Dense Star Fields Research Articles

CSST Dense Star Field Preparation: A Framework for Astrometry and Photometry for Dense Star Field Images Obtained by the China Space Station Telescope (CSST)

The China Space Station Telescope (CSST) is a telescope with 2 m diameter, obtaining images with high quality through wide-field observations. In its first observation cycle, to capture time-domain observation data, the CSST is proposed to observe the Galactic halo across different epochs. These data have significant potential for the study of properties of stars and exoplanets. However, the density of stars in the Galactic center is high, and it is a well-known challenge to perform astrometry and photometry in such a dense star field. This paper presents a deep learning-based framework designed to process dense star field images obtained by the CSST, which includes photometry, astrometry, and classifications of targets according to their light curve periods. With simulated CSST observation data, we demonstrate that this deep learning framework achieves photometry accuracy of 2% and astrometry accuracy of 0.03 pixel for stars with moderate brightness mag = 24 (i band), surpassing results obtained by traditional methods. Additionally, the deep learning based light curve classification algorithm could pick up celestial targets whose magnitude variations are 1.7 times larger than magnitude variations brought by Poisson photon noise. We anticipate that our framework could be effectively used to process dense star field images obtained by the CSST.

Research on Guide Star Distribution of Sub-Arcsecond Attitude Determination for Microsatellites Reusing Scientific Cameras

Onboard scientific cameras are reused in attitude determination to meet the sub-arcsecond attitude determination accuracy requirements of microsatellites. This approach does not require an additional payload for microsatellites. It involves reusing high-quality optical lenses from the scientific camera and utilizing the peripheral high-quality imaging areas of its square-shaped detector. Separate detectors are placed within these areas as attitude determination detectors to obtain star patterns for closed-loop attitude determination, thereby achieving high-precision attitude determination for microsatellites. The star patterns obtained using this method may pose specific issues due to the relative positions of stars. Through an analysis of the theoretical model that examines the relationship between attitude determination accuracy and the main influencing factors, it is indicated that guide star distribution is one of the main, complex factors determining attitude determination accuracy. A further simulation analysis was conducted on the specific impact of two guide star distribution characteristics—namely, the coverage of guide stars in the attitude determination areas and the proportion of the average field of view occupied by the guide star triangles to the total field of view of the attitude determination areas—on attitude determination accuracy. This study concludes that when the measurement error of the guide stars is bigger than the attitude determination accuracy requirement for its area configuration, four attitude determination areas should be configured. Four attitude determination areas should be prioritized when the measurement error is equal to or smaller than the attitude determination accuracy requirement, followed by the option to configure three attitude determination areas or two symmetric attitude determination areas. When selecting guide stars for star pattern recognition, the guide stars should cover the attitude determination areas as much as possible, and guide stars with a higher proportion of the average field of view occupied by the guide star triangles to the total field of view should be chosen. Finally, experimental validation was conducted using star patterns from dense star fields and sparse star fields. The research results provide an important reference for the optimization of attitude determination area configuration, navigation star catalog construction, and star pattern recognition algorithm research for microsatellites equipped with scientific cameras.

Reliable stellar abundances of individual stars with the MUSE integral-field spectrograph

ABSTRACT We present a novel approach to deriving stellar labels for stars observed in MUSE fields making use of data-driven machine learning methods. Taking advantage of the comparable spectral properties (resolution and wavelength coverage) of the LAMOST and MUSE instruments, we adopt the data-driven Payne (DD-Payne) model used on LAMOST observations and apply it to stars observed in MUSE fields. Remarkably, in spite of instrumental differences, according to the cross-validation of 27 LAMOST-MUSE common stars, we are able to determine stellar labels with precision better than 75K in Teff, 0.15 dex in log g, and 0.1 dex in abundances of [Fe/H], [Mg/Fe], [Si/Fe], [Ti/Fe], [C/Fe], [Ni/Fe], and [Cr/Fe] for current MUSE observations over a parameter range of 3800 < Teff < 7000 K, −1.5 < [Fe/H] < 0.5 dex. To date, MUSE has been used to target 13 000 fields across the southern sky since it was first commissioned 6 yr ago and it is unique in its ability to study dense star fields such as globular clusters or the Milky Way bulge. Our method will enable the automated determination of stellar parameters for all stars in these fields. Additionally, it opens the door for applications to data collected by other spectrographs having resolution similar to LAMOST. With the upcoming BlueMUSE and MAVIS, we will gain access to a whole new range of chemical abundances with higher precision, especially critical s-process elements, such as [Y/Fe] and [Ba/Fe], that provide key age diagnostics for stellar targets.

Simulation on the Measurement Method of Geometric Distortion of Telescopes

The accurate measurement on the effect of telescope geometric distortion is conducive to improving the astrometric positioning accuracy of telescopes, which is of significant importance for many disciplines of astronomy, such as stellar clusters, natural satellites, asteroids, comets, and other celestial bodies in the solar system. For this reason, the predecessors have developed an iterative self-calibration method to measure the telescope geometric distortion by dithering observations in a dense star field, and achieved fine results. However, the previous work did not make constraints on the density of star field, and the dithering mode, but chose empirically some good conditions (for example, a denser star field and a larger dithering number) to observe, which took up much observing time, and caused a rather low efficiency. In order to explore the validity of the self-calibration method, and optimize its observational conditions, it is necessary to carry out the corresponding simulations. In this paper, we introduce first the self-calibration method in detail, then by the simulation method, we verify the effectiveness of the self-calibration method, and make further optimizations on the observational conditions, such as the density of star field and the dithering number, to achieve a higher accuracy of geometric distortion measurement. Finally, taking consideration of the practical application for correcting the geometric distortion effect, we have analyzed the relationship between the number of reference stars in the field of view and the astrometric accuracy by virtue of the simulation method.

Toward Understanding the Anisotropic Point Spread Function of Suprime-Cam and Its Impact on Cosmic Shear Measurement

We examined the anisotropic point spread function (PSF) of Suprime-Cam data utilizing dense star field data. We decomposed the PSF ellipticities into three components—the optical aberration, atmospheric turbulence, and chip misalignment in an empirical manner—and evaluated the amplitude of each component. We then tested a standard method for correcting the PSF ellipticities used in weak lensing analysis against a mock simulation. We found that, for long-exposure data, the optical aberration has the largest contribution to the PSF ellipticities, which could be modeled well by a simple analytic function based on the lowest-order aberration theory. The statistical properties of PSF ellipticities resulting from atmospheric turbulence were investigated by using numerical simulations. The simulation results are in a reasonable agreement with the observed data. It follows from these findings that the spatial variation of PSF ellipticities consists of two components: one is a smooth and parametrizable component arising from the optical PSF, and the other is a non-smooth and stochastic component resulting from the atmospheric PSF. The former can be well corrected by the standard correction method with a polynomial fitting function. However, for the latter, its correction is affected by the common limitation caused by sparse sampling of PSFs due to a limited number of stars. We also examined the effects of the residual PSF anisotropies on Suprime-Cam cosmic shear data (5.6-degree2 of i′-band data). We found that the shape and amplitude of the B-mode shear variance are broadly consistent with those of the residual PSF ellipticities measured from the dense star field data. This indicates that most of the sources of residual systematic are understood, which is an important step for cosmic shear statistics to be a practical tool of the precision cosmology.

Constraining the Adaptive Optics Point‐Spread Function in Crowded Fields: Measuring Photometric Aperture Corrections

The point-spread function (PSF) of an adaptive optics (AO) system is often poorly known. This ignorance can lead to significant systematic errors in photometry. Since the degree of AO correction is sensitive to the observing conditions: seeing, wind speed, brightness of the wavefront reference, etc., it would be desirable to estimate the PSF from the data themselves. We have developed a method to estimate the PSF delivered by an AO system in the case where the scene consists of a crowded star field. We model the modulation transfer function (MTF) of several key components of the imaging system. The power spectrum of the image, even a dense star field, can then be used to constrain our model, which in turn can be used to reconstruct the PSF. We show that the method yields reasonable fit parameters and a useful approximation to the PSF when applied to data from laser guide star (LGS) AO system at the Keck Observatory. Comparison of Keck LGS/AO and HST/NICMOS data shows that photometric accuracy of a few percent can be achieved for data with Strehl ratios as low as 4%.

Matched Filter Processing for Asteroid Detection

Matched filter (MF) processing has been shown to provide significant performance gains when processing stellar imagery used for asteroid detection, recovery, and tracking. This includes extending detection ranges to fainter magnitudes at the noise limit of the imagery and operating in dense cluttered star fields as encountered at low Galactic latitudes. The MF software has been shown to detect 40% more asteroids in high-quality Spacewatch imagery relative to the currently implemented approaches, which are based on moving target indicator (MTI) algorithms. In addition, MF detections were made in dense star fields and in situations in which the asteroid was collocated with a star in an image frame, cases in which the MTI algorithms failed. Thus, using legacy sensors and optics, improved detection sensitivity is achievable by simply upgrading the image-processing stream. This in turn permits surveys of the near-Earth asteroid (NEA) population farther from opposition, for smaller sizes, and in directions previously inaccessible to current NEA search programs. A software package has been developed and made available on the NASA data services Web site that can be used for asteroid detection and recovery operations utilizing the enhanced performance capabilities of MF processing.

Photometry of neglected open clusters in the first and fourth Galactic quadrants

CCD BVI photometry is presented for eight previously unstudied star clusters located in the first and fourth Galactic quadrants: AL 1, BH 150, NGC 5764, Lynga 9, Czernik 37, BH 261, Berkeley 80 and King 25. Colour‐magnitude diagrams of the cluster regions suggest that several of them (BH 150, Lynga 9, Czernik 37, BH 261 and King 25) are so embedded in the dense stellar population toward the galactic centre that their properties, or even their existence as physical systems, cannot be confirmed. Lynga 9, BH 261 and King 25 appear to be slight enhancements of dense star fields, BH 150 is probably just a single bright star in a dense field and Czernik 37 may be a sparse, but real cluster superimposed on the galactic bulge population. We derive preliminary estimates of the physical parameters for the remaining clusters. AL 1 appears to be an intermediate-age cluster beyond the solar circle on the far side of the galaxy and the final two clusters, NGC 5764 and Berkeley 80, are also of intermediate age but located inside the solar ring. This set of clusters highlights the difficulties inherent in studying the stellar populations toward the inner regions of the galaxy.

Characterizing the Adaptive Optics Off‐Axis Point‐Spread Function. II. Methods for Use in Laser Guide Star Observations11Based on observations obtained at the Lick Observatory, which is operated by the University of California.

ABSTRACTMost current astronomical adaptive optics (AO) systems rely on the availability of a bright star to measure the distortion of the incoming wave front. Replacing the guide star with an artificial laser beacon alleviates this dependency on bright stars and therefore increases sky coverage, but it does not eliminate another serious problem for AO observations. This is the issue of PSF variation with time and field position near the guide star. In fact, because a natural guide star is still necessary for correction of the low‐order phase error, characterization of laser guide star (LGS) AO PSF spatial variation is more complicated than for a natural guide star alone. We discuss six methods for characterizing LGS AO PSF variation that can potentially improve the determination of the PSF away from the laser spot; that is, off‐axis. Calibration images of dense star fields are used to determine the change in PSF variation with field position. This is augmented by AO system telemetry and simple computer simulations to determine a more accurate off‐axis PSF. We report on tests of the methods using the laser AO system on the Lick Observatory Shane Telescope. We observed with offsets typical of separations between dim science targets and the nearest suitably bright tip‐tilt guide star, up to 20″. If the tip‐tilt guide star is used as the PSF reference, the predicted Strehl ratio within an 8″ radius of the LGS can be more than a factor of 2 too high, as it does not take into account the field‐dependent anisoplanatism. A better result may be obtained by an improved empirical approach: repositioning the tip‐tilt guide star to compensate for the additional 8″ offset. This would result in a 27% relative error in Strehl ratio. A simple Gaussian model of PSF variation can reduce the error in the prediction to ∼20%. Some further improvement may be obtained semianalytically by using telemetry from the AO system (14% error) plus a more sophisticated theoretical model of PSF variation (13% error). It should be noted, however, that all but the first method achieve comparable accuracy in predicting PSF FWHM. This is important because the last method, unlike the others, is not directly applicable to 10 m apertures without further computational complexity.

Interacting supergiant shells in the Large Magellanic Cloud

We seek evidence for secondary star formation in the Large Magellanic Cloud (LMC) arising in gas compressed by the interactions between H supergiant shells (SGS) identified from the Australia Telescope Compact Array (ATCA) survey of Kim et al., using mid-infrared (MIR) images from the Midcourse Space Experiment (MSX), and radio continuum images from the Molonglo Observatory Synthesis Telescope (MOST). We identify a dense photodissociation region shaped like a bar that lies between the rims of LMC 4 and LMC 5 as a strong candidate for this process, and argue that it has produced the young associations LH 52 and 53 in the dense starfield of NGC 1948 within the past few Myr. No obvious stellar candidate matches the coincident 8.3-, 21.3-μm, and 843-MHz peaks in this bar, although this bright spot lies between two nebulous Ha patches. At least one other such H i-MIR bar exists in the LMC so that this mode of star formation is unlikely to have been a unique occurrence. We have found a correlation between 8.3-μm radiance and H I column density for the LMC in general that can be predicted from the Schmidt law for spiral galaxies.

Characterizing the Adaptive Optics Off‐Axis Point‐Spread Function. I. A Semiempirical Method for Use in Natural Guide Star Observations1,1Based, in part, on observations obtained at the Lick Observatory, which is operated by the University of California. 22All authors except D. C. are affiliated with the Center for Adaptive Optics.

ABSTRACTEven though the technology of adaptive optics (AO) is rapidly maturing, calibration of the resulting images remains a major challenge. The AO point‐spread function (PSF) changes quickly in both time and position on the sky. In a typical observation, the star used for guiding will be separated from the scientific target by 10″–30″. This is sufficient separation to render images of the guide star by themselves nearly useless in characterizing the PSF at the off‐axis target position. A semiempirical technique is described that improves the determination of the AO off‐axis PSF. The method uses calibration images of dense star fields to determine the change in PSF with field position. It then uses this information to correct contemporaneous images of the guide star to produce a PSF that is more accurate for both the target position and the time of a scientific observation. We report on tests of the method using natural guide star AO systems on the Canada‐France‐Hawaii Telescope and Lick Observatory Shane Telescope, augmented by simple atmospheric computer simulations. At 25″ off‐axis, predicting the PSF FWHM using only information about the guide star results in an error of 60%. Using an image of a dense star field lowers this error to 33%, and our method, which also folds in information about the on‐axis PSF, further decreases the error to 19%.

Correcting for Blending Problems in Gravitational Microlensing Events by UsingHubbleSpaceTelescope

The biggest uncertainty in determining microlensing parameters comes from the blending of source star images because the current experiments are being carried out toward very dense star fields: the Galactic bulge and Magellanic Clouds. The experiments try to correct the blending effects for individual events by introducing an additional lensing parameter, the residual flux, but this method suffers from very large uncertainties in the derived lensing parameters due to the degeneracies among the parameters. In this paper, I propose to use the Hubble Space Telescope (HST) to correct blending effects. With the high resolving power of the HST combined with the color information from ground-based observations, one can uniquely identify the lensed source star in the blended seeing disk, and thus the uncertainty in the derived time scale can be significantly reduced.

Astronomical Objective Gratings

This article deals with the use of objective gratings in contemporary astronomy. It is shown that their use in photometry may help considerably toward reducing the photoelectric effort in calibrating the magnitudes of faint stars. Their usefulness in dense star fields is also explained. The article contains a discussion of the precautions that have to be taken to ensure first-class results. The use of objective gratings in astrometry for the elimination of magnitude equation is explained. The effects of atmospheric dispersion in work with objective gratings are discussed.

Star Chains and Dark Filaments in Galactic Nebulae

When investigating sufficiently dense star fields, one often finds some short chains of stars oriented with a certain regularity and in a few cases connected with dark filaments of the galactic nebulae. It is necessary to be very cautious in the interpretation of this phenomenon, because even completely accidental clustering of some objects can sometimes lead to interesting deviations from homogeneity.

Variable stars in Variable Star Field 193.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Variable stars in Variable Star Field 193. Hoffleit, Dorrit Abstract Magnitudes On 30 or more plates of the Harvard MF series, taken with the io-inch Metcalf telescope, have been estimated for 450 unpublished variable stars in VSF 193 at 18h24m, -23?3 in Sagittarius. Periods have been determined for 65 of the long-period variables, or about half the total number of this type I discovered in the region. Their distances, based on the Wilson- Merrill period-luminosity relation, range from one to 13 kpc. Only those with periods shorter than 250 days have been found at distances beyond the galactic center. Selection effects are discussed which hinder the discovery of the stars of longer period either at great distances or in dense star-fields. Cambridge, Mass. Publication: The Astronomical Journal Pub Date: 1957 DOI: 10.1086/107646 Bibcode: 1957AJ.....62S..18H full text sources ADS |