The Rockets for Extended-source X-ray Spectroscopy (tREXS) is a suborbital rocket program that uses a wide-field grating spectrograph to obtain spectroscopic data on extended, soft-X-ray sources. The multi-channel tREXS spectrograph uses passive, mechanical focusing optics and stacks of reflection gratings to achieve a spectral resolution of R ≈ 50 from ≈15 – 40 Å over a >10 deg2 field of view. The dispersed spectra are read out by an array of 11 X-ray CMOS detectors that form a 97-megapixel focal-plane camera. tREXS was launched for the first time in September, 2022 to observe the Cygnus Loop supernova remnant. Though a failure in the rail pumping system led to a non-detection of emission from the Cygnus Loop during the flight, the rest of the instrument performed nominally and was recovered successfully. We present here an update on the instrument, results from the first flight, and a discussion of the future outlook.
This presentation "Fabrication of custom astronomical gratings for the next decade (and beyond)," took place during the conference on Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray, part of SPIE Astronomical Telescopes + Instrumentation symposium (2022).
Diffraction gratings used in ultraviolet astronomical spectrographs have been made using mechanical ruling or interference lithography. However, required performance for newly developed EUV (10-90 nm) and FUV (100-180 nm) spectrographs can benefit from groove densities, blaze angles, and low-scatter enabled with electron-beam lithography patterning and chemical etching. We report on the fabrication of custom grating prototypes developed at the Nanofabrication Laboratory at Penn State University. The gratings in development for the ESCAPE NASA Small Explorer (Univ. of Colorado/Boulder) involve writing specific patterns of curved grooves with variable line density on flat substrates. The design of the grating within the DEUCE sounding rocket payload involves writing straight grooves on a spherically curved substrate. All gratings are subsequently etched to achieve the specified blaze in the silicon. These efforts are enabling new applications in the field of astronomical UV spectroscopy.
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 Water Recovery X-Ray Rocket (WRXR) was a suborbital rocket payload that was launched and recovered in April 2018. The WRXR flew two technologies being developed for future large x-ray missions: x-ray reflection gratings and a hybrid CMOS detector (HCD). The large-format replicated gratings on the WRXR were measured in ground calibrations to have absolute single-order diffraction efficiency of ∼60 % , ∼50 % , and ∼35 % at CVI, OVII, and OVIII emission energies, respectively. The HCD was operated with ∼6 e − read noise and ∼88 eV energy resolution at 0.5 keV. The WRXR was also part of a two-payload campaign that successfully demonstrated NASA sounding rocket water recovery technology for science payloads. The primary instrument, a soft x-ray grating spectrometer, targeted diffuse emission from the Vela supernova remnant over a field-of-view >10 deg2. The flight data show that the detector was operational during flight and detected x-ray events from an on-board calibration source, but there was no definitive detection of x-ray events from Vela. Flight results are presented along with a discussion of factors that could have contributed to the null detection.
Astronomical X-ray diffraction gratings are a key technology under development for current and future NASA missions. X-ray reflection gratings, developed at Penn State University, have recently demonstrated both leading diffraction efficiency and high spectral resolving power. However, recent results are the result of different fabrication techniques and a single technique has not yet been developed to yield a grating that satisfies both the diffraction efficiency and resolving power required by future missions. Here we seek to leverage exiting electron-beam lithographic techniques to produce a grating with groove groove pattern capable of high resolving power. We then introduce new ion-milling techniques to create custom groove profiles capable of high diffraction efficiency. The goal is to produce a radial groove pattern with precisely blazed facets that are customizable based on ion mill input parameters. The process should be insensitive to groove density (ranging from ~150 nm to 400+ nm), facet size, and desired facet angle. Initial efforts in this study have concentrated on constraining various parameters in ion milling to fully characterize the effect of each parameter on the grating groove profile. We present here initial results and discuss experimental verification and future work.
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