The limitations of photometric techniques

The photometric all-sky surveys conducted in the EUV by the ROSAT WFC and EUVE have been sources of important information concerning the general properties of groups of objects contained in the EUV source population, including late-type stars, white dwarfs, cataclysmic variables and active galactic nuclei. However, when considering individual objects in detail, the amount of information that can be extracted from three or four such data points is limited. For example, if heavy elements are present in the atmosphere of a hot white dwarf, the survey data can only give an indication of the level of opacity and are unable to distinguish between the possible species responsible and especially whether or not helium is present. Similarly, in studying the emission from stellar coronae, only crude estimates of conditions in the plasma can be made and usually only when simplifying assumptions such as the existence of a single temperature component are incorporated into the analysis. The overwhelming advantage of spectroscopic observations lies in the ability to study individual spectral features or blends of features, giving a more detailed picture of the underlying physical processes responsible for the EUV emission.

The Extreme Ultraviolet Explorer spectrometer

The main components of EUVE have been described in detail in chapter 4 with the exception of the spectrometer. This instrument made use of part of the converging beam of the Wolter type II deep survey telescope, intercepting this with three variable line space reflection gratings (Hettrick et al. 1985).