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The abundance of helium relative to hydrogen has been measured with the “Coronal Helium Abundance Spacelab Experiment” (CHASE) from the space shuttle Challenger in 1985. Previous solar measurements have proved difficult due to the temperature-sensitivity of the electron excitation rates for the observed lines. In this approach scattered Lyman Alpha (Lyα) radiation of belium and hydrogen formed in the corona were measured with a grazing-incidence spectrometer and compared with the intensity of the illuminating flux from the solar chromosphere. The abundance ratio by number of atoms was found to be 0.070 with an uncertainty of 0.011. Scattered light in the telescope is the main source of error.

Following a successful launch on 1990 June 1, the ROSAT spacecraft and its payload, consisting of an X-ray and an EUV telescope, underwent two months of in-orbit calibration and detailed performance checks. A preliminary observation of a small section of the sky, carried out over July 11-16 (the mini-survey) showed all ROSAT systems to be functioning well and has allowed predictions to be made on the ultimate productivity of both ROSAT all-sky surveys. An analysis of the mini-survey data from the UK Wide Field Camera, has revealed 35 EUV sources, including several white dwarf stars, a variety of active cool stars and several other objects. In all, 23 sources have probable optical counterparts

DETAILED understanding of the evolution of main-sequence stars into white dwarfs depends on knowledge of the chemical composition of white-dwarf atmospheres, but the hottest white dwarfs emit much of their radiation in the extreme ultraviolet (EUV), a part of the spectrum that has been generally inaccessible to astronomers. We report here the discovery of a new white dwarf as a bright EUV source found during the first observations using the Wide Field Camera1 on the Rosat satellite. Within the 1-arcmin error box of the EUV source is an unusually blue star, which we show to be a binary system consisting of a DA white dwarf and a cool companion in the classification range dM2–dM5. There are some similarities between this new object and the binary system Feige 24 (ref. 2), but spectral differences in the EUV emission can be attributed to significantly different atmospheric compositions for the two white dwarfs.

The Coronal Diagnostic Spectrometer is designed to probe the solar atmosphere through the detection of spectral emission lines in the extreme ultraviolet wavelength range 150–800 A. By observing the intensities of selected lines and line profiles we may derive temperature, density, flow and abundance information for the plasmas in the solar atmosphere. Spatial and temporal resolutions of down to a few arcseconds and seconds, respectively, allow such studies to be made within the fine-scale structure of the solar corona. Furthermore, coverage of large wavelength bands provides the capability for simultaneously observing the properties of plasmas across the wide temperature ranges of the solar atmosphere.

We present early results from observations made by the UK’s imaging EUV telescope, the Wide Field Camera (WFC). This telescope is being flown aboard the West German satellite ROSAT. In conjunction with a large X-ray telescope, built by the West Germans, the WFC is conducting a survey of the whole sky. This is the first in the EUV waveband. Afterwards ROSAT will be devoted to pointed observations of specific objects. We use results from the earliest phases of the ROSAT mission to highlight the importance of WFC observations for the study of white dwarfs and related objects such as the central stars of planetary nebulae.

The main design features and the early findings of the Rosat XUV wide field camera (WFC) are discussed. The most important data on the WFC telescope and detectors are presented. The WFC operational features, observing efficiency, filter performance, thermal performance star tracker performance, and single-event upsets are discussed. The first WFC images are compared with preflight calibration data.