Slitless spectroscopy is astronomical spectroscopy done without a small slit to allow only light from a small region to be diffracted. It works best in sparsely populated fields, as it spreads each point source out into its spectrum, and crowded fields will be too confused to be useful. It also faces the problem that for extended sources, nearby emission lines will overlap.
The Crossley telescope utilized a slitless spectrograph that was originally employed by Nicholas Mayall.
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Slitless spectroscopy Wikipedia(Text) CC BY-SA