Yunnan Observatory Research Articles

A CONVENIENT SOLUTION TO GEOMETRIC DISTORTION AND ITS APPLICATION TO PHOEBE'S OBSERVATIONS

A simple but effective approach is proposed for measuring the geometric distortion of a CCD field of view of a ground-based telescope. For three open clusters (M35, M67, and NGC 2324), 425 CCD frames taken by a 1 m telescope at the Yunnan Observatory are used to test this approach. It is found that the geometric distortion pattern depends strongly on the corresponding filter used. The geometric distortion is then used to correct the pixel positions for Phoebe, the ninth satellite of Saturn, and its reference stars imaged in 220 CCD frames taken by the same telescope. The standard deviation of the (O - C; observed minus computed) residuals of Phoebe is significantly improved after correcting the geometric distortions.

New Period Investigation of Near Contact Binary BF Virginis

Abstract The near contact binary BF Virginis was monitored by us from 2007 to 2011 with the 60-cm telescope of Yunnan Observatory. Eight new observed times of light minimum were derived. All of the available eclipse times, spanning more than 100 yr, were analyzed. It is found that there exists a cyclic variation with a semi-amplitude of 0.034 d and a period of 63.3 yr, superposed by an upward parabolic variation. The upward parabolic change can be explained as a continuous period increase due to mass transfer from the secondary to the primary component of BF Vir, which supports the secondary filling near contact configuration. The period oscillation can be explained either by the light-time effect due to the presence of a third body or by magnetic activity. If it is caused by the light-time effect, the minimum mass and the corresponding orbital radius of the additional body should be $ m_{3}$$ =$ 0.93 $ M_{\odot}$ and $ a_{3}$$ =$ 19.4 AU, respectively. On the other hand, the period oscillation may be explained as a consequence of magnetic cycles in its secondary component. In this case, the required variation of the quadrupole moment is calculated to be $ \Delta{Q}$$ =$ 2.1 $ \times$ 10$ ^{50}$ g cm$ ^2$ .

Preliminary results of solving the problem of geometric distortion for the 2.4 m telescope at Yunnan Observatory

We observed the open clusters NGC 1664 (43 exposures) and M35 (42 exposures) by the Yunnan Faint Object Spectrograph and Camera in the 2.4 m telescope at Yunnan Astronomical Observatory on 2011 January 3, and processed them by a method recently proposed by us. The result shows that there is a geometric distortion effect in the field of view and the maximum distortion is ∼ 0.25″ (i.e. 1 pixel). After correcting the geometric distortion, the precision of stellar positional measurement is significantly improved. The best precision in each direction is 6 mas for well-exposed stars.

The New Photometric Observations for Transiting Exoplanet HAT-P-24b

AbstractThe transiting exoplanetary system HAT-P-24 was observed by using CCD cameras at Yunnan Observatory and Hokoon Astronomical Centre, China in 2010 and 2012. Three new transit light curves are analyzed by means of MCMC technique, and the new physical parameters of the system are derived, which are compatible with the old ones in the discovery paper. The orbital period of HAT-P-24b is refined and no obvious TTV signal can be found from five transit events during 2010-2012.

Precise CCD positions of Galilean satellite-pairs

In this paper, we present 526 precise CCD positions of Galilean satellite-pairs of Jupiter, which have been extracted from 441 CCD frames captured by a 1-m telescope at the Yunnan Observatory from 2002 to 2010. The four Galilean satellites (Io, Europa, Ganymede and Callisto) are used to calibrate the CCD field of view by comparing their pixel positions with their theoretical positions computed from two modern ephemerides of the Galilean satellites, L2 and JUP230, which have been developed by the Institut de Mechanique Celeste et de Calcul des Ephemerides and the Jet Propulsion Laboratory, respectively. In this paper, we focus on the relative position of a pair of satellites with short separation (less than 85 arcsec) for good internal precision. The mean (O − C) (observed minus computed) values of all these satellite-pairs in right ascension and declination are found to be no larger than 6 mas and 2 mas, respectively, for each ephemeris. The estimated precision for one single observation is better than 30 mas in each direction.

Mutual eclipses of J2 Europa by J1 Io observed at Yunnan Observatory in 2009

Mutual events between natural satellites include mutual occultation and mutual eclipse. Mutual eclipse is another kind of mutual occultation as viewed from the center of the Sun instead of the Earth. Two mutual eclipses of J2 Europa by J1 Io (2009 Aug. 28 and Sept. 12) were observed at Yunnan Observatory during the PHEMU09 international campaign. We will calculate the astrometric data of these Galilean satellites by analyzing and fitting the light curves we obtained. The limb-darkening was considered during modeling the light intensity of eclipsed satellites in the penumbra zone, by taking the Lommel-Seeliger scattering law into account. Several dynamical quantities, such as the relative coordinates of the eclipsing satellite from the eclipsed one Δαcosδ and Δδ, impact parameter and mid-time corresponding to the impact parameter and the deviations O — C of observed Δαcosδ and Δδ relative to ephemerides, were obtained for each event respectively.

Preliminary results of solving geometric distortions for a CCD image

It is crucial to solve accurately the geometric distortion of a field of view for high precision astrometry.A convenient approach is proposed in this paper using the reference for the solution of equivalent to the Hubble Space Telescope(HST).Based on the 72 CCD exposures taken for the open cluster M67(NGC2682) on April 2-3,2011 by the 1-m telescope at the Yunnan Observatory,obvious geometric distortions are found for the telescope with maximum distortion ～0.6 pixel(～120 mas) at the corner of field of view.After correcting the distortion,the measurement precision of stellar positions has significantly improved.

Mutual occultations between Galilean satellites observed at Yunnan Observatory in 2009

Context. Photometry of mutual events between natural satellites has been shown to be a most effective and accurate ground-based method for obtaining accurate astrometric data by fitting the light curves observed during these events, and is invaluable for developing the orbital models of the natural satellites. Aims. Mutual occultations of J2 Europa by J1 Io (Aug. 28, 2009), J1 Io by J2 Europa (Nov. 9 and Dec. 11, 2009), and J1 Io by J3 Ganymede (Nov. 28, 2009) were observed at Yunnan Observatory during the PHEMU09 international campaign. We calculate the astrometric data of satellites by analyzing and fitting the light curves that we derived. Methods. The model proposed by Emelianov (2003, Sol. Syst. Res., 37, 314) was used to fit the light curves, while taking the Lommel-Seeliger scattering law into account. Results. For each event, we derive dynamical quantities such as the deviation of the observed relative satellite motion from the theoretical motion provided by the relevant ephemeris denoted as D(x) and D(y), and the impact parameter and its corresponding mid-time. These results have an accuracy of between about several mas and 90 mas for D(x) and D(y), and 0.31-2.97 s for the mid-time. The residuals of the longitude shifts (delta l(1) for J1 Io and delta l(2) for J2 Europa) that represent the shifts of the mid-times of the events, are also calculated to be around 10 km with the most accurate available ephemerides.

Preliminary Results of Atmospheric Extinction Coefficient Measurements at Gaomeigu Observational Station

The measurement of the atmospheric extinction coefficients is one of the important and fundamental works for astronomical observatories. By using the newly-built 2.4 m telescope at Gaomeigu Observational Station of Yunnan Observatory, the calibration works, such as the measurement of the atmospheric extinction coefficient, are carried out and their preliminary results are presented. By observing the Landolt standard stellar fields during April and May 2009, the atmospheric extinction coefficients of five bands UBVRI at two nights are given. In addition, the components which contribute to the atmospheric extinction coefficients of this site are discussed. The measured extinction coefficients are found to be basically consistent with the previous results obtained in the site selection some years ago. And they are also compared with the values of some other well-known observational stations worldwide.

The observation of exoplanet transit events in China

We have carried out a research project on the exoplanet transit events at Yunnan Observatory. By using CCD cameras attached to 1m telescope of Yunnan Observatory and 85cm telescope of Xinglong station, NAOC, a group of exoplanet systems with transit events have been observed photometrically. By means of MCMC method, the preliminary results of the systems WASP-11 and XO-2 are derived. Finally, we give out the future plan on this research topic in China.

Accurate determination of the plate model based on CCD images with different orientations

The images of the open cluster NGC2168 are taken using the newly-fitted 2k×2k CCD attached to the 1-m telescope at the Yunnan Observatory with two different orientations. After the catalogue position of a star from UCAC2 is compared with its corresponding measured position, a surprising result shows that a small field of view (about 7’×7’) exhibits an obvious distortion. Specifically, the polynomial between the standard coordinate (ξ,η) and its corresponding measured one (x,y) for a star in each field of view has significant terms as high as 4th order. Accordingly, the positional measurement accuracy of a bright UCAC2 star can be derived as well as ˜7 mas in each direction (60 s of exposure time and near the zenith). This new finding would be important for a CCD image to deliver a highly accurate position in its whole field of view.

Determination of the Instantaneous Values of Astronomical Atmospheric Refraction and Local Observational Modeling

A new application of astronomical atmospheric refraction in space geodesy is utilized. It is pointed out that in order to meet the high needs of this new application there must be an effective method by means of which the instantaneous value of atmospheric refraction can be directly determined. An atmospheric refraction model fitting in the geographical environment surrounding the observing station is established and then transformed into the neutral atmospheric refraction delay correction model. In this article the necessary conditions for the determination of the value of atmospheric refraction are briefly described. A method for the direct determination of the values of instantaneous atmospheric refraction in various directions and at various zenith distances by taking advantage of the observational principle of the low latitude meridian circle, explored by the Yunnan Observatory, is expounded and the atmospheric refraction observational models built on the basis of stellar spectral type classification in the 4 directions of east, south, west and north and by making use of the observed data are given.

Adaptive Optics Image Restoration Based on Frame Selection and Multi-frame Blind Deconvolution

Restricted by the observational condition and the hardware, adaptive optics can only make a partial correction of the optical images blurred by atmospheric turbulence. A postprocessing method based on frame selection and multi-frame blind deconvolution is proposed for the restoration of high-resolution adaptive optics images. By frame selection we mean we first make a selection of the degraded (blurred) images for participation in the iterative blind deconvolution calculation, with no need of any a priori knowledge, and with only a positivity constraint. This method has been applied to the restoration of some stellar images observed by the 61-element adaptive optics system installed on the Yunnan Observatory 1.2m telescope. The experimental results indicate that this method can effectively compensate for the residual errors of the adaptive optics system on the image, and the restored image can reach the diffraction-limited quality.

Occurrence of bidirectional type III bursts in solar flares

The 13 pairs of type III bursts with the bidirectional drift structures recorded with the spectrograph in the frequency ranges of 230-300 MHz and 625-1500 MHz at the Yunnan Observatory and 2600-3800 MHz at the Beijing National Astronomical Observatories are analyzed in this present article and the outstanding characteristics of these events are obtained. These bursts respectively reveal that the separatrix frequency between the bursts with positive and negative drifts comes between 250 MHz and 3420 MHz, with a gap being between 0.6 MHz and 110 MHz; the duration is 53 ms-1880 ms and the frequency drift rate is between 45 MHz/s and 56000 MHz/s. The drift rate at metric wavelengths is relatively low, only a few decades of MHz while it is comparatively high at microwave wavelengths, reaching 56000 MHz/s. The qualitative explanation of these events is given in this paper.

CCD POSITIONS OF SATURN AND ITS MAJOR SATELLITES FROM 2002-2006

This paper presents 2154 precise positions of Saturn and its major satellites from 359 CCD exposures taken with the 1 m telescope at the Yunnan Observatory during the years 2002-2006. It also describes the improved image-processing techniques for the pixel positional measurement of Saturn's rings and its major satellites, especially for Mimas and Enceladus. The four bright satellites S3-S6 (i.e., Tethys, Dione, Rhea, and Titan) of Saturn are used to calibrate the CCD field of view by comparing their pixel positions with their theoretical ones from the theory TASS1.7. The observational positions of these major satellites, when measured with respect to Rhea, usually have a good agreement with their theoretical ones except for Mimas, which has the biggest systematic difference of about -0.3 arcsec in R.A. in its 2002 observational data sets. However, these differences of Mimas become much smaller when the recent Jet Propulsion Laboratory ephemeris is replaced. The rms errors in each coordinate are about 40 mas for Saturn and its bright satellites S2-S6, and 90 mas for Mimas. These positional observations are comparable to the best ground-based CCD observations.

New evidence of precision premium for Galilean satellites from CCD imaging

After a CCD image of the four Galilean satellites of Jupiter is obtained by a long focal length telescope, we can compare the theoretical positions of these satellites with their pixel positions so as to obtain the calibration parameters of the CCD field of view. In theory, when two of the four satellites have small enough separation, their relative positional measurement will have a good accuracy since the error existing in the solved calibration parameters has a direct proportional effect on the separation of the two satellites. The 347 CCD images taken by 1-m long focal length telescope at Yunnan Observatory in 2002–2005 are used to perform the experimental test. After we improve the centroid algorithm for the satellites and our former halo-removal technique, the results show that the positional measurement of two small-separation satellites has an external precision as good as 0.01–0.03 arcsec. This precision has comparability as that from rarely occurring mutual events of the Galilean satellites. This experiment confirms the finding of the “precision premium” firstly presented by Pascu [1994. An appraisal of the USNO program of photographic astrometry of bright planetary satellites. In: Morrison, L.V., Gilmore, G.F., (Eds.), Galatic and Solar System Optical Astrometry, pp. 304–311] using photographic observations. We believe that this type of observations, besides mutual event observations, might also be used to improve our knowledge of the orbital motions of the Galilean satellites because of its much more opportunities.

Correlation analysis between [formula omitted] and SFT/MTK vector magnetograms

Three components of the vector magnetic field, flux density B, inclination γ and azimuth χ of the active region NOAA10507 are derived from the two-dimensional Stokes spectral data obtained by the S 3 T at the Yunnan Observatory. The distributions of the longitudinal magnetic field and the transverse magnetic field are contoured on the basis of the three components. The distributions indicate that the active region is a very complicated sunspot group which is mainly composed of five sunspots, including one of negative polarity and four positive ones. Comparing the vector magnetograms obtained by S 3 T and the SFT/MTK, it is found that there is basic agreement on the longitudinal fields of S 3 T and SFT/MTK magnetograms with a correlation coefficient ρ Bz = 0.842 , and the two distributions of transverse magnetic field obtained by S 3 T and SFT/MTK have correlation coefficients, ρ Bt = 0.423 and ρ azimuth = 0.72 .

Constructing an Observational Model of the Neutral Atmospheric Refraction Delay from Measured Values of the Astronomical Refraction

Discussing the requirements for an accurate correction of the neutral atmospheric refraction delay for the Global Positioning System, our paper points out that the existing theoretical correction models and the old postprocessing methods cannot attain the anticipated accuracy because the atmospheric refraction index has not been directly determined precisely. It is necessary to adopt atmospheric refraction delay models to improve the correction accuracy. The models are dependent on the observing station and the observation azimuth. By the new method we propose in this paper, the observational data of astronomical refractions at different azimuths and different zenith distances are grouped according to azimuth and used to calculate the refractivity and mapping functions. As an example of the newmethod, we introduce the configuration of the Lower Latitude Meridian Circle and the measurement method of instantaneous astronomical refractions, and present the observational astronomical refraction model toward the east, south, west, and north of the Yunnan Observatory. We estimate that, without the use of any atmospheric distribution models, the correction accuracy of the zenith delay will be within 1 mm, the correction accuracy of the refraction delay at the lower elevation will be improved to a centimeter magnitude, and the cutoff elevation angle will be reduced to 5 degrees or less.

Photometric observation of the transiting exoplanet WASP-1b

AbstractThe extra-solar planet WASP-1b, discovered by SuperWASP consortium in 2006, was observed through R filter using the 1-meter telescope with CCD camera at Yunnan Observatory on November 11, 2006. The systematic errors in photometric data were reduced by means of Tamuz et al. (2005) and Collier Cameron et al. (2006)'s algorithms. In order to estimate the parameters of the system, the MCMC (Markov Chain Monte Carlo) analysis is applied to fit the observed light curve. The following parameters of the system are derived: Rp=1.44RJ, Mp=0.88MJ, R*=1.52R⊙, a=0.0396AU. The new parameters of the planet imply its low density, which agrees to the previous results.

Photometric follow-up observation of some SuperWASP transiting planet candidates

AbstractThree SuperWASP transiting planet candidates were observed through R or I filters using the 1-meter telescope and CCD camera of Yunnan Observatory from 2006 to 2007. The relative photometric data were corrected for the systematic errors by means of Tamuz et al. (2005) and Collier Cameron et al. (2006)'s algorithms. The resulting light curves demonstrate that one of three targets is a potential exoplanet candidate, which is worthy to perform further follow-up observation to clarify.