Outstanding high-resolution gratings have now been obtained by ruling under interferometric control on the M.I.T. ruling engine, as a result of considerable improvements which have followed the application of two separate studies carried out for this purpose: (1) a general theory of the effects of grating errors on the distribution of light in the spectral images; and (2) a theory of the uncalled-for corrections introduced into an interferometric servomechanism by incorrect translation of the grating-blank motion into representative interferometric fringe signals. The errors found to be most detrimental to high-resolution grating, in that they affect the close neighborhood of the line centers, are rather extended small-amplitude deviations in the wave fronts. These can be caused in particular by errors of run in the ruling, as a result of inadequate temperature and servocontrol, by defects in flatness of the grating blanks and of the aluminum coatings, as well as by incorrect adjustments of the control interferometers. Reduction of these errors to less than λ/10 at 5461 A and corresponding improvements in the diamond-carriage adjustments have resulted in improvements on the order of 4 to 9 in the spectral quality of the high-angle gratings when compared to the 10-in. gratings previously described by G. R. Harrison and G. W. Stroke [ J. Opt. Soc. Am. 50, 1153 ( 1960)]. Precision replicas of these gratings permit high-resolution studies in spectrometers as short as 1 m and in spectrographs with as few as 1012 atoms in the source. Considerable further gains in luminosity, compactness and resolution should result from the use of gratings and echelles blazed at 76° in autocollimation rather than at 64°, as has been customary so far.
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