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The launch of the first imaging X-ray satellite, Einstein, had a profound impact on our understanding of clusters.

3.1. Detailed Imaging and Spectra

The Imaging Proportional Counter (IPC) and High Resolution Imager (HRI) provided images of unprecedented quality (up to 5" FWHM). These two instruments provided a great deal of detailed information on individual clusters from targeted observations (Fabian et al. 1981; Jones & Forman 1984, 1999; Stewart et al. 1984; White, Jones, & Forman 1997). Most of the observations were of Abell clusters or other optically selected clusters, but radio galaxies in clusters were also targets. Given the nature of the targeted observations it is not possible to derive any stringent limits on the statistical properties of clusters, but a luminosity function was derived for Abell clusters (Burg et al. 1994).

Einstein also carried the first semi-conductor detector (the forerunner to today's CCDs), the Solid State Spectrometer (SSS) and a deployable Bragg Crystal, the Focal Plane Crystal Spectrometer (FPCS). Both of these instruments provided important results for clusters (Canizares et al. 1979; White et al. 1991), but were not used systematically.

3.2. The EMSS

The Einstein survey that has had the most impact on cluster research is undoubtedly the Extended Einstein Medium Sensitivity Survey (EMSS). By combining most of the serendipitous detections from IPC observations, it was possible to survey 980 square° and detect 99 clusters (Stocke et al. 1991). This sample has since been the basis for a great deal of work at all wavelengths (Donahue, Stocke, & Gioia 1992; Le Févre et al. 1994; Carlberg et al. 1997; Luppino et al. 1999).

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