering separately. We test the method by comparing stray light corrections derived from both the aureole and from intensity profiles across Mercury's disc obtained during the transit of November 10, 1973.     

Measurement of Aureole and Suppression of Internal Stray Light of Aureole Photometer

A modern aureole photometer (AP) was developed for the site survey inWest China, in preparation for the installation of future large solar equipments. The performance of this new AP was tested in preliminary observations, and a lot of sky brightness data were accumulated at a few sites in Yunnan Province. The result of data analysis shows that the aureole near the noon time on Jiaozi Snow Mountain is as low as a few millionths of the intensity at the solar disk center, indicating the low internal stray light level of our instrument. The internal stray light of the AP comes mainly from two parts: the edge diffraction of the ferrule for fixing the ND4 filter in the front end of the telescope tube, causing the stray light distributed in the inner region of the field of view, and the edge diffractions of the diaphragms placed inside the telescope tube, causing the stray light distributed in the outer region of the field of view. In order to suppress the stray light of the latter part, the experiment to change the aperture size of an additional diaphragm was performed. The result shows that the stray light in the outer region of the field of view can be effectively suppressed by reducing properly the aperture size of the diaphragm.     

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.     

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.     

Stray-Light Effect on Magnetograph Observations

To examine the stray-light effect in magnetograph observations, we have determined the point spread functions of the vector magnetograph mounted on the Japanese Solar Flare Telescope based on two indirect methods, one analyzing the solar limb intensity profile, and the other using the Fourier power spectra of photospheric intensity distributions. Point spread functions consist of two parts: a blurring part which describes seeing and small-spread-angle stray light, and a scattering part which describes large-spread-angle stray light. The FWHM spatial resolution is typically 3.0, and the amount of scattered light is about 15% on clear days. We find that the blurring part is well described by a Moffat function whose Fourier transform is given by an exponential function. Our results indicate that polarization measurements of low-intensity magnetic elements like sunspots may be significantly underestimated due to the large-spread-angle stray light, and polarization measurements of magnetic elements which are smaller than 5–7 appear to be disturbed by small-spread-angle stray light.     

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.     

Initial CHIP He I Observations of Solar Limb Activity

A new instrument capable of 3-min time resolution full-disk and limb observations in the Hei 1083 nm spectral line has been in operation at the High Altitude Observatory's Mauna Loa Solar Observatory (MLSO) since April 1996. We discuss instrument capabilities and performance and present some initial observations of limb activity from the first year of instrument operation. We compare limb Hei and Hα observations of quiescent and active prominences, comment on the role of Doppler shifts in interpreting the Hei observations, and illustrate the use of disk/limb Hei observations of a CME-associated eruptive filament in mass-ejection studies.     

Zonal Differences in Jupiter's Atmosphere from Continuum CCD Photometry of Its Disk

Based on our 1997 observations with a CCD camera and narrow-band filters attached to the 1-m Assy Obsev vatory telescope, we extensively study the latitudinal variations in limb darkening and normal albedo on Jupiter's disk at wavelengths of 387, 445, 502, 619, and 702 nm. In addition, we carried out observations in 1998 with broad-band red, green, and blue filters. Apart from a general regularity—the increase in darkening coefficient with normal albedo of Jupiter's cloud cover—there is an appreciable scatter of darkening coefficients for the same albedo, which is most pronounced in the ultraviolet. This scatter may result from differences in the optical depth of the aerosol haze above the clouds. The lack of any wavelength dependence of the darkening coefficients is confirmed for Jupiter's polar regions, while at other latitudes, they decrease with decreasing wavelength.     

The role of telescopic stray light in limb-darkening scans obtained in April 1981 (and later)

The suspicion of Elste (1990), that telescopic stray light together with imperfect collimation of telescope and spectrograph could be a possible explanation for the systematic differences and variations found by Neckel and Labs (1987) in many limb-darkening scans, proves to be unfounded for the following reasons: (1)The collimation was performed very precisely; (2) the telescope mirrors remained fixed in position and direction during most of the observing period; (3) stray light effects depending on hour angle were not detectable; (4) in the same collimation status, also many almost symmetric scans had been recorded; (5) the observed east-west differences in the solar intensities are partly even larger than the total amount of stray light (from telescope and sky!) observed as sky-background just outside the limb; (6) any east-west differences in the sky-background near the limb are just a few 0.01% of the disk center intensity; (7) the differences of the average intensities along eastern and western radius appear to be correlated with the east-west differences of the intensity's R.M.S.     

Probing the Fundamental Physics of the Solar Corona with Lunar Solar Occultation Observations

Imaging and spectroscopy of the solar corona, coupled with polarimetry, are the only tools available at present to capture signatures of physical processes responsible for coronal heating and solar wind acceleration within the first few solar radii above the solar limb. With the recent advent of improved detector technology and image processing techniques, broad-band white light and narrow-band multi-wavelength observations of coronal forbidden lines, made during total solar eclipses, have started to yield new views about the thermodynamic and magnetic properties of coronal structures. This paper outlines these unique capabilities, which until present, have been feasible primarily with observations during natural total solar eclipses. This work also draws attention to the exciting possibility of greatly increasing the frequency and duration of solar eclipse observations with Moon orbiting observatories utilizing lunar limb occultation of the solar disk for coronal measurements.     

Observations of extrasolar planet transits with the automated telescopes of the Pulkovo Astronomical Observatory

Exoplanet observations have been performed on the automated Pulkovo Observatory telescopes. We have obtained 33 transit light curves for 16 known exoplanets and six transit observations for three exoplanet candidates discovered by the Kepler telescope. Based on our observations, we have reliably confirmed the existence of an exoplanet with an extremely large radius, R _pl = 1.83 ± 0.16R _Jup, in the system KOI 256 and detected a strong deviation of its orbital revolution from the theoretically predicted one. During the transit of the exoplanet WASP-12b across the stellar disk, we detected bursts that could be caused by the planet transit across spots on the star or by the presence of a satellite around this exoplanet. We detected possible periodic variations in the duration of the exoplanet transit across the stellar disk with time for HAT-P-12b that could be caused by variations in orbital inclination. The transit duration and depth, the central transit time, and the radius and orbital inclination of the planet have been estimated. The equilibrium temperature and albedo have been estimated for several exoplanets.     

The effect of scattered light on solar intensity observations as derived from 9 May, 1970 Mercury transit

The method for correcting observed sunspot intensities for scattered light has been tested using the transit of Mercury. It is found that the correction method of Zwaan (1965), Staveland (1970) has an uncertainty (rms value) of 0.05 times the photospheric intensity. During good observing conditions the uncertainty is 0.02 (rms value) with the scanning speed used in this study. A simpler and quicker correction method is suggested.Observations during Mercury's occultation of a sunspot as well as measurements close to the solar limb are briefly described.     

抚仙湖一米红外太阳望远镜Hα窄带滤光器扫描轮廓的检测与修正

抚仙湖1 m红外太阳望远镜的重要终端之一是多通道高分辨成像系统,主要由两路宽带和一路窄带成像系统组成。目前窄带成像系统的工作谱线为Hα。主要介绍了窄带成像系统扫描轮廓的检测和修正。主要检测内容包括扫描轮廓的中心波长位置、扫描轮廓对称性、前置滤光片对扫描轮廓的影响、滤光器工作温度稳定性等问题。检测结果显示:扫描轮廓在656.281-0.15 nm到656.281+0.4 nm的范围内与理论轮廓较好地吻合,而在656.281-0.15 nm到656.281-0.4 nm的范围内明显衰减。同时轮廓中心波长位置(即强度最低点的波长位置)相对于滤光器显示的"0 nm"偏带点蓝移了0.013 nm。针对上述检测结果,将滤光器的工作温度提高了约0.3℃。在温度调整之后,扫描轮廓的整体特征不变,轮廓中心波长位置与"0 nm"偏带点偏差小于0.004 nm,同时红蓝翼对称偏带点的强度差异小于10%(对应1.8 km/s的多普勒速度测量误差)。目前可以明确,扫描轮廓的蓝翼衰减是由前置滤光片造成,对于常用工作范围(656.281±0.1 nm),可以忽略前置滤光片的影响。滤光器工作温度比较稳定,1个月内温度变化幅度的标准方差约0.001 7℃。目前,该滤光器仍存在的问题是扫描轮廓在"0 nm"偏带点略有突起,幅度在6%~8%。建议在以后的使用过程中,需要定期定量地对滤光器的扫描轮廓以及前置滤光片的透过率曲线进行检测。     

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.     

Observations of Solar EUV Radiation with the CORONAS-F/SPIRIT and SOHO/EIT Instruments

The SPIRIT complex onboard the CORONAS-F satellite has routinely imaged the Sun in the 171, 175, 195, 284, and 304 Å spectral bands since August 2001. The complex incorporates two telescopes. The Ritchey-Chretien telescope operates in the 171, 195, 284, and 304 Å bands and has an objective similar to that of the SOHO/EIT instrument. The Herschel telescope obtains solar images synchronously in the 175 and 304 Å bands with two multilayer-coated parabolic mirrors. The SPIRIT program includes synoptic observations, studies of the dynamics of various structures on the solar disk and in the corona up to 5 solar radii, and coordinated observations with other spaceborne and ground-based telescopes. In particular, in the period 2002–2003, synoptic observations with the SPIRIT Ritchey-Chretien telescope were coordinated with regular 6-hour SOHO/EIT observations. Since June 2003, when EIT data were temporarily absent (SOHO keyholes), the SPIRIT telescope has performed synoptic observations at a wavelength of 175 A. These data were used by the Solar Influence Data Analysis Center (SIDC) at the Royal Observatory of Belgium for an early space weather forecast. We analyze the photometric and spectral parameters of the SPIRIT and EIT instruments and compare the integrated (over the solar disk) EUV fluxes using solar images obtained with these instruments during the CORONAS-F flight from August 2001 through December 2003.     

Near-Limb Zeeman and Hanle Diagnostics

“Weak” magnetic-field diagnostics in faint objects near the bright solar disk are discussed in terms of the level of non-object signatures, in particular, of the stray light in telescopes. Calculated dependencies of the stray light caused by diffraction at the 0.5-, 1.6-, and 4-meter entrance aperture are presented. The requirements for micro-roughness of refractive and reflective primary optics are compared. Several methods for reducing the stray light (the Lyot coronagraphic technique, multiple stages of apodizing in the focal and exit pupil planes, apodizing in the entrance aperture plane with a special mask), and reducing the random and systematic errors are noted. An acceptable level of stray light in telescopes is estimated for the V-profile recording with a signal-to-noise ratio greater than three. Prospects for the limb chromosphere magnetic measurements are indicated.     

Spatial variations of the sodium/potassium ratio in Mercury’s exosphere uncovered by high-resolution spectroscopy

High-resolution spectroscopy of Mercury has been obtained with two different instruments in 2006: the EMMI instrument at the 3.6-m NTT telescope of ESO La Silla Chile and the ESPADON spectrograph at the 3.6-m CFHT telescope on top of Mauna Kea (Hawaii). The disk of the planet has been scanned for spatial variation of the exospheric species. The large spectral range and high resolution allow simultaneous measurements of the integrated column density of Na and K.We measure Na/K ratio between 80 and 400 with values between 60 and 90 when the telescope was pointed towards the subsolar region of Mercury’s disk and much larger value when we looked to other part of the exosphere. Moreover, we observed that the Na and K exospheres display very different spatial distributions. Even if these two species are probably ejected with very similar mechanisms from the surface, their differences in mass and sensitivity to solar pressure acceleration imply very different behavior in Mercury’s exosphere.     

Model-independent restoration of the brightness distribution across the disks of stars from high precision eclipsing light curves

Estimates are made of the accuracy with which the brightness distributions across the disks of stars can be reconstructed through analysis of data from high precision space-based photometry of classical eclipsing systems and observations of the transit of planets across stellar disks. The ill-posed reconstruction problem was solved on a compact set of monotonically nonincreasing, upwardly convex, non-negative functions. One of the difficulties with this method in the case of stars with thin photospheres is the poor convergence of the solution at the point where the brightness distribution has a discontinuity at the edge of the star's disk. Nevertheless, the use of this method for analysis of high precision observational data is justified, since it can be used to obtain an estimate of the limb darkening that is independent of any model assumptions. The reconstructed brightness distribution for the star HD 209458, for which the transit of a planet over its disk was observed with the HST space telescope, is in good agreement with the results of a nonlinear model fit. Translated from Astrofizika, Vol. 51, No. 4, pp. 595–606 (November 2008).     

Sunspot intensity observations during the 9 May 1970 Mercury transit

The intensity of a sunspot was measured in eight wavelength regions during the Mercury transit of 9 May 1970. The observations have been corrected for scattered light in the Earth's atmosphere as well as in the instrument using two different methods plus a combination of these. One method consists of using Mercury as a calibration spot. In the second method the corrections for scattered light are determined from solar limb observations.