A 21-ft original concave diffraction grating, ruled in aluminum and blazed for 5500 Å, has been overcoated with a 1000-Å layer of aluminum followed by a 265-Å layer of magnesium fluoride. The relative grating efficiency has been evaluated in spectral orders 2 through 6 at 1335 Å. It was found that the efficiency was increased 12 times at 1335 Å (4th order) and 20 times at 1215 Å (5th order). The blaze was considerably sharper after coating than before coating. No impairment in resolving power resulted.
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