OPTICAL FABRICATION: Replication produces 840-mm diffraction grating

Optical engineers at the Richardson Grating Laboratory of the Milton Roy Company (Rochester, NY) have adopted a mosaic approach that allowed them to use existing technology to produce a pair of 400 mm gratings that act as one large grating without resorting to oversized ruling engines and coating facilities. Fabricated for the European Southern Observatory (ESO), the record-dimension, high-performance echelle diffraction grating disperses light at wavelengths between 400 and 1000 nm. Two segments were replicated onto a single Zerodur substrate to create a finished element 840 × 214 × 125 mm in size.

Using a diamond tool mounted on a servo-controlled ruling engine, engineers fabricated a grating master by burnishing grooves into a 20-µm layer of aluminum vacuum-deposited on a 400-mm-long Zerodur substrate. This master was reproduced using an optical replication technique in which the grating is coated with a layer of polymer release, followed by a coating of aluminum, and a Zerodur backing plate. The release allows the replicated grating to be removed from the master, yielding a high-resolution, economical duplicate.

Two grating segments were produced from the master for the echelle grating. The groove spacing of the completed elements is 31.6 mm^-1, with a 75.1° blaze angle. The two ruled segments were aligned on the monolithic Zerodur substrate such that they imaged their spectra to the same point, with an angular alignment on the order of arc seconds and required coplanarity within a few microns. This application did not require that the grooves of the two segments be in phase with one another.

The ESO Very Large Telescope Project is an array of four 8-m telescopes to be located in Chile. Optimized for the red end of the spectrum, the echelle grating will be one of the dispersing elements in a two-arm ultraviolet/visible light spectrograph being constructed for the ESO. A second grating optimized for the blue spectral region is currently under fabrication. The large size of the grating is necessary to accommodate the 200-mm beam diameter produced by the telescopes. The observatory is expected to be operational in 1997.