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Simultaneous imaging and spectroscopy of the solar atmosphere: advantages and challenges of a 3-order slitless spectrograph

The dynamic solar atmosphere poses a severe observational challenge for imaging spectroscopy in EUV. The traditional method of building up images by rastering a slit spectrograph has so far proven too slow to keep up with the Sun's rapidly changing transition region and corona. We describe a new approach, using a slitless imaging spectrograph operating in a narrow band, with imaging detectors at three orders. This arrangement offers cotemporal imaging and spectroscopy at high spatial, spectral, and temporal resolution. The prospect of disentangling spatial and spectral information is greatly improved by choosing a narrow band containing only two spectral lines, and by imaging at several spectral orders. This paper discusses several advantages and challenges of the multi-order slitless approach. We derive a mathematical description of the null space of spatial-spectral signatures to which an ideal three-order slitless spectrograph has zero response. An exploration of the null space helps to clarify the capabilities and limitations of this instrument type. We infer that the three-order slitless spectrograph is sensitive to line intensity, doppler shift and line width; but insensitive to line asymmetry. Strategies are developed to minimize the ambiguity in interpreting the multi-order data. A proof of concept sounding rocket payload, the Multi-Order Solar EUV Spectrograph (MOSES), is under development with an anticipated launch in Spring, 2004.