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Implications of Atmospheric Differential Refraction for Adaptive Optics Observations
Many adaptive optics systems operate by measuring the distortion of thewave front in one wavelength range and performing the scientificobservations in a second, different wavelength range. One commontechnique is to measure wave front distortions at wavelengths less than~1 μm while operating the science instrument at wavelengths greaterthan ~1 μm. The index of refraction of air decreases sharply fromshorter visible wavelengths to near-infrared wavelengths. Therefore,because the adaptive optics system is measuring the wave frontdistortion in one wavelength range and the science observations areperformed at a different wavelength range, residual image motion occursand the maximum exposure time before smearing of the image can besignificantly limited. We demonstrate the importance of atmosphericdifferential refraction, present calculations to predict the effect ofatmospheric differential refraction, and finally discuss theimplications of atmospheric differential refraction for several currentand proposed observatories. Data presented herein were obtained at theW. M. Keck Observatory, which is operated as a scientific partnershipamong the California Institute of Technology, the University ofCalifornia, and the National Aeronautics and Space Administration. TheObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| Lunar occultation summary. II
Results of a second series of 196 two-color photoelectric lunaroccultation observations made within the interval from May 1972 toDecember 1973 are reported. Each observation has been employed toestimate the minimum magnitude difference between the observed objectand possible undetected companions. The study contains the analyses of143 disappearances and 53 reappearances. Eighteen of these eventsmanifest some degree of multiplicity. Included in the study are 18occultation events for Pleiades members.
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