This conference presentation was prepared for the Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray conference at SPIE Astronomical Telescopes + Instrumentation, 2022.
The Suborbital Imaging Spectrograph for Transition-region Irradiance from Nearby Exoplanet host stars (SIS- TINE) sounding rocket payload is an f /30 imaging spectrograph designed to measure the far ultraviolet (1000 - 1275 and 1300 - 1565 Å) output of exoplanet host stars. The instrument is composed of an f /14 Cassegrain telescope with a 500 mm diameter primary mirror feeding a 2.1x magnifying spectrograph. Light is dispersed by a blazed, holographically ruled grating, reflected off a powered fold mirror, and recorded on a large format microchannel plate (MCP) detector. The instrument incorporates enhanced LiF (eLiF) protected aluminum on the primary, secondary, and fold mirrors. The secondary mirror also has a protective AlF3 capping layer, applied using atomic layer deposition (ALD). The detector is composed of two windowless 113 x 42 mm segments with cross delay line anodes and CsI photocathodes. The detector utilizes ALD processed borosilicate plates, and additionally serves as a flight test for detectors on future astrophysics missions. The instrument reaches a peak effective area of 99.9 cm2 at 1145 Å. The assembly of SISTINE-2 included the application of new photocathodes to the detector, showing improvements in quantum efficiency after laboratory tests. SISTINE-2 will observe the nearby F star Procyon in late 2021, making the first simultaneous observation from O VI through C IV and setting new empirical constraints on the radiation fields experienced by planets orbiting mid-F stars.
We describe new results from the ALD application of fluoride mirror coatings that offer high performance over the UV bandpass (90 -235 nm). Such coatings provide an alternative to PVD and have broad applications in the UV, particularly for UV astronomy. The fabrication process is carried out at the JPL Microdevices Lab, beginning with an optically thick layer of evaporated Al which is capped with an ALD protective fluoride layer, such as LiF. Capping the Al is necessary because it otherwise oxidizes quickly and loses significant reflectance. Immediately before applying the ALD protective thin film we use atomic layer etching (ALE) to remove any native oxides that manifested after Al deposition. FUV reflectivity measurements are conducted in the vacuum ultraviolet space hardware characterization facilities at the University of Colorado at Boulder. We present the results of reflectivity testing of five samples from 90 to 230 nm; samples include Al+LiF (3), Al+LiF+MgF2 (1), and Al+LiF+LiF3 (1). We vary the number of ALE and ALD cycles across several Al+LiF samples to begin exploring the optimal amount of etching and film thickness. Preliminary results confirm <80% reflectivity at 1200 Å with one Al+LiF sample yielding 86 ± 2%. Future work will include testing how layer thickness and ALD deposition temperature impact FUV reflectivity with a focus on optimizing reflectivity below Lyβ.
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