Ruling of Large Diffraction Gratings with Interferometric Control*

Abstract

The 14-inch MIT ruling engine, operated under interferometric servo control as previously described, now rules excellent diffraction gratings up to 8 inches in width. The more recent of the 70 test gratings produced approach in quality the best yet ruled on any engine, showing resolving powers of about 600 000 in the green, and giving crisp spectral lines with low local background even at high angles. Rowland ghost intensities appear lower than any yet reported, ranging from 1/700 to 1/800 at 74°, corresponding to 1/25 000 to 1/29 000 in the first order of a 15 000 groove/in. grating. The balls on which the blank carriage rolls were found to introduce irregularities which were finally eliminated by a rotation-control servo mechanism which permits a carriage to be moved on curved or irregular ways over distances of 10 inches or more without rotation greater than 0.01 sec of arc. Servo controls are described which reduce both periodic and cumulative errors to new low levels, and also eliminate “fanning” of grooves.A device for compensating the variations in the control fringe field which are produced by barometric pressure changes has now been made completely automatic. No failure of electronic components has yet occurred in the thousands of hours during which the engine has been operated. The continuous control of engine motion with interferometers makes feasible a new method for ruling wider gratings.