New far-UV instrumentation enabled by recent advances in mirror coating processes

Recent technological advances have opened up new instrument capabilities for ultraviolet (UV) astronomy. Of particular interest are advanced deposition processes that have increased the performance of Lithium Fluoride (LiF) overcoated mirrors while mitigating the procedures required for their handling, raising the reflectivity from ∼ 65% to greater than 80% in the Lyman UV (λ > 1000 A). Traditional magnesium fluride (MgF 2 ) protected aluminum mirrors have a reflectivity truncated at 1150 A, missing crucial tracers of warm gas and molecules. The hygroscopic sensitivity of LiF has traditionally added to mission risk and cost. The addition of a thin capping layer of another material (AlF 3 , MgF 2 ) on top of the LiF has been shown to mitigate the degradation by providing a barrier against moisture. These advances open up a new paradigm in UV astronomy by enabling multi-passed optical designs without the crippling 1000 - 1150 A throughput losses inherent to conventional mirror coatings. We present recent progress in the testing of enhanced lithium fluoride (eLiF) coated optics, and discuss potential instrument concepts for UV astronomy in the next decade.