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The purpose of this paper is to survey some of the recent advances in the state of the art of soft X-ray spectrometers, particularly as they might be applied to Solar Observations. The discussions will center on the windowless region from roughly 1 to 100 A, and will cover both grating and crystal instruments. During the interval that has elapsed since the launches of the Solar Maximum Mission and P 78-1 satellites in the early 1980's, there have been a number of significant technological developments. Among these, are multi-layer mirrors, large format CCD detectors sensitive to X-rays, position sensitive photon counting detectors, new kinds of X-ray films, and the discovery of a number of ingenious new optical systems based on gratings with non-uniform ruling spacings. There have also been a number of improvements in the extent and accuracy of the atomic physics data sets on which the analysis of spectroscopic observations depend. Finally, the analysis of the data from the two missions mentioned above, from the earlier Skylab mission, and from recent sounding rocket flights, have raised a number of interesting questions that will greatly benefit from a new generation of instruments and the observations that they will produce. I will begin with a short discussion of the solar soft X-ray spectrum and its interpretation, followed by a few general comments on problems peculiar to soft X-ray instruments. The main portion of the paper will be devoted to a review of recent developments in spectrometer optical design, which has been a lively field during the last dozen years. This is particularly true in the case of grating spectrometers, because of the development of diffraction gratings with variable groove spacings or with curved rulings. The availability of multi-layer mirror coatings means that designs are not necessarily restricted to the glancing incidence region, giving additional design flexibility. The paper will conclude with a short section on telescope considerations, and some hopeful remarks on future flight opportunities.
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