The Mid-infrared Instrument for JWST and Its In-flight Performance

The Mid-Infrared Instrument (MIRI) extends the reach of the James Webb Space Telescope (JWST) to 28.5 μm. It provides subarcsecond-resolution imaging, high sensitivity coronagraphy, and spectroscopy at resolutions of λ/Δλ ∼ 100-3500, with the high-resolution mode employing an integral field unit to provide spatial data cubes. The resulting broad suite of capabilities will enable huge advances in studies over this wavelength range. This overview describes the history of acquiring this capability for JWST. It discusses the basic attributes of the instrument optics, the detector arrays, and the cryocooler that keeps everything at approximately 7 K. It gives a short description of the data pipeline and of the instrument performance demonstrated during JWST commissioning. The bottom line is that the telescope and MIRI are both operating to the standards set by pre-launch predictions, and all of the MIRI capabilities are operating at, or even a bit better than, the level that had been expected. The paper is also designed to act as a roadmap to more detailed papers on different aspects of MIRI

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

Modelling the path length of aluminium seen by the detectors in the MIRI instrument on the JWST

The MIRI instrument on the James Webb Space Telescope is equipped with detectors which are susceptible to signal disruption by the charge deposited from impacting cosmic rays. In order to quantify the degree to which the structure of MIRI will shield the detectors, we have used an opto-mechanical ray tracing approach, whereby the solid bodies in a detailed 3D model of the instrument are substituted with an absorptive glassy material. By importing this modified model into a ray tracing program (Tracepro) and then launching many rays from the detector, we have been able to generate a map of aluminium path length as a function of direction. We find that there is a minimum thickness of 2 to 3 mm over a few patches which subtend no more than 1.5 % of the sky for the worst case, imager detector. We discuss the performance of the shielding provided by the MIRI structure, concluding that this minimum thickness of aluminium is sufficient to suppress the impact of low energy protons below the level of the unavoidable flux due to high energy cosmic rays.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Space-compatible strain gauges as an integration aid for the James Webb Space Telescope Mid-Infrared Instrument

Space instruments are designed to be highly optimised, mass efficient hardware required to operate in extreme environments. Building and testing is extremely costly, and damage that appears to have no impact on performance at normal ambient conditions can have disastrous implications when in operation. The Mid-Infrared Instrument is one of four instruments to be used on the James Webb Space Telescope which is due for launch in 2018. This telescope will be successor to the Hubble Space Telescope and is the largest space-based astronomy project ever to be conceived. Critical to operation of the Mid-Infrared Instrument is its primary structure, which provides both a stable platform and thermal isolation for the scientific instruments. The primary structure contains strain-absorbing flexures and this article summarises how these have been instrumented with a novel strain gauge system designed to protect the structure from damage. Compatible with space flight requirements, the gauges have been used in both ambient and cryogenic environments and were successfully used to support various tasks including integration to the spacecraft. The article also discusses limitations to using the strain gauge instrumentation and other implications that should be considered if such a system is to be used for similar applications in future

Design and implementation of a prototype head and neck phantom for the performance evaluation of gamma imaging systems

BACKGROUND: A prototype anthropomorphic head and neck phantom has been designed to simulate the adult head and neck anatomy including some internal organs and tissues of interest, such as thyroid gland and sentinel lymph nodes (SLNs). The design of the head and neck phantom includes an inner jig holding the simulated SLNs and thyroid gland. The thyroid gland structure was manufactured using three-dimensional (3D) printing taking into consideration the morphology and shape of a healthy adult thyroid gland. RESULT: The head and neck phantom was employed to simulate a situation where there are four SLNs distributed at two different vertical levels and at two depths within the neck. Contrast to noise ratio (CNR) calculations were performed for the detected SLNs at an 80 mm distance between both pinhole collimators (0.5 and 1.0 mm diameters) and the surface of the head and neck phantom with a 100 s acquisition time. The recorded CNR values for the simulated SLNs are higher when the hybrid gamma camera (HGC) was fitted with the 1.0 mm diameter pinhole collimator. For instance, the recorded CNR values for the superficially simulated SLN (15 mm depth) containing 0.1 MBq of (99m)Tc using 0.5 and 1.0 mm diameter pinhole collimators are 6.48 and 16.42, respectively (~87% difference). Gamma and hybrid optical images were acquired using the HGC for the simulated thyroid gland. The count profiles through the middle of the simulated thyroid gland images provided by both pinhole collimators were obtained. The HGC could clearly differentiate the individual peaks of both thyroid lobes in the gamma image produced by the 0.5-mm pinhole collimator. In contrast, the recorded count profile for the acquired image using the 1.0-mm-diameter pinhole collimator showed broader peaks for both lobes, reflecting the degradation of the spatial resolution with increasing the diameter of the pinhole collimator. CONCLUSIONS: This anthropomorphic head and neck phantom provides a valuable tool for assessing the imaging ability of gamma cameras used for imaging the head and neck region. The standardisation of test phantoms for SFOV gamma systems will provide an opportunity to collect data across various medical centres. The phantom described is cost effective, reproducible, flexible and anatomically representative

Elfen: A CubeSat mission to measure heavy ions on both the dayside and nightside of Earth's magnetosphere

International audienceThe Elfen mission has two science goals: (1) To understand the heavy ion composition of the solar wind and its influence on the dayside of the Earth’s magnetosphere, (2) To understand the origin of and processing involving ions in the plasma sheet. We propose a mission using two instruments; a mass spectrometer to determine the in situ heavy ion composition, and a magnetometer to help determine the contemporaneous plasma regime of the Earth’s magnetosphere traversed by the spacecraft.The influence of solar wind heavy ions, such as C6+, or O7+ in the solar-terrestrial system is poorly understood, and no dedicated mission to understand these ions has been undertaken. It is unknown, for example, if charged iron atoms in the magnetosphere are of ionospheric or solar wind origin, and of their influence on the mesosphere. Recently analysis from Cluster has suggested that both the inflow and outflow contribute to ions in the magnetosphere. Ion spatial segregation, particularly in areas such as the magnetospheric cusps, has unknown influence on magnetosphere-ionosphere coupling. In addition, temporal changes in solar wind composition will lead to changes in the total X-ray emission via charge exchange that can occur on impact of an ion with the neutral hydrogen exosphere. This has implications for current and future X-ray imaging missions, such as the joint ESA and Chinese Academy of Sciences SMILE mission, or for NASA’s LEXI mission. Heavy ion processes in the plasma sheet have only been sampled by an extremely limited number of previous missions. The origin of the plasma sheet is hotly debated. Diffusion may occur through the flanks of the magnetosheath, e.g. through the Kelvin Helmholtz instability, or ions may be injected via nightside reconnection in the tail. Alternatively, plasma may be injected sporadically under certain magnetospheric conditions.Elfen, in its current configuration as a ~16U CubeSat, from its 12 RE circular orbit, will make composition measurements at 10 second cadence on the nightside, and at a minimum of 1 minute on the dayside. The magnetometer will have sensitivity of 2 nT at a cadence of 10 Hz. The Elfen team comprises partners from the UK, US, France, and Norway. We will present a recent Concurrent Design Facility study that has significantly matured the initial science concept. See: https://elfen.le.ac.uk

Elfen: A CubeSat mission to measure heavy ions on both the dayside and nightside of Earth's magnetosphere

International audienceThe Elfen mission has two science goals: (1) To understand the heavy ion composition of the solar wind and its influence on the dayside of the Earth’s magnetosphere, (2) To understand the origin of and processing involving ions in the plasma sheet. We propose a mission using two instruments; a mass spectrometer to determine the in situ heavy ion composition, and a magnetometer to help determine the contemporaneous plasma regime of the Earth’s magnetosphere traversed by the spacecraft.The influence of solar wind heavy ions, such as C6+, or O7+ in the solar-terrestrial system is poorly understood, and no dedicated mission to understand these ions has been undertaken. It is unknown, for example, if charged iron atoms in the magnetosphere are of ionospheric or solar wind origin, and of their influence on the mesosphere. Recently analysis from Cluster has suggested that both the inflow and outflow contribute to ions in the magnetosphere. Ion spatial segregation, particularly in areas such as the magnetospheric cusps, has unknown influence on magnetosphere-ionosphere coupling. In addition, temporal changes in solar wind composition will lead to changes in the total X-ray emission via charge exchange that can occur on impact of an ion with the neutral hydrogen exosphere. This has implications for current and future X-ray imaging missions, such as the joint ESA and Chinese Academy of Sciences SMILE mission, or for NASA’s LEXI mission. Heavy ion processes in the plasma sheet have only been sampled by an extremely limited number of previous missions. The origin of the plasma sheet is hotly debated. Diffusion may occur through the flanks of the magnetosheath, e.g. through the Kelvin Helmholtz instability, or ions may be injected via nightside reconnection in the tail. Alternatively, plasma may be injected sporadically under certain magnetospheric conditions.Elfen, in its current configuration as a ~16U CubeSat, from its 12 RE circular orbit, will make composition measurements at 10 second cadence on the nightside, and at a minimum of 1 minute on the dayside. The magnetometer will have sensitivity of 2 nT at a cadence of 10 Hz. The Elfen team comprises partners from the UK, US, France, and Norway. We will present a recent Concurrent Design Facility study that has significantly matured the initial science concept. See: https://elfen.le.ac.uk