Echelle gratings: their testing and improvement

Tests are described on new master echelles of very large size and on others having unusually high blaze angles, produced on interferometrically controlled ruling engines. Defects arising from the gravitational distortion of large grating blanks during ruling were reduced by the use of calibrated edge-supporting springs to diminish sag. The sixth echelle in a series of the largest yet produced (400 x 600 mm, or 16 x 24 in.), ruled on a fused silica blank 127 mm thick with 79 grooves/mm blazed at 63.4 degrees , gives high speed at resolutions well in excess of 10(6). A series of 254-mm echelles ruled at blaze angles whose tangents range from 2 to 8 were compared. Because an echelle blazed at 63 degrees can theoretically give 90% of the maximum resolving power available at any blaze angle, little resolution is gained by using steeper angles, and optical defects are likely to increase with groove asymmetry. Very high blaze angles are useful for raising intrinsic dispersion, however, making possible construction of echelle spectrometers of extreme compactness.