Differential Atmospheric Refraction Research Articles

Trigonometric Parallaxes of Two T Dwarfs With Keck and ShaneAO Astrometry

We present trigonometric parallax and proper motion measurements for two T-type brown dwarfs. We derive our measurements from infrared laser guide star adaptive optics observations spanning five years from the ShaneAO/SHARCS and NIRC2/medium-cam instruments on the Shane and Keck telescopes, respectively. To improve our astrometric precision, we measure and apply a distortion correction to our fields for both instruments. We also transform the Keck and ShaneAO astrometric reference frames onto the ICRS using five-parameter parallax and proper motion solutions for background reference stars from Gaia DR2. Fitting for parallax and proper motion, we measure parallaxes of 73.5 ± 9.2 mas and 70.1 ± 6.7 mas for WISEJ19010703+47181688 (WISE1901) and WISEJ21543294+59421370 (WISE2154), respectively. We utilize Monte Carlo methods to estimate the error in our sparse field methods, taking into account overfitting and differential atmospheric refraction. Comparing to previous measurements in the literature, all of our parallax and proper motion values fall within 2σ of the published measurements, and 4 of 6 measurements are within 1σ. These data are among the first parallax measurements of these T dwarfs and serve as precise measurements for calibrating stellar formation models. These two objects are the first results of an ongoing survey of T dwarfs with Keck/NIRC2 and the Shane Adaptive Optics system at Lick Observatory.

Improvements to Pan-STARRS1 Astrometry Using Gaia

Abstract We use the Gaia DR2 catalog to improve the astrometric accuracy of about 1.7 billion objects in Pan-STARRS1 Data Release 2 (PS1 DR2). We also obtain proper motions for these PS1 objects. The cross-match between Gaia and PS1 reveals residuals that are correlated on a scale of about 1′. We apply a spatially adaptive correction algorithm for all PS1 objects having more than two detections to reduce these residuals and align the object positions to Gaia. For point-like PS1 objects that cross-match to Gaia, the algorithm reduces PS1/Gaia residuals by 33% in position (median value of 13.5 mas reduced to 9.0 mas) and by 24% in proper motion (median value of 6.3 mas yr−1 reduced to 4.8 mas yr−1). The residuals for the corrected positions are smallest for objects with the most point-like morphologies and with intermediate magnitudes of about 17 mag. The residual errors in decl. are systematically larger than those in R.A.; the decl. errors increase with zenith angle in proportion to the air mass of the observations. The decl. positional residuals at a given decl. generally vary with color and are consistent with the effects of differential atmospheric refraction. In principle, these residuals could be reduced further by taking into account object color.

Ground-based Spectroscopy of the Exoplanet XO-2b Using a Systematic Wavelength Calibration

Abstract Exoplanets orbiting close to their host star are expected to support a large ionosphere, which extends to larger pressures than witnessed in our solar system. These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals, which are the source of the ions. However, most ground-based transit spectra do not systematically resolve the wings of the features and continuum, as needed to constrain the alkali abundances. Here we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2b. To assess the consistency of our results, observations were obtained from two separate platforms: Gemini/GMOS and Mayall/KOSMOS. To mitigate the systematic errors, we chose XO-2, because it has a binary companion of the same brightness and stellar type, which provides an ideal reference star to model Earth’s atmospheric effects. We find that interpretation of the data is highly sensitive to time-varying translations along the detector, which change according to wavelength and differ between the target and reference star. It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction. This approach allows us to resolve the wings of the Na line across five wavelength bins at a resolution of ∼1.6 nm and limit the abundance of Na. We obtain consistent results from each telescope with an Na amplitude of 521 ± 161 and 403 ± 186 ppm for GMOS and KOSMOS, respectively. The results are analyzed with a radiative transfer model that includes the effects of ionization. The data are consistent with a clear atmosphere between ∼1 and 100 mbar that establishes a lower limit on Na at ppm ([Na/H] = ), consistent with solar. However, we cannot rule out the presence of clouds at ∼10 mbar that allow for higher Na abundances, which would be consistent with the stellar metallicity measured for the host star ([Na/H] = 0.485 ± 0.043).

A treatment procedure for GMOS/IFU data cubes: application to NGC 2835

We present a set of treatment techniques for GMOS/IFU data cubes, including: correction of the differential atmospheric refraction; Butterworth spatial filtering, to remove high spatial-frequency noise; instrumental fingerprint removal; Richardson-Lucy deconvolution, to improve the spatial resolution of the observations. A comparison with HST images shows that the treatment techniques preserve the morphology of the spatial structures in the data cubes, without introducing distortions or ghost features. The treatment procedure applied to the data cube of the central region of the late-type galaxy NGC 2835 allowed the detection of a nuclear emitting region with emission-line ratios indicating a probable LINER, although the classifications of Transition Object or Seyfert galaxy cannot be discarded. Such a detection was not possible without the data treatment. This shows that the benefits provided by this treatment procedure are not merely cosmetic, but actually affect the quality of different types of analyses, like the determination of the fraction of galaxies, of different morphological types, with a central Seyfert/LINER/Transition object emission.

Strategies for flux calibration in massive spectroscopic surveys

AbstractOptical large-scale medium-resolution spectroscopic surveys such as SDSS, LAMOST, DESI, WEAVE or 4MOST are subject to constraints that limit the choice of flux calibrators, and the attained precision. The use of optical fibers, a large but limited field of view, the tiling strategies and tight schedules, are all factors that call for a careful evaluation of the flux calibration procedures.The density of stars with well-known spectral energy distributions is so low that makes them unsuitable for flux calibration of large scale spectroscopic surveys. The alternative is to use stars with relatively simple spectra, which can be approximated well by synthetic spectra based on model atmospheres. One example are white dwarfs (Bohlin 1996), but their density is also too low for practical purposes: a few per square degree down to 19th magnitude. An alternative choice, exploited by the SDSS, are halo turn-off F-type stars (Stoughton et al.2002). A-type stars offer another option, albeit with lower densities at high Galactic latitudes (Allende Prieto del Burgo 2016). Ideally, one would use stars of various spectral types. The most common type, halo turn-off stars, can be used for the actual calibration, and the others for quality assessment.The spectral typing needs to be performed before spectra are flux calibrated. Our group has explored various strategies for continuum normalization (the removal of the instrument response), finding good results using a running mean filter (Aguado et al.2017; Allende Prieto et al.2014). Interpolation in the models speeds up the model fitting process, but it is important to ensure that interpolations are sufficiently accurate (see, e.g. Mészáros Allende Prieto 2013).Fiber-fed spectrographs are particularly challenging, since errors in positioning fibers, guiding errors, or differential atmospheric refraction, add up. In our tests with data from the Baryonic Oscillations Spectroscopic Survey (BOSS; Dawson et al.2016), we conclude that while the flux calibration is statistically accurate (<5%), individual spectra can exhibit much larger excursions, in excess of 20%.

A two-stage outflow in NGC 1068

We present an analysis of the Seyfert 2 galaxy NGC 1068 with archive data from SINFONI-VLT, in the HK bands with pixel scales of 0.1 (data set 1 - DS1) and 0.025 (DS2) arcsec. The data are revisited with a sophisticated data treatment, such as the differential atmospheric refraction correction and the application of a Butterworth filtering and deconvolution. The gain in the process is quantified by a significant improvement in the Strehl ratio and it shows that an unprecedented high spatial resolution is achieved. For DS1, a detailed study of the H$_{2}$, [Fe II] and [Si VI] emission lines reveals a three-phase gas morphology: (1) the low-velocity [Fe II] emission representing the glowingwall of an hourglass structure, (2) the high-velocity compact blobs of low and high ionization emissions filling the hourglass volume and (3) the distribution of H$_{2}$ molecular gas defines the thick and irregular walls of a bubble surrounding a cavity. Both the hourglass and the molecular emissions have an asymmetry caused by the fragmentation of the north-eastern molecular wall, closest to the active galactic nucleus, resulting in highvelocity compact blobs of ionized gas outside the bubble. The south-western part of the bubble is excavated by the jet, where the blobs remain confined and are blown along the bubble's inner boundary. We propose that those blobs are driven by a hot "secondary wind" coming from the spot where the jet interacts and injects its energy in the molecular gas. The combination of a primary wind launched by the central source and the secondary wind is what we call a two-stage outflow. For DS2, we detected a [Si VI] outflow nearly coplanar to the maser disc and orthogonal to the CO outflow found by a previous study. Such unexpected scenario is interpreted as the interaction between the central radiation field and a two-phase gas density torus.

IMPROVED SPECTROPHOTOMETRIC CALIBRATION OF THE SDSS-III BOSS QUASAR SAMPLE

ABSTRACT We present a model for spectrophotometric calibration errors in observations of quasars from the third generation of the Sloan Digital Sky Survey Baryon Oscillation Spectroscopic Survey (BOSS) and describe the correction procedure we have developed and applied to this sample. Calibration errors are primarily due to atmospheric differential refraction and guiding offsets during each exposure. The corrections potentially reduce the systematics for any studies of BOSS quasars, including the measurement of baryon acoustic oscillations using the Lyα forest. Our model suggests that, on average, the observed quasar flux in BOSS is overestimated by ∼19% at 3600 Å and underestimated by ∼24% at 10,000 Å. Our corrections for the entire BOSS quasar sample are publicly available.

Spectra of faint sources in crowded fields with FRODOSpec on the Liverpool Robotic Telescope

AbstractWe check the performance of the FRODOSpec integral‐field spectrograph for observations of faint sources in crowded fields. Although the standard processing pipeline L2 yields too noisy fibre spectra, we present a new processing software (L2LENS) that gives rise to accurate spectra for the two images of the gravitationally lensed quasar Q0957+561. Among other things, this L2LENS reduction tool accounts for the presence of cosmic‐ray events, scattered‐light backgrounds, blended sources, and chromatic source displacements due to differential atmospheric refraction. Our non‐standard reduction of Q0957+561 data shows the ability of FRODOSpec to provide useful information on a wide variety of targets, and thus, the big potential of integral‐field spectrographs on current and future robotic telescopes. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A treatment procedure for Gemini North/NIFS data cubes: application to NGC 4151

We present a detailed procedure for treating data cubes obtained with the Near-Infrared Integral Field Spectrograph (NIFS) of the Gemini North telescope. This process includes the following steps: correction of the differential atmospheric refraction, spatial re-sampling, Butterworth spatial filtering, 'instrumental fingerprint' removal and Richardson-Lucy deconvolution. The clearer contours of the structures obtained with the spatial re-sampling, the high spatial-frequency noise removed with the Butterworth spatial filtering, the removed 'instrumental fingerprints' (which take the form of vertical stripes along the images) and the improvement of the spatial resolution obtained with the Richardson-Lucy deconvolution result in images with a considerably higher quality. An image of the Br{\gamma} emission line from the treated data cube of NGC 4151 allows the detection of individual ionized-gas clouds (almost undetectable without the treatment procedure) of the narrow-line region of this galaxy, which are also seen in an [O III] image obtained with the Hubble Space Telescope. The radial velocities determined for each one of these clouds seem to be compatible with models of biconical outflows proposed by previous studies. Considering the observed improvements, we believe that the procedure we describe in this work may result in more reliable analysis of data obtained with this instrument.

Microarcsecond Astrometry with MCAO Using a Diffractive Mask

AbstractWe present a new ground-based technique to detect or follow-up long-period, potentially habitable exoplanets via precise relative astrometry of host stars using Multi-Conjugate Adaptive Optics (MCAO) on 8 meter telescopes equipped with diffractive masks. MCAO improves relative astrometry both by cancellation of high-altitude atmospheric layers, which induce dynamic focal-plane distortions, and the improvement of centroiding precision with sharper PSFs. However, mass determination of habitable exoplanets requires multi-year reference grid stability of ~1–10 μas or nanometer-level stability on the long-term average of out-of-pupil phase errors, which is difficult to achieve with MCAO (e.g., Meyer et al. 2011). The diffractive pupil technique calibrates dynamic distortion via extended diffraction spikes generated by a dotted primary mirror, which are referenced against a grid of background stars (Guyon et al. 2012). The diffractive grid provides three benefits to relative astrometry: (1) increased dynamic range, permitting observation of V < 10 stars without saturation; (2) calibration of dynamic distortion; and (3) a spectrum of the target star, which can be used to calibrate the magnitude of differential atmospheric refraction to the microarcsecond level. A diffractive 8-meter telescope with diffraction-limited MCAO in K-band reaches < 3–5 μas relative astrometric error per coordinate perpendicular to the zenith vector in one hour on a bright target star in fields of moderate stellar density (~10–40 stars arcmin−2). We present preliminary on-sky results of a test of the diffractive mask on the Nickel telescope at Lick Observatory.

IMPROVING GALACTIC CENTER ASTROMETRY BY REDUCING THE EFFECTS OF GEOMETRIC DISTORTION

We present significantly improved proper motion measurements of the Milky Way's central stellar cluster. These improvements are made possible by refining our astrometric reference frame with a new geometric optical distortion model for the W. M. Keck II 10 m telescope's Adaptive Optics camera (NIRC2) in its narrow field mode. For the first time, this distortion model is constructed from on-sky measurements, and is made available to the public. When applied to widely dithered images, it produces residuals in the separations of stars that are a factor of ~3 smaller compared to the outcome using previous models. By applying this new model, along with corrections for differential atmospheric refraction, to widely dithered images of SiO masers at the Galactic center, we improve our ability to tie into the precisely measured radio Sgr A*-rest frame. The resulting infrared reference frame is ~2-3 times more accurate and stable than earlier published efforts. In this reference frame, Sgr A* is localized to within a position of 0.6 mas and a velocity of 0.09 mas/yr, or ~3.4 km/s at 8 kpc (1 sigma). While earlier proper motion studies defined a reference frame by assuming no net motion of the stellar cluster, this approach is fundamentally limited by the cluster's intrinsic dispersion and therefore will not improve with time. We define a reference frame with SiO masers and this reference frame's stability should improve steadily with future measurements of the SiO masers in this region. This is essential for achieving the necessary reference frame stability required to detect the effects of general relativity and extended mass on short-period stars at the Galactic center.

Spatially resolved spectroscopy of planetary nebulae and their halos

Strong mass loss off stars at the tip of the asymptotic giant branch (AGB) profoundly affects properties of these stars and their surroundings, including the subsequent planetary nebula (PN) stage. With this study we wanted to determine physical properties of mass loss by studying weakly emitting halos, focusing on objects in the galactic disk. Halos surround the, up to several thousand times, brighter central regions of PNe. Young halos, specifically, still contain information of the preceeding final mass loss stage on the AGB. In the observations we used the method of integral field spectroscopy with the PMAS instrument. This is the first committed study of halos of PNe that uses this technique. We improved our data analysis by a number of steps. In a study of the influence of scattered light we found that a moderate fraction of intensities in the inner halo originate in adjacent regions. As we combine line intensities of distant wavelengths, and because radial intensity gradients are steep, we corrected for effects of differential atmospheric refraction. In order to increase the signal-to-noise of weak emission lines we introduced a dedicated method to bin spectra of individual spatial elements. We also developed a general technique to part the temperature-sensitive oxygen line [O III] λ4363 from the adjacent telluric mercury line Hg4358 – without using separate sky exposures. By these steps we avoided introducing errors of several thousand Kelvin to our temperature measurements in the halo. For IC 3568 we detected a halo. For M 2–2 we found a halo radius that is 2.5 times larger than reported earlier. We derived radially densely sampled temperature and density structures for four nebulae, which all extend from the central regions and out into the halo. NGC 7662, IC 3568, and NGC 6826 show steep radially increasing temperatures and a hot halo, indicating that the gas in the halo is not in thermal equilibrium. M 2–2 shows a larger temperature in the central region and an otherwise constant value. From the density structures we made estimates of core and halo masses and – for the first time reliable – mass loss rates at the tip of the AGB. All four objects show inwards radially increasing mass loss rate structures, which represent a rise by a factor of about 4–7, during the final mass loss phase, that covers a time period of approximately 104 years. Within a factor of two, the average of the maximum mass loss rates, which are distance dependent, is 10-4 yr-1.

Accurate Astrometry and Photometry of Saturated and Coronagraphic Point Spread Functions

Accurate astrometry and photometry of saturated and coronagraphic point spread functions (PSFs) are fundamental to both ground- and space-based high contrast imaging projects. For ground-based adaptive optics imaging, differential atmospheric refraction and flexure introduce a small drift of the PSF with time, and seeing and sky transmission variations modify the PSF flux distribution. For space-based imaging, vibrations, thermal fluctuations and pointing jitters can modify the PSF core position and flux. These effects need to be corrected to properly combine the images and obtain optimal signal-to-noise ratios, accurate relative astrometry and photometry of detected objects as well as precise detection limits. Usually, one can easily correct for these effects by using the PSF core, but this is impossible when high dynamic range observing techniques are used, like coronagrahy with a non-transmissive occulting mask, or if the stellar PSF core is saturated. We present a new technique that can solve these issues by using off-axis satellite PSFs produced by a periodic amplitude or phase mask conjugated to a pupil plane. It will be shown that these satellite PSFs track precisely the PSF position, its Strehl ratio and its intensity and can thus be used to register and to flux normalize the PSF. A laboratory experiment is also presented to validate the theory. This approach can be easily implemented in existing adaptive optics instruments and should be considered for future extreme adaptive optics coronagraph instruments and in high-contrast imaging space observatories.

Image Field Deformation of LAMOST due to Differential Atmospheric Refraction

A vectorial expression of the image field deformation of LAMOST due to the differential atmospheric refraction is presented. The calculated results are compared with those from previous analyses based on the traditional spherical trigonometric formulas. It is demonstrated that different tangential displacements of star images during the observation tracking given by various authors are simply due to different reference points adopted. It is pointed out that the observational celestial pole is the center of the apparent diurnal motion, that, by referring to the observational celestial pole, the effect of the differential refraction on the image field of LAMOST during the 1.5-hour tracking period is approximately equivalent to a constant rotation of −13.65'' for all declination belts. It is therefore unnecessary to design a particular tracking velocity for each observation, and this will be obviously advantageous to the observation implementation. If the maximum tracking error of the fibers is 0.2'' , then the fibers are required to be able to re-position during observational tracking for sky regions south of declination +20° and north of declination +60°

Titan’s Atmosphere at High Resolution

Fig. 1.—Example images of some of the phenomena discovered and observed during this work. (a) Accumulation of ethylene in Titan’s south polar stratosphere is apparent in this 10.7 mm image, which is sensitive to thermal emission from haze and ethylene (C2H4) in Titan’s stratosphere. (b) Titan’s troposphere and surface are obscured from view because of strong methane absorption in this mm filter image. The global stratospheric haze and the bright south polar 1.702 0.008 collar of haze are seen in reflected sunlight. (c) and (e) In sections of the K filter (1.95–2.30 mm), Titan’s atmosphere is relatively transparent and numerous surface features are seen in reflected sunlight. (d) and ( f ) In a 2.108–2.140 mm filter, the surface is blocked from view because of moderate methane absorption. This filter probes down into the troposphere, and cloud activity near the south pole is apparent. The resolution in (b)–( f ) is ∼300 km. Titan’s apparent diameter is 0 .83 in (a) and is 0 .87 in (b)–( f ). Titan stands out in our solar system as the sole moon with a substantial atmosphere. Until recently, the only spatially resolved observations of Titan were from the Voyager spacecraft flybys in 1980 and 1981, when the season was early northern spring on Titan. The advent of large ground-based telescopes and adaptive optics allows us to study Titan and its atmosphere with spatial resolutions of a few hundred kilometers. We report nearand mid-infrared observations of Titan’s atmosphere with the Keck and Gemini telescopes in late southern spring on Titan. These observations represent the start of a long-term ground-based monitoring campaign that will extend over a significant fraction of Titan’s 30 Earth-year seasonal cycle, and in doing so will lead us to a better understanding of the seasonal variations. Observing at mid-infrared wavelengths (8–13 mm) with the Long-Wavelength Spectrometer (LWS) on the Keck I Telescope, we found a large buildup of ethylene (C2H4) in the south polar stratosphere (see Fig. 1a, which is from H. G. Roe, I. de Pater, & C. P. McKay 2003, Icarus, submitted). We link this buildup to chemistry that occurs in the shadow of the winter pole. We also developed a new line-by-line radiative-transfer model for Titan at 8–13 mm. This model can easily be extended to longer wavelengths and will in the future be useful for several Titan-related projects, including long-term monitoring of south polar ethylene. At near-infrared wavelengths (1–2.5 mm) using adaptive optics on the Keck II Telescope, we found a collar of haze near the tropopause at 70 –75 south latitude (see Fig. 1b, which is from H. G. Roe et al. 2002, Icarus, 157, 254). This collar is most likely the remnant of stratospheric nitrile chemistry during the long polar winter night. We report a spatially resolved detection of clouds on Titan (see Figs. 1c–1f, which are from H. G. Roe et al. 2002, ApJ, 581, 1399). These observations were made using adaptive optics on the Gemini North and Keck II telescopes. We found the discrete cloud features to vary on timescales of hours to days. All clouds were near the south pole, with the most northern cloud appearing at 61 south. We find each cloud covers an area of Titan’s surface of to km. From a limited 4 5 2 # 10 3 # 10 temporal sample, these clouds appear to be common in the south polar region in late southern spring. We discuss several possible seasonal triggering mechanisms for these clouds. Finally, in appendices we present a new telluric transmission model and discuss the implications of differential atmospheric refraction for adaptive-optics observations (H. G. Roe 2002, PASP, 114, 450).

Implications of Atmospheric Differential Refraction for Adaptive Optics Observations11Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

ABSTRACTMany adaptive optics systems operate by measuring the distortion of the wave front in one wavelength range and performing the scientific observations in a second, different wavelength range. One common technique is to measure wave front distortions at wavelengths less than ∼1 μm while operating the science instrument at wavelengths greater than ∼1 μm. The index of refraction of air decreases sharply from shorter visible wavelengths to near‐infrared wavelengths. Therefore, because the adaptive optics system is measuring the wave front distortion in one wavelength range and the science observations are performed at a different wavelength range, residual image motion occurs and the maximum exposure time before smearing of the image can be significantly limited. We demonstrate the importance of atmospheric differential refraction, present calculations to predict the effect of atmospheric differential refraction, and finally discuss the implications of atmospheric differential refraction for several current and proposed observatories.

Analysis of astronomical data from optical superconducting tunnel junctions

Currently operating optical superconducting tunnel junction (STJ) detectors, developed at the European Space Agency (ESA), can simultaneously measure the wavelength (Dl550 nm at 500 nm) and arrival time (to within ;5 ms) of individual photons in the range 310 to 720 nm with an efficiency of;70%, and with count rates of the order of 5000 photons s 21 per junction. A number of STJs placed in an array format generates 4-D data: photon arrival time, energy, and array ele- ment (X,Y). Such STJ cameras are ideally suited for, e.g., high-time- resolution spectrally resolved monitoring of variable sources or low- resolution spectroscopy of faint extragalactic objects. The reduction of STJ data involves detector efficiency correction, atmospheric extinction correction, sky background subtraction, and, unlike that of data from CCD-based systems, a more complex energy calibration, barycentric ar- rival time correction, energy range selection, and time binning; these steps are, in many respects, analogous to procedures followed in high- energy astrophysics. We discuss these calibration steps in detail using a representative observation of the cataclysmic variable UZ Fornacis; these data were obtained with ESA's S-Cam2 636-pixel device. We furthermore discuss issues related to telescope pointing and guiding, differential atmospheric refraction, and atmosphere-induced image mo- tion and image smearing (''seeing'') in the focal plane. We also present a simple and effective recipe for extracting the evolution of atmospheric seeing with time from any science exposure and discuss a number of caveats in the interpretation of STJ-based time-binned data, such as light curves and hardness ratio plots. © 2002 Society of Photo-Optical Instrumen-

Differential atmospheric refraction in integral-field spectroscopy: Effects and correction

The basic concepts for determining the effect of differential atmospheric refraction on integral-field (two-dimensional) spectroscopic observations are presented, and a method for correcting the spectra from this effect is given. The method is demonstrated with real data.

Differential Atmospheric Refraction and Limitations on the Relative Astrometric Accuracy of Large Telescopes

ABSTRACTRecent advances have made ground‐based relative astrometry accurate to 100 microarcseconds (μas) or better a possibility. An important systematic effect on such measurements is the refraction induced by the atmosphere, which tends to alter the apparent separation of two stars. For stars separated by 30″ and observed in the K band at a zenith angle of 45°, this change in observed separation can amount to as much as 12,000 μas, or some 120 times the desired accuracy. Given a model of the atmosphere, the magnitude of this effect on observations made with large telescopes can be calculated and corrected. We have demonstrated that the differential refraction can effectively be divided into two independent components: one that is determined by the separation of the stars on the sky, and another that depends on the difference in their colors, the former component dominating in most cases. According to the atmospheric model we have adopted, there are seven quantities that must be measured in order to perform a calculation of the differential refraction ΔR. These are the zenith angle of the first star, the observed separation of the stars along the zenith direction, the ground‐level atmospheric parameters of temperature, pressure, and relative humidity, as well as the effective surface temperatures of the two stars. We discuss how accurately these must be measured to limit the error in the ΔR correction to 10 μas or less per input parameter. The most stringent of these requirements is that the stellar surface temperature should be known accurately (≈100 K) if the star is cool.

A Computational Study of the Star-Image Displacement due to Differential Atmospheric Refraction During Observations : Su Ding-qiang & Wang Ya-nan Nanjing Astronomical Instruments Research Center, Chinese Academy of Sciences, Nanjing 210042

A Computational Study of the Star-Image Displacement due to Differential Atmospheric Refraction During Observations : Su Ding-qiang & Wang Ya-nan Nanjing Astronomical Instruments Research Center, Chinese Academy of Sciences, Nanjing 210042