In this paper we address the problem of the non-LTE (NLTE) inversion of line profiles by means of a nonlinear least-squares minimization procedure combined with very efficient multilevel transfer methods. Our approach is based on the concept of response functions, which measure the first-order response of the emergent profiles to changes in the

Daily measurements of the solar aureole were made at the Vacuum Newton Telescope (/D=40 cm) at the Teide Observatory (Tenerife) for various airmasses. We use these measurements to understand how the aureole is produced and to extrapolate visible measurements to the infrared range (1.6 /μm). This information will be used to correct sunspot

We present observations and analyses of some of the most massive stars in the Milky Way, showing the potential and limits of current model atmospheres, and the consequences for the evolution theory of massive stars. We then show how these techniques can be applied to very bright stars in M31 and M33 to derive radial abundance gradients and get

We have constructed an instrument able to map the sky at 10 GHz using circular scans with an angular resolution of 1deg. This instrument consist of a spinning plate working as a primary mirror for a parabolic dish with two cryogenic receptors at its focus. The area observed is a band of 20deg in Dec and the full sky in RA, centered at Dec 28 (the

Seeing statistics from several year-long site-testing campaigns carried out at different locations at the Roque de los Muchachos Observatory (ORM) are reviewed (this contribution contains results previously published by Munoz-Tunon et al. (1997) [Estudio de Sitio para el Gran Telescopio: Informe final; A&AS, 125, 183]). In particular, tests at the

We present monthly statistics for a two-year meteorological site-testing campaign at the two candidate sites for the location of the GTC. The windroses presented here are a fairly complete representation of the wind regimes prevailing at the lower site and indicate that at night the predominant wind direction for the site is from the NE, whereas