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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

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 A) 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 A. 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.

Modern grating manufacturing techniques suffer from inherent issues that limit their peak efficiencies. We describe work in collaboration with the Nanofabrication Lab at Penn State University to design and characterize etched silicon gratings optimized for the extreme (EUV; 10 { 90 nm) and far ultraviolet (FUV; 90 { 180 nm) bandpasses. We develop this technology by fabricating a variety of gratings that operate over these bandpasses. We present analyses for two different grating designs in this work. The first is an FUV echelle that has similar parameters to the grating own on the CHESS sounding rocket. CHESS was an FUV spectrograph that utilized a mechanically ruled echelle grating. We compare the efficiency and in-instrument performance of the gratings, finding a ~ 50% increase in groove efficiency and an 80% decrease in inter-order scatter for the etched gratings compared to their mechanically ruled counterpart. The FUV echelle improvements can ultimately benefit the faint source sensitivity and high-resolution performance of future UV observatories, such as LUVOIR, by reducing the non-uniform inter-order backgrounds that have historically plagued echelle spectrographs. We additionally provide a description of how this lithographic process can be extended to gratings with holographic solutions by discussing our procedure for generating a map of groove traces from holographic recording parameters. This discussion is provided in the context of the creation of a grating sample that was developed in support of the ESCAPE Small Explorer Phase A study.

The Far-UV Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS) has been successful in maturing technologies for carrying out multi-object spectroscopy in the far-UV, including: the successful implementation of the Next Generation of Microshutter Arrays; large-area microchannel plate detectors; and an aspheric "dual-order" holographically ruled diffraction grating with curved, variably-spaced grooves with a laminar (rectangular) profile. These optical elements were used to construct an efficient and minimalist "two-bounce" spectro-telescope in a Gregorian configuration. However, the susceptibility to Lyman alpha (Ly$\alpha$) scatter inherent to the dual order design has been found to be intractably problematic, motivating our move to an "Off-Axis" design. OAxFORTIS will mitigate its susceptibility to Ly$\alpha$ by enclosing the optical path, so the detector only receives light from the grating. The new design reduces the collecting area by a factor of 2, but the overall effective area can be regained and improved through the use of new high efficiency reflective coatings, and with the use of a blazed diffraction grating. This latter key technology has been enabled by recent advancements in creating very high efficiency blazed gratings with impressive smoothness using electron beam lithography and chemical etching to create grooves in crystalline silicon. Here we discuss the derivation for the OAxFORTIS grating solution as well as methods used to transform the FORTIS holographic grating recording parameters (following the formalism of Noda et al.1974a,b), into curved and variably-spaced rulings required to drive the electron beam lithography write-head in three dimensions. We will also discuss the process for selecting silicon wafers with the proper orientation of the crystalline planes and give an update on our fabrication preparations., Comment: 10 pages, 6 figures, to appear in Proc. of SPIE Vol. 11821, UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXII

The Colorado Ultraviolet Transit Experiment (CUTE) is a near-UV (NUV), 6U CubeSat designed to characterize the interaction between exoplanetary atmospheres and their host stars. CUTE is dedicated to observing multiple transits of short period planets with a range of masses to measure the transit depths of atomic and molecular nearUV features. These observations will enable us to quantify as a function of wavelength the transit ingress, egress, and depth of exoplanet light curves in order to determine the presence of bow shocks and strong atmospheric mass loss. The CUTE optical system combines a novel rectangular Cassegrain telescope and a holographically-ruled, aberration-correcting diffraction grating. The high-throughput optical system is projected to obtain an average effective area of ≈24 cm 2 , comparable to previous Explorer class missions (GALEX) in a CubeSat package. This proceeding provides an overview of the science motivation, the final telescope and spacecraft design, and an outline of the mission operation.

In this proceeding, we describe the scientific motivation and technical development of the Colorado Highresolution Echelle Stellar Spectrograph (CHESS), focusing on the hardware advancements and testing of components for the fourth and final launch of the payload (CHESS-4). CHESS is a far ultraviolet rocket-borne instrument designed to study the atomic-to-molecular transitions within translucent cloud regions in the interstellar medium. CHESS is an objective echelle spectrograph, which uses a mechanically-ruled echelle and a powered (f/12.4) cross-dispersing grating; it is designed to achieve a resolving power R > 100,000 over the band pass λλ 1000–1600 A. CHESS-4 utilizes a 40 mm-diameter cross-strip anode readout microchannel plate detector, fabricated by Sensor Sciences LLC, to achieve high spatial resolution with high global count rate capabilities (∼ MHz). An error in the fabrication of the cross disperser limited the achievable resolution on previous launches of the payload to R ∼ 4000. To remedy this for CHESS-4, we physically stress the echelle grating, introducing a shallow toroidal curvature to the surface of the optic. Preliminary laboratory measurements of the resulting spectrum show a factor of 4–5 improvement to the resolving power. Results from final efficiency and reflectivity measurements for the optical components of CHESS-4 are presented, along with the pre-flight laboratory spectra and calibration results. CHESS-4 launched on 17 April 2018 aboard NASA/University of Colorado Boulder sounding rocket mission 36.333 UG. We present flight results for the observation of the γ Ara sightline.

Recent advances in the physical vapor deposition (PVD) of protective fluoride films have raised the far-ultraviolet (FUV: 912-1600 {\AA}) reflectivity of aluminum-based mirrors closer to the theoretical limit. The greatest gains, at more than 20%, have come for lithium fluoride-protected aluminum, which has the shortest wavelength cutoff of any conventional overcoat. Despite the success of the NASA FUSE mission, the use of lithium fluoride (LiF)-based optics is rare, as LiF is hygroscopic and requires handling procedures that can drive risk. With NASA now studying two large mission concepts for astronomy, Large UV-Optical-IR Surveyor (LUVOIR) and the Habitable Exoplanet Imaging Mission (HabEx), which mandate throughput down to 1000 {\AA}, the development of LiF-based coatings becomes crucial. This paper discusses steps that are being taken to qualify these new enhanced LiF-protected aluminum (eLiF) mirror coatings for flight. In addition to quantifying the hygroscopic degradation, we have developed a new method of protecting eLiF with an ultrathin (10-20 {\AA}) capping layer of a non-hygroscopic material to increase durability. We report on the performance of eLiF-based optics and assess the steps that need to be taken to qualify such coatings for LUVOIR, HabEx, and other FUV-sensitive space missions., Comment: Published by the Optical Society of America in Applied Optics

We present a progress report on the development of new broadband mirror coatings that demonstrate g 80% reflectivities from 1020−5000A. Four different coating recipes are presented as candidates for future far-ultraviolet (FUV) sensitive broadband observatories. Three samples were first coated with aluminum (Al) and lithium fluoride (LiF) at the NASA Goddard Space Flight Center (GSFC) using a new high-temperature physical vapor deposition (PVD) process. Two of these samples then had an ultrathin (10−20 A) protective coat of either magnesium fluoride (MgF2) or aluminum fluoride (AlF3) applied using atomic later deposition (ALD) at the NASA Jet Propulsion Laboratory (JPL). A fourth sample was coated with Al and a similar high temperature PVD coating of AlF3. Polarized reflectivities into the FUV for each sample were obtained through collaboration with the Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology. We present a procedure for using these reflectivities as a baseline for calculating the optical constants of each coating recipe. Given these results, we describe plans for improving our measurement methodology and techniques to develop and characterize these coating recipes for future FUV missions.

The University of Colorado ultraviolet sounding rocket program presents the motivation and design capabilities of the new Suborbital Imaging Spectrograph for Transition Region Irradiance from Nearby Exoplanet host stars (SISTINE). SISTINE is a pathfinder for future UV space instrumentation, incorporating advanced broadband refl ective mirror coatings and large format borosilicate microchannel plate detectors that address technology gaps identified by the NASA Cosmic Origins program. The optical design capitalizes on new capabilities enabled by these technologies to demonstrate optical pathlengths in a sounding rocket envelope that would otherwise require a prohibitive effective area penalty in the 1020 - 1150 A bandpass. This enables SISTINE to achieve high signal-to-noise observations of emission lines from planet-hosting dwarf stars with moderate spectral resolution (R ~ 10,000) and sub-arcsecond angular imaging. In this proceedings, we present the scientific motivation for a moderate resolution imaging spectrograph, the design of SISTINE, and the enabling technologies that make SISTINE, and future advanced FUV-sensitive instrumentation, possible.

In this proceeding, we describe the scientific motivation and technical development of the Colorado High-resolution Echelle Stellar Spectrograph (CHESS), focusing on the hardware advancements and testing supporting the second flight of the payload (CHESS-2). CHESS is a far ultraviolet (FUV) rocket-borne instrument designed to study the atomic-to-molecular transitions within translucent cloud regions in the interstellar medium (ISM). CHESS is an objective f/12.4 echelle spectrograph with resolving power $>$ 100,000 over the band pass 1000 $-$ 1600 {\AA}. The spectrograph was designed to employ an R2 echelle grating with "low" line density. We compare the FUV performance of experimental echelle etching processes (lithographically by LightSmyth, Inc. and etching via electron-beam technology by JPL Microdevices Laboratory) with traditional, mechanically-ruled gratings (Bach Research, Inc. and Richardson Gratings). The cross-dispersing grating, developed and ruled by Horiba Jobin-Yvon, is a holographically-ruled, "low" line density, powered optic with a toroidal surface curvature. Both gratings were coated with aluminum and lithium fluoride (Al+LiF) at Goddard Space Flight Center (GSFC). Results from final efficiency and reflectivity measurements for the optical components of CHESS-2 are presented. CHESS-2 utilizes a 40mm-diameter cross-strip anode readout microchannel plate (MCP) detector fabricated by Sensor Sciences, Inc., to achieve high spatial resolution with high count rate capabilities (global rates $>$ 1 MHz). We present pre-flight laboratory spectra and calibration results. CHESS-2 launched on 21 February 2016 aboard NASA/CU sounding rocket mission 36.297 UG. We observed the intervening ISM material along the sightline to $\epsilon$ Per and present initial characterization of the column densities, temperature, and kinematics of atomic and molecular species in the observation., Comment: 19 pages, 16 figures, to be submitted to SPIE Astronomical Telescopes + Instrumentation 2016 (9905-138)

We present the results of a preliminary aging study of new enhanced broadband reflectivity lithium fluoride mirror coatings under development at the thin films laboratory at GSFC. These coatings have demonstrated greater than 80% reflectivity from the Lyman ultraviolet (~1020 A) to the optical, and have the potential to revolutionize far-ultraviolet instrument design and capabilities. This work is part of a concept study in preparation for the fight qualification of these new coatings in a working astronomical environment. We outline the goals for TRL advancement, and discuss the instrument capabilities enabled by these high reflectivity broadband coatings on potential future space missions. We also present the early design of the first space experiment to utilize these coatings, the proposed University of Colorado sounding rocket payload SISTINE, and show how these new coatings make the science goals of SISTINE attainable on a suborbital platform.

Observations of molecular hydrogen (H$_2$) fluorescence are a potentially useful tool for measuring the H$_2$ abundance in exoplanet atmospheres. This emission was previously observed in M$\;$dwarfs with planetary systems. However, low signal-to-noise prevented a conclusive determination of its origin. Possible sources include exoplanetary atmospheres, circumstellar gas disks, and the stellar surface. We use observations from the "Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanet Host Stars" (MUSCLES) Treasury Survey to study H$_2$ fluorescence in M$\;$dwarfs. We detect fluorescence in Hubble Space Telescope spectra of 8/9 planet-hosting and 5/6 non-planet-hosting M$\;$dwarfs. The detection statistics, velocity centroids, and line widths of the emission suggest a stellar origin. We calculate H$_2$-to-stellar-ion flux ratios to compare flux levels between stars. For stars with planets, we find an average ratio of 1.7$\,\pm\,$0.9 using the fluxes of the brightest H$_2$ feature and two stellar C IV lines. This is compared to 0.9$\,\pm\,$0.4 for stars without planets, showing that the planet-hosting M$\;$dwarfs do not have significant excess H$_{2}$ emission. This claim is supported by the direct FUV imaging of GJ 832, where no fluorescence is observed at the expected star-planet separation. Additionally, the 3-$\sigma$ upper limit of 4.9$\,\times\,$10$^{-17}$ erg$\;$cm$^{-2}\;$s$^{-1}$ from these observations is two orders of magnitude below the spectroscopically-observed H$_2$ flux. We constrain the location of the fluorescing H$_2$ using 1D radiative transfer models and find that it could reside in starspots or a $\sim$2500-3000$\;$K region in the lower chromosphere. The presence of this emission could complicate efforts to quantify the atmospheric abundance of H$_2$ in exoplanets orbiting M$\;$dwarfs., Comment: 20 pages, 7 figures, 7 tables; Accepted by ApJ