Nature © Macmillan Publishers Ltd 19988 radius apertures following the standard prescription 28 . Additional corrections were made as follows: (1) +0.05 mag to correct for the so-called 'long exposure' effect 29 ; (2) −0.04 mag, which is the appropriate colour term to convert to standard Cousin's I magnitudes for red stars with V Ϫ I Ϸ 1:5 (ref. 28); and (3) −0.04 mag to allow for the expected foreground extinction 30 . We estimate that the combined uncertainty from the zero-point calibration and the various correction terms amounts to 0.06 mag. Clearly extended sources were removed from the catalogue by restricting our analysis to sources with a DAOPHOTsharpness parameter Ϫ 0:6 Ͻ s Ͻ 0:4. A similar procedure was also followed for the HDF control field. Finally, we created 12 test data sets in which 265 simulated stars were added to the real data frames. The input magnitudes of the simulated stars covered the range from 23.5 to nearly 30. We then processed these frames in the same way as the original data, and produced a matrix relating the recovered magnitudes and detection efficiency to the input magnitudes of the simulated stars. From these simulations we estimate that the catalogue is ϳ80% complete at I ¼ 27:9, and use this as the limiting magnitude in our analysis.

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Global acquisition of atmospheric wind profiles using a spaceborne direct-detection Doppler wind lidar is being accomplished following the launch of European Space Agency’s Aeolus mission. One key part of the instrument is a single-frequency, ultraviolet laser that emits nanosecond pulses into the atmosphere. High output energy and frequency stability ensure a sufficient signal-to-noise ratio of the backscatter return and an accurate determination of the Doppler frequency shift induced by the wind. This Letter discusses the design of the laser transmitter for the first Doppler wind lidar in space and its performance during the first year of the Aeolus mission, providing valuable insights for upcoming space lidar missions.

ESA deployed the first Doppler Wind lidar in space within its Earth Explorer Mission Aeolus. The objective of Aeolus is to provide tropospheric and lower stratospheric wind profiles globally for the improvement of weather forecasts on short and medium term. Spin-off products are profiles of atmospheric backscatter and extinctions coefficients and lidar ratio. The observations will also be used as input to air quality models and to verify climate model parameterization and predictability. After the successful launch in late August this year an intensive commissioning phase is taking place in the first three month of the mission, including the first switch on of the instrument ALADIN and its calibration in flight. First preliminary results will be presented during the talk.

A tunable mid-IR source obtained by difference-frequency generation is demonstrated in a selectively oxidized GaAsAlAs multilayer waveguide. We designed the waveguide to present the required form birefringence for phase matching of the nonlinear interaction. We took special care to lower losses for the mid-IR radiation. IR tunability from 5.2 to 5.6 mum was achieved by variation of the waveguide temperature and one pump wavelength. IR output power as great as 0.12 muW was obtained with the product of two pump powers of 7 mW(2). Losses of ~50 cm(-1) were measured for the mid-IR radiation. These losses are attributed to surface scattering.