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It is suggested that high-dispersion long-slit spectra or very narrow-band etalon images of ‘blank’ sky could reveal patches of Ly-<it>α</it> line emission from the population of clouds whose absorption produces the ‘Ly-<it>α</it> forest’ in QSO spectra. A non-observation can put limits on the ionizing background at high redshift which are better than those obtainable by direct measurements of background light.
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Hogan et al. (1987) studied this question.
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