Last week, the MOONS team at UKATC fitted all the fixed optics into their mounts and subsystems. This is an exciting milestone towards the full assembly of the instrument!

The two fixed optics assemblies inside MOONS splits the light up to the three cameras on each side of the main bench. It does this using 2 pieces of glass with special coatings that reflect all light with a shorter wavelength (bluer light) than a certain value, and let the longer wavelength (redder) light go through. By doing this twice, you can split the white light coming in from the optical fibres into the three channels.

The other pieces mounted in these units is a VPH (Volume Phase Holographic) grating. These are two pieces of glass, with a layer of gelatine in between that has been exposed to a laser. By doing this in a very specific way, you create groves in the material, meaning it works as a grating, diffracting the light into a spectrum (essentially making a rainbow).

With the size of MOONS each of these pieces of glass is around 300mm on each side, and weighs 30kg or more. Before assembling the units they were also measured using a laser tracker to ensure that when we put them together everything would be in the right places to 0.1mm.

All the parts are held in place with springs and sprung clamps to ensure the glass is held gently enough not to distort it, but also firmly enough that it will not move when being shipped, or should it be effected by an earthquake at the telescope.

Both of these are now fully assembled and stored safely in a clean environment so that they can be fitted into the main instrument later this year.

[Story and (great!) photos prepared by UKATC Optical Engineer Martin Black]

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