We present a technique to model the instrumental polarization in the focal plane of a telescope. It takes into account that different rays of an incoming beam suffer different variation of the original polarization in their paths through the system. It also considers that the net effect depends on the way in which the different rays interfere with

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The discovery of lithium in the spectrum of the binary system V404 Cygni is described. A high lithium abundance in the secondary star, close to that of very young stars, is derived. Without lithium production, the secondary would have to be comparable in age to the Pleiades cluster. If the system is not being seen at an early moment in its lifetime

CCD narrow-band H-alpha photometry of the Sextans A Local Group dwarf irregular galaxy is presented. Fourteen H II regions have been recorded. Their identification charts, H-alpha luminosities and sizes are given, and their luminosity and size distributions are discussed. Twenty-five stars with masses between 15 and 30 solar masses, distributed

A detector test bench is described which encompasses among its capabilities a wide, IR-visible spectral range, and a wide range of radiation response. The optical, cryogenic, mechanical and electronic devices used are sufficiently flexible for data acquisition that characterizes the detector in terms of linearity, dark noise, spectral and time

We report the results of a systematic study on the spatial variation of the electron density in a selected group of bright giant H II complexes in the galaxies M 101, M 51, and NGC 6822. Density values for the ionized gas of the regions have been obtained from the forbidden S II 6717/6731 ratio, using long-slit spectrophotometry at high spectral