Color Refraction Research Articles

Astrometric Observations of NEA 1998 HH49 Using the Daocheng 50 cm Telescope

This study details an astrometric observation campaign of the Near-Earth Asteroid 1998 HH49, conducted with the aim of refining our understanding of its physical characteristics. Utilizing the 50 cm telescope located at the Wumingshan Mountain in Daocheng, Sichuan, images were obtained over four nights, from 2023 October 19 to October 22. These observations were processed using Astrometrica software, facilitating the precise determination of the asteroid’s position. The observational results were compared with the ephemerides from three distinct sources to verify accuracy: the Jet Propulsion Laboratory (JPL) Horizons System, the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) Miriade, and the Near-Earth Objects Dynamic Site (NEODyS-2). When compared with the JPL ephemeris, a mean observed-minus-calculated (O-C) result of 0.″07 in the R.A. direction and −0.″35 in the decl. direction was yielded. Furthermore, the comparison with the IMCCE ephemeris yielded mean O-C results of 0.″08 in the R.A. direction and −0.″06 in the decl. direction. The comparison with the NEODyS-2 ephemeris yielded the mean O-C results of 0.″06 in R.A. and −0.″49 in decl. direction. The study’s findings demonstrate a general consistency between the observed data and the ephemeris predictions, with minor discrepancies observed across the data sets. Notably, both the JPL and NEODyS-2 ephemerides show that the residuals in the decl. direction exceed those in the R.A. direction. The disparities may result from atmospheric differential color refraction, ephemeris discrepancies, observational errors, and other factors. Additionally, it is worth noting that further investigation is required due to the potential influence of additional factors. Overall, the Daocheng 50 cm Telescope exhibits the ability to conduct high-precision positional measurements.

TRANSLIENT: Detecting Transients Resulting from Point-source Motion or Astrometric Errors

Detection of moving sources over a complicated background is important for several reasons. First is measuring the astrophysical motion of the source. Second is that such motion resulting from atmospheric scintillation, color refraction, or astrophysical reasons is a major source of false alarms for image-subtraction methods. We extend the Zackay, Ofek, and Gal-Yam image-subtraction formalism to deal with moving sources. The new method, named the translient (translational transient) detector, applies hypothesis testing between the hypothesis that the source is stationary and that the source is moving. It can be used to detect source motion or to distinguish between stellar variability and motion. For moving source detection, we show the superiority of translient over the proper image subtraction, using the improvement in the receiver-operating characteristic curve. We show that in the small translation limit, translient is an optimal detector of point-source motion in any direction. Furthermore, it is numerically stable, fast to calculate, and presented in a closed form. Efficient transient detection requires both the proper image-subtraction statistics and the translient statistics: When the translient statistic is higher, then the subtraction residual is likely due to motion. We test our algorithm both on simulated data and on real images obtained by the Large Array Survey Telescope. We demonstrate the ability of translient to distinguish between motion and variability, which has the potential to reduce the number of false alarms in transients detection. We provide the translient implementation in Python and MATLAB.

Speckle Observations of Binary Stars with a 0.5 m Telescope

ABSTRACTWe present 36 observations of 17 visual binaries of moderate separation (ranging from 0.″15 to 0.″790) made with the 50 cm Cassegrain telescope of the Jagiellonian University in Kraków. The speckle interferometry technique was combined with modest optical hardware and a standard photometric CCD camera. We used broadband VRI filters without a Risley prism to reduce differential color refraction. Thus, we performed a model analysis to investigate the influence of this effect on the results of the measurements. For binary components of spectral type O–F, the difference of three spectral classes between them should bias their relative positions by no more than a couple of tens of milliarcseconds (mas) for moderate zenith distances. The statistical analysis of our results confirmed this conclusion. A cross‐spectrum approach was applied to resolve the quadrant ambiguity. Our separations have rms deviations of 0.″012, and our position angles have rms deviations of 1.°8. Relative photometry in V, R, and I filters appeared to be the less accurately determined parameter. We discuss our errors in detail and compare them to other speckle data. This comparison clearly shows the quite good accuracy of our measurements. We also present an example of the enhancement of image resolution for an extended object with an angular size that is greater than the atmospheric coherence patch, using speckle interferometry techniques.

Astrometry of International Celestial Reference Frame Sources Using the Second US Naval Observatory CCD Astrograph Catalog

We present results of a pilot investigation on the astrometry of International Celestial Reference Frame (ICRF) sources using small- to medium-sized telescopes and the second US Naval Observatory CCD Astrograph Catalog (UCAC2). For this purpose, 31 ICRF sources were observed, mostly south of the equator, during 1997–2000. We used the automated 0.6 and 1.6 m Cassegrain telescopes equipped with CCD detectors located at Laboratorio Nacional de Astrofisica, Brazil. The source positions were referred to UCAC2, with fainter 0.6 m telescope stars serving as a reference frame to the reductions of the 1.6 m telescope CCD fields. Observations were made in the V band in a compromise between the 579–643 nm bandpass (between V and R) of UCAC2 and the bluer ICRF sources. To ensure that UCAC2, with its magnitude bandpass system, is a reliable reference catalog for our V-band CCD frame reductions, we have also compared it against an independent set of star positions with similar characteristics obtained in the V band with the Valinhos CCD Meridian Circle, Brazil. Average values and errors for the optical - radio position offsets using the 0.6 m telescope were +4 ± 8 mas (41 mas) and +1 ± 8 mas (42 mas) for right ascension and declination, respectively. (Parenthetical values refer to standard deviation, i.e., to the typical error of a single measurement given the quantity of sources.) For the 1.6 m telescope, offsets were -12 ± 9 mas (45 mas) and +8 ± 9 mas (46 mas). An expected random error that increases with magnitude and affects the positions of the fainter 0.6 m telescope secondary stars is verified. No systematic errors were found within the attained position precision, including differential color refraction. External comparisons with independent telescope/catalog sets of precise source positions were also made, showing consistent results within the respective errors.

The Solar Neighborhood. XIII. Parallax Results from the CTIOPI 0.9 Meter Program: Stars with μ ≥ 1."0 yr-1(MOTION Sample)

We present the first set of definitive trigonometric parallaxes and proper motions from the Cerro Tololo Inter-American Observatory Parallax Investigation. Full astrometric reductions for the program are discussed, including methods of reference star selection, differential color refraction corrections, and conversion of relative to absolute parallax. Using data acquired at the 0.9 m telescope at CTIO, full astrometric solutions and VRIJHKs photometry are presented for 36 red and white dwarf stellar systems with proper motions faster than 10 yr-1. Of these, 33 systems have their first ever trigonometric parallaxes, which comprise 41% of MOTION systems (those reported to have proper motions greater than 10 yr-1) south of δ = 0° that have no parallaxes. Four of the systems are new members of the RECONS 10 pc sample for which the first accurate trigonometric parallaxes are published here: DENIS J1048-3956 (4.04 ± 0.03 pc), GJ 1128 (LHS 271, 6.53 ± 0.10 pc), GJ 1068 (LHS 22, 6.97 ± 0.09 pc), and GJ 1123 (LHS 263, 9.02 ± 0.16 pc). In addition, two red subdwarf–white dwarf pairs, LHS 193AB and LHS 300AB, are identified. The white dwarf secondaries fall in a previously uncharted region of the H-R diagram.

WIYN Open Cluster Study. XVII. Astrometry and Membership toV=21 in NGC 188

We present techniques for obtaining precision astrometry using old photographic plates from assorted large-aperture reflectors in combination with recent CCD Mosaic Imager frames. At the core of this approach is a transformation of plate/CCD coordinates into a previously constructed astrometric reference frame around the open cluster NGC 188. This allows us to calibrate independently the optical field angle distortion for all telescopes and field correctors used in this study. Particular attention is paid to computing the differential color refraction, which has a marked effect in the case of NGC 188 as a result of the large zenith distances at which this cluster has been observed. Our primary result is a new catalog of proper motions and positions for 7812 objects down to V = 21 in the 0.75 deg2 area around NGC 188. The precision for well-measured stars is 0.15 mas yr-1 for proper motions and 2 mas for positions on the system of the Tycho-2 catalog. In total, 1490 stars have proper-motion membership probabilities Pμ ≥ 10%. The sum of membership probabilities indicates that NGC 188 contains ~1050 stars down to V = 21. Comprehensive lists of the candidate blue stragglers and red giant stars substantially enlarge the number of such stars known in NGC 188. We have also obtained a small correction to the proper motions from the mean of background galaxies. Thus, the absolute proper motion of NGC 188 is μ = -2.56 ± 0.2 and μ = +0.18 ± 0.2 mas yr-1.

Upgrades to the Flagstaff Astrometric Scanning Transit Telescope: A Fully Automated Telescope for Astrometry

The Flagstaff Astrometric Scanning Transit Telescope (FASTT) is a fully automated telescope that takes about 41,000 CCD frames of data a year for various research projects. All aspects of the telescope's operation have been automated (e.g., target selection, observing, reduction of data, and collation of results), and manpower needs are now under one person per year, mostly involved with routine maintenance and the dissemination of data. This paper describes the FASTT instrumental system, methods used with its automated operation, and the various FASTT research projects. Among the projects, astrometry is provided in support of various spacecraft missions, to predict occultation events, calculate dynamical masses for selective asteroids, and improve the ephemerides for thousands of asteroids, the planets Jupiter to Pluto, and 17 satellites of Jupiter through Neptune. Although most of the FASTT observing program involves the solar system, FASTT stellar astrometry was used to set up a number of astrometric calibration regions along the celestial equator, verify the Hipparcos link to the International Celestial Reference Frame, determine accurate positions for a large sample of radio stars, and investigate systematic errors in the FK5 star catalog. Furthermore, the FASTT produces accurate magnitudes that are being used to investigate the shapes of thousands of asteroids. By the end of year 2003, the FASTT will have produced over 190,000 positions of solar system objects in a program to provide a very large and homogeneous database for each object that will extend over many years and include positions accurate to ±47 to ±300 mas, depending on the magnitude of each observed object (3.5 < V < 17.5). Moreover, extensive efforts have been undertaken to improve the systematic accuracy of FASTT equatorial positions by applying corrections in the reductions for differential color refraction, distortions in the focal plane, and correcting for a positional error that is dependent on magnitude. The systematic accuracy of FASTT observations is now about ±20 mas in both right ascension and declination. FASTT data have contributed very significantly to recent successful spacecraft missions and to a dramatic improvement in the predictions made for occultation events.

Infrared Parallaxes for Methane T Dwarfs

We report final results from our 2.5 yr infrared parallax program carried out with the European Southern Observatory 3.5 m New Technology Telescope and the SOFI infrared camera. Our program targeted precision astrometric observations of 10 T-type brown dwarfs in the J band. Full astrometric solutions (including trigonometric parallaxes) for nine T dwarfs are provided along with proper-motion solutions for a further object. We find that HgCdTe-based infrared cameras are capable of delivering precision differential astrometry. For T dwarfs, infrared observations are to be greatly preferred over the optical, both because they are so much brighter in the infrared, and because their prominent methane absorptions lead to similar effective wavelengths through the J filter for both target and reference stars, which in turn results in a dramatic reduction in differential color refraction effects. We describe a technique for robust bias estimation and linearity correction with the SOFI camera, along with an upper limit to the astrometric distortion of the SOFI optical train. Color-magnitude and spectral type?magnitude diagrams for both L and T dwarfs are presented that show complex and significant structure, with major import for luminosity function and mass function work on T dwarfs. Based on the width of the early L dwarf and late T dwarf color magnitude diagrams, we conclude the brightening of early T dwarfs in the J passband (the early T hump) is not an age effect, but due to the complexity of brown dwarf cooling curves. Finally, empirical estimates of the turn on magnitudes for methane absorption in field T dwarfs and in young stars clusters are provided. These make the interpretation of the T6 dwarf ? Ori J053810.1-023626 as a ? Ori member problematic.

Calculated atmospheric color refraction and observed stellar positions

AbstractAstrometric observations at different zenith distances have been performed in Dresden in an area centered atNGC 6791 where there are some stars with reliable color information (widely dispersed spectral types in the MK systemand color indices BT – VT) as well as with accurate positions from Tycho‐2 catalog. The results are used to estimate how significant improvements in stellar positions may be when accurate corrections for color refraction are taken into account. We have treated two cases for refraction calculations: (1) a photometric case for color indices and (2) a spectral case for spectral types and luminosity classes. To calculate refraction we use Stone's modified computer code (Malyuto &amp; Meinel 2000). To treat the photometric case we have calculated the synthetic color indices for the spectral energy distributions of Sviderskiene (1988). The positional improvements due to including color refraction corrections are significant and slightly larger in the spectral case. An improvement of about 15% is reached at a zenith distance of 65°. Our basic conclusion is that color refraction should be taken into account for obtaining accurate stellar positions from ground based observations at larger zenith distances. Reliable refraction corrections may be calculated from spectral and/or photometric data.

A New Method for Computing Differential Color Refraction

ABSTRACTA new method for computing differential color refraction (DCR) is presented in this paper that uses observations taken through a conventional filter as well as through a narrowband interference filter that serves as a template for the reductions. The method is tested with observations taken through Johnson BV and Cousins RI filters, and, in general, there is good agreement (excellent in all but one case) between observed and predicted values of DCR. A full description of the method is given as well as a discussion of the refraction model used to make the predictions. As expected, DCR is strongest in the B passband and weakest in the I band because of the wavelength dependence of atmospheric refraction. Furthermore, the common practice of calibrating DCR against a stellar color is investigated, finding that the procedure is justified, albeit the intrinsic scatter in the calibration can vary widely with the choice for the color. This scatter can be minimized with a judicious choice for the color system, and examples are given for the BVRI passbands considered in the paper. Nonetheless, there will always be some remaining error caused by the calibrating process, which will affect mostly star positions determined at large zenith distances. Further gains in positional accuracy are possible by choosing a passband for which the DCR is very small. Two such passbands are the Cousins I band (FWHM = 1900 Å) and the passband (FWHM = 630 Å) chosen for the USNO CCD Astrograph Catalog project. The ranges in the DCR for these passbands are, respectively, only 19 and 12 mas at a zenith distance of ZD = 60°. When calibrated against stellar color, remaining errors in the DCR calibrations for both of these passbands are expected to be under 5 mas. In the final section of the paper, numerical results are given for other possible passbands with small ranges (≤20 mas) in DCR.

U.S. Naval Observatory CCD parallaxes of faint stars. I - Program description and first results

The U.S. Naval Observatory CCD trigonometric parallax program is described in detail, including the instrumentation employed, observing procedures followed, and reduction procedures applied. Astrometric results are presented for 72 stars ranging in apparent brightness from V = 15.16 to 19.58. Photometry (V and V−I on the Kron-Cousins system) is presented for the parallax stars and for all 426 individual reference stars employed in the astrometric solutions. Corrections for differential color refraction, calibrated to the observed V−I colors, have been applied to all astrometric measures