Astrometric Reduction of Saturnian Satellites with Cassini-ISS Images Degraded by Trailed Stars

The Imaging Science Subsystem(ISS)mounted on the Cassini spacecraft has taken a lot of images,which provides an important source of high-precision astrometry of some planets and satellites.However,some of these images are degraded by trailed stars.Previously,these degraded images cannot be used for astrometry.In this paper,a new method is proposed to detect and compute the centers of these trailed stars automatically.The method is then performed on the astrometry of ISS images with trailed stars.Finally,we provided 658 astrometric positions between 2004 and 2017 of several satellites that include Enceladus,Dione,Tethys,Mimas and Rhea.Compared with the JPL ephemeris SAT427,the mean residuals of these measurements are 0.11 km and 0.26 km in R.A.and decl.,respectively.Their standard deviations are 1.08 km and 1.37 km,respectively.The results show that the proposed method performs astrometric measurements of Cassini ISS images with trailed stars effectively.

The Astrometric Performance Test of 80 cm Telescope at Yaoan Station and Precise CCD Positions of Apophis

The 80 cm azimuthal telescope has newly been mounted at Yaoan Station,Purple Mountain Observatory since2018.The astrometric performance of the telescope is tested in the following three aspects.(a)The geometric distortion of its CCD attached.It is stable in both a single epoch and multi epochs.Eight distortion solutions are derived over about one year.The maximum values range from 0.75 to 0.79 pixel and the median values range from 0.14 to 0.16 pixel.(b)The limit magnitude of stars.About 20.5 mag(Gaia-G)stars can be detected with Johnson-V filter exposured in 300 s.The astrometric error of about 20.5 mag stars is estimated at O".14 using the fitted sigmoidal function.(c)The astrometric accuracy and the precision of stacked fast-moving faint object.24 stacked frames of the potentially hazardous asteroid(99942)Apophis were derived on 2021 April 14 and 15(fainter than18 mag)based on the ephemeris shifts.During data reduction,the newest Gaia EDR3 Catalog and Jet Propulsion Laboratory Horizons ephemeris are referenced as theoretical positions of stars and Apophis,respectively.Our results show that the mean(O-C)s(observed minus computed)of Apophis are-O".018 and O".020 in R.A.and decl.,and the dispersions are estimated at O".094 and O".085,respectively,which show the consistency of the stacked results by Astrometrica.

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.

Matching CCD images to a stellar catalog using locality-sensitive hashing

The usage of a subset of observed stars in a CCD image to find their corresponding matched stars in a stellar catalog is an important issue in astronomical research. Subgraph isomorphic-based algorithms are the most widely used methods in star catalog matching. When more subgraph features are provided, the CCD images are recognized better. However, when the navigation feature database is large, the method requires more time to match the observing model. To solve this problem, this study investigates further and improves subgraph isomorphic matching algorithms. We present an algorithm based on a locality-sensitive hashing technique, which allocates quadrilateral models in the navigation feature database into different hash buckets and reduces the search range to the bucket in which the observed quadrilateral model is located. Experimental results indicate the effectivity of our method.

Automatic removal of false image stars in disk-resolved images of the Cassini Imaging Science Subsystem

Taking a large number of images,the Cassini Imaging Science Subsystem(ISS)has been routinely used in astrometry.In ISS images,disk-resolved objects often lead to false detection of stars that disturb the camera pointing correction.The aim of this study was to develop an automated processing method to remove the false image stars in disk-resolved objects in ISS images.The method included the following steps:extracting edges,segmenting boundary arcs,fitting circles and excluding false image stars.The proposed method was tested using 200 ISS images.Preliminary experimental results show that it can remove the false image stars in more than 95%of ISS images with disk-resolved objects in a fully automatic manner,i.e.,outperforming the traditional circle detection based on Circular Hough Transform(CHT)by 17%.In addition,its speed is more than twice as fast as that of the CHT method.It is also more robust(no manual parameter tuning is needed)when compared with CHT.The proposed method was also applied to a set of ISS images of Rhea to eliminate the mismatch in pointing correction in automatic procedure.Experiment results showed that the precision of final astrometry results can be improve by roughly 2 times that of automatic procedure without the method.It proved that the proposed method is helpful in the astrometry of ISS images in a fully automatic manner.

Preliminary results of CCD observations targeting Himalia acquired at Yunnan Observatories in 2015

In order to study the potential associated with high precision CCD astrometry of irregular satellites, we have acquired experimental observations of Himalia, the sixth and irregular satellite of Jupiter.A total of 185 CCD observations were obtained by using the 2.4 m and 1 m telescopes administered by Yunnan Observatories over ten nights. Preliminary analysis of the observations were made, including geometric distortion, atmospheric refraction, and also the phase effect. All positions of Himalia are measured relative to the reference stars from the catalog UCAC4 in each CCD field of view. The theoretical positions of Himalia were retrieved from the Institute de M e′chanique Ce′leste et de Calcul des E′ph e′m e′rides, while the positions of Jupiter were obtained based on the planetary ephemeris INPOP13 c. The results show that the means of observed minus computed(O- C) residuals are-0.004′′and-0.002′′in right ascension and declination, respectively. The standard deviations of(O- C) residuals are estimated to be about 0.04′′in each direction.

An improved solution to geometric distortion using an orthogonal method

The geometric distortion of a CCD field of view has a direct influence on the positional measurements of CCD observations. In order to obtain high precision astrometric results, the geometric distortion should be derived and corrected precisely. As presented in our previous work, a convenient solution has been carried out and has also been applied to observations of Phoebe. In order to further improve the solution, an orthogonal method based on Zernike polynomials is used in this work.Four nights of CCD observations including Himalia, the sixth satellite of Jupiter, and open clusters(NGC 1664 or NGC 2324) on each night have been processed as an application. The observations were obtained from the 2.4 m telescope administered by Yunnan Observatories. The catalog UCAC4 was used to match reference stars in all of the CCD frames. The ephemeris of Himalia is retrieved from the Institut de M e′canique Ce′leste et de Calcul des E′ph e′m e′rides(IMCCE). Our results show that the means of observed minus calculated(O- C) positional residuals are-0.034 and-0.026 arcsec in right ascension and declination, respectively. The corresponding standard deviations are 0.031′′and 0.028′′.The measurement dispersion is significantly improved compared to that by using our previous solution.

Contour detection in Cassini ISS images based on Hierarchical Extreme Learning Machine and Dense Conditional Random Field

In Cassini ISS(Imaging Science Subsystem)images,contour detection is often performed on disk-resolved objects to accurately locate their center.Thus,contour detection is a key problem.Traditional edge detection methods,such as Canny and Roberts,often extract the contour with too much interior details and noise.Although the deep convolutional neural network has been applied successfully in many image tasks,such as classification and object detection,it needs more time and computer resources.In this paper,a contour detection algorithm based on H-ELM(Hierarchical Extreme Learning Machine)and Dense CRF(Dense Conditional Random Field)is proposed for Cassini ISS images.The experimental results show that this algorithm’s performance is better than both traditional machine learning methods,such as Support Vector Machine,Extreme Learning Machine and even deep Convolutional Neural Network.The extracted contour is closer to the actual contour.Moreover,it can be trained and tested quickly on the general configuration of PC,and thus can be applied to contour detection for Cassini ISS images.