The Grating

The heart of any spectrograph is the dispersive element, which separates out an object's light to see how much power there is in any given wavelength/color. Analysis of the 'spectra' resulting from this separation can give you a wealth of information about the object you are observing, from its temperature to its distance. Spectra are the backbone of astronomy.

The grating is therefore a critical part of the DEUCE payload. Our grating was made of fused silica with 3800 grooves per millimeter, and is about 11 cm square by 1.5cm high. The producer was Horiba JY, in France, one of the few companies around the world that are able to make these precise UV gratings.

The grating was originally coated with a thin layer of gold on top of chromium, which helped it better adhere to the glass. While gold is a decent all-around reflector in the EUV, Silicon Carbide is better. Thus, we had our grating over-coated with another layer of chromium and then SiC at Goddard Space Flight Center in Maryland. To do this, the grating is placed upside down in a vacuum chamber and ion beam sputtering is used to coat the grating in a thin layer of silicon carbide. Silicon carbide is fairly robust, and doesn't react with water vapor, so we only had to protect the grating from silicates and contamination, not humidity (this isn't the case for many other coatings!)

Thermal expansion and a small protrusion in the metal mounting hardware led to a small chip occurring on a corner of the grating during coating. This had the potential to be very serious; if any small cracks existed, they would propagate during the intense vibration of launch and our grating would likely shatter. However, examination by an optical expert showed no propagating cracks, and the grating has happily survived two flights and at least 10 vibration tests.