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 Å). Traditional magnesium fluride (MgF2) protected aluminum mirrors have a reflectivity truncated at 1150 Å, 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 (AlF3, MgF2) 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 Å 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.
We present the design for a far-UV integral field spectrograph for astronomy enabled by two-axis digital micromirror arrays. Techniques used for integral field spectroscopy in the Optical-IR bandpass either do not apply to the far-UV due to low material transmission, or have other UV-specific challenges. In order to circumvent this limitation, we have designed an all-reflective method of dynamically reformatting the focal plane of a telescope with micro-opto-electro-mechanical systems (MOEMS). The Adaptive Micromirror Array Demonstration Experiment for Ultraviolet Spectroscopy (AMADEUS) is a benchtop far-UV/Optical spectrograph designed to demonstrate that the stability, repeatability, and scattered light contamination are all sufficiently controlable to use these devices in a high sensitivity astronomical instrument. The use of MOEMS devices enables the focal plane mapping to be reconfigured at will, providing some field sampling and path length control advantages relative to conventional Optical/IR techniques for integral field spectroscopy. We report on the design of AMADEUS and present a spectrograph concept for a future sub-orbital mission.
We present a progress report on the development of new broadband mirror coatings that demonstrate ⪆ 80% reflectivities from 1020−5000Å. 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 Å) 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.
This paper presents the design and theory of operation for a semi-automated reflectivity chamber for ultraviolet optimized optics. A graphical user interface designed in LabVIEW controls the stages, interfaces with the detector system, takes semi-autonomous measurements, and monitors the system in case of error. Samples and an optical photodiode sit on an optics plate mounted to a rotation stage in the middle of the vacuum chamber. The optics plate rotates the samples and diode between an incident and reflected position to measure the absolute reflectivity of the samples at wavelengths limited by the monochromator operational bandpass of 70 nm to 550 nm. A collimating parabolic mirror on a fine steering tip-tilt motor enables beam steering for detector peak-ups. This chamber is designed to take measurements rapidly and with minimal oversight, increasing lab efficiency for high cadence and high accuracy vacuum UV reflectivity measurements.