In principle, the use of longer slits is indicated in spectroscopy wherever the resolution is energy limited. Various problems arise, however, as slits are lengthened beyond the conventional limits. This paper sets forth practical solutions to some of these difficulties through the use of an image transformation method, and discusses the advantages and limitations of the schemes suggested.
WILLIAM BENESCH and JOHN STRONG, "The Optical Image Transformer," J. Opt. Soc. Am. 41, 252-254 (1951)