High reflectivity coatings

[Robin Leadbeater]

There have indeed been various discussions on the ARAS spectroscopy forum about some modern coatings (on mirrors, lenses and camera windows) which while very efficient in the visible, drop off rapidly in the UV, for example this plot for the GSO RC telescope which effectively cuts off steeply at 4000A

https://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2398&p=13202#p13470

This has been a big issue for the UVEX spectrograph, summed up by  Christian Buil in the same thread

“What always worries me is to see reflectivity of 99%, via dielectric treatments. These things are really our enemies in spectrography when we want to go to slightly extreme wavelengths. Besides that 99%, it is only for a narrow area of ​​the spectrum – it is misleading. Long live the coating with a single protective layer, even if we are at 96% reflection in the visible.”