Sodium laser guide star technique, spectroscopy and imaging with adaptive optics

Abstract

A sodium laser guide star (LGS) adaptive optics (AO) system developed at Stewart Observatory is to be used at the 6.5m MMT. Annual measurements at Kitt Peak show that the mean mesospheric sodium column density varies from ∼2x10⁹cm⁻² (summer) to ∼5x10⁹cm⁻² (winter). The sodium column density also varies by a factor of two during a one hour period. The first simultaneous measurements of sodium LGS brightness, sodium column density and laser power were obtained. The absolute sodium return for a continuous wave circularly polarized beam is 1.2(±0.3)x10⁶ photons s⁻¹m⁻²W⁻¹ for the sodium column density of 3.7x10⁹cm⁻². Theoretical studies demonstrate that the 6.5m MMT LGS AO can provide Strehl ratios better than 0.15 and about 50% flux concentration within 0.2" aperture for 1-5.5μm under median seeing. This correction will be available for the full sky. Better Strehl and higher flux concentration can be achieved with natural guide stars, but limited sky coverage. The AO corrected field-of-view is about 60". The Arizona IR Imager and Echelle Spectrograph (ARIES) was designed to match the 6.5m MMT AO. Detection limits of more than 2 magnitude fainter can be reached with the AO over without the AO. A pre-ARIES wide field near-IR camera was designed, built and tested. The camera provides 1" images in the near-IR over an 8.5 x 8.5arcmin² field. The 10-σ detection limit with one minute exposures is 17.9 mag. in the K band. A prototype very high resolution cross-dispersed optical echelle spectrograph was designed and built to match the Starfire Optical Range 1.5m AO images. Interstellar KI 7698Å absorption lines have been detected in the spectra of αCyg and ζPer. The spectral resolution is 250.000. About 300Å wavelengths were covered in a single exposure. Total detection efficiency of 1% has been achieved. For the first time, a near-single-mode fiber with 10μm core size was applied to transmit the Mt. Wilson 100inch AO corrected beams to a spectrograph. The coupling efficiency of the fiber reached up to 70%. Spectra of αOri were recorded. The spectral resolution is 200,000. The total wavelength coverage is about 650Å per exposure.