Volume 7, Issue 6 - Space Optics / Guest Editors: Stefan Bäumer, Henri Werij

The James Webb Space Telescope (JWST) is frequently referred to as the follow-on mission to the Hubble Space Telescope (HST). The ‘Webb’ will be the biggest space telescope ever built and is expected to enable astounding new science. The observatory comprises a 6.5-m-diameter telescope with a segmented primary mirror and four high-performance optical science instruments. The JWST has mostly been optimized to work in the near- (0.6–5.0 μm) and mid-infrared (5.0–29 μm) wavelength regions. The project is a strong international partnership led by the National Aeronautics and Space Administration (NASA) with contributions from the European Space Agency (ESA) and the Canadian Space Agency (CSA). The observatory is currently scheduled for launch in early 2021 from Kourou, French Guyana, by an ESA-provided Ariane 5 rocket. This paper will focus on the European optical contribution to the mission, which mainly consists of two highly advanced optical science instruments: The multi-object near-infrared spectrograph (NIRSpec) and the mid-infrared instrument (MIRI). The opto-mechanical design considerations and the realization of both instruments will be described, and we will conclude with a short JWST project status report and future outlook.

The increase of spatial and temporal resolution for Earth observation (EO) is the ultimate driver for science and societal applications. However, the state-of-the-art EO missions like DigitalGlobe’s Worldview-3, are very costly. Moreover, this system has a high mass of 2800 kg and limited swath width of about 15 km which limits the temporal resolution. In this article, we present the status of the deployable space telescope (DST) project, which has been running for 6 years now at the Delft University of Technology, as a cutting-edge solution to solve this issue. Deployable optics have the potential of revolutionising the field of high resolution EO. By splitting up the primary mirror (M1) of a telescope into deployable segments and placing the secondary mirror (M2) on a deployable boom, the launch volume of a telescope can be reduced by a factor of 4 or more, allowing for much lower launch costs. This allows for larger EO constellations, providing image data with a much better revisit time than existing solutions. The DST specification baseline, based on Wordview-3, aims to provide images with a ground resolution of 25 cm (panchromatic 450–650 nm) from an orbital altitude of 500 km. In this paper, the current status of the optical, thermo-mechanical, and active optics systems design are described. The concurrent design approach combined with a strict bottom-up and top-down compliant systems engineering approach show that the DST is a healthy system concept.

The accurate simulation of stray light is essential for the verification of the contrast requirements in optical instruments. In a spectrometer, the scattering from reflective gratings is difficult to characterize while contributing significantly to the overall system stray light and reduction of the spectrometer contrast. In addition, the multiple diffraction orders create a ghost sensitive environment, which must be considered in the design of the instrument. In this article, we present an experimental setup for, and measurement results from, the characterization of the bidirectional scattering distribution function (BSDF) of a holographic grating for a spectrometer applied in a typical earth observation mission with demanding stray light requirements. We observed distinct stray light peaks out of the diffraction plane, which are called ‘satellites.’ The main challenges in the measurement of grating BSDFs arise from the near angle limit, the determination of the instrument signature and the selection of the appropriate sampling (2D or 3D). Following the grating characterization, the next step is to introduce these measured BSDFs into stray light simulation. We have done that by fitting appropriate functions to the measured BSDF and defining them in the optical analysis software ASAP as a user-defined BSDF. Ghost analysis is done at the spectrometer level as a sensitivity analysis of the tilts of the optical elements. Due to the ghosting of higher diffraction orders of the grating, a high sensitivity to the tilts of some of the optical elements can be seen.

The Absorbing Aerosol Sensor (AAS) will be launched aboard the GaoFen-5B satellite in China. The main purpose of AAS is to monitor absorbing aerosols by measuring the solar backscatter radiation. AAS is an ultraviolet-visible imaging spectrometer that uses a single charge coupled device to capture both the spectrum and the cross-track direction with a 114° wide swath. The large field of view enables daily global coverage with 4-km spatial resolution. The spectral range of the instrument extends from 340 to 550 nm with spectral resolution (full width at half maximum) of 2 nm. This paper provides details of the instrument design, including system design, optical design, and mechanical design, as well as detector and calibration unit on orbit. The numerous simulations show that all design results satisfy the specification and vibration requirements of the instrument.

Gravitational waves are a prediction of Einstein’s general relativity theory. In autumn 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO; https://www.ligo.caltech.edu/) experiment reported the first detection of gravitational waves in addition to electromagnetic radiation from the collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light and opens the door to a new type of astronomical observatory based on gravitational waves. The gravitational wave spectrum covers a broad span of frequencies and requires both space- and ground-based observatories to cover the full range. Space-based gravitational wave observatories, such as the proposed Laser Interferometer Space Antenna (LISA), operate at frequencies between 0.1 mHz and 1 Hz and complement the frequency range of 30–1000 Hz accessible by ground-based gravitational wave observatories, such as LIGO. A rich array of high-energy astrophysical sources is expected in the LISA measurement band. LISA was selected in 2017 as the third large mission of the Cosmic Vision program of the European Space Agency. The National Aeronautics and Space Administration will collaborate on both the scientific and technical aspects of this mission. This paper addresses the design of the optical telescope as an essential component of LISA’s long-distance interferometric measurement system.