The Twin ANthropogenic Greenhouse Gas Observers (TANGO) mission will monitor and quantify greenhouse gas emissions at the level of individual facilities. A consortium consisting of ISISpace, TNO, SRON and KNMI are developing the TANGO mission for the ESA Scout program. ISISpace is the prime contractor and responsible for the spacecraft, SRON and KNMI are responsible for the atmospheric science, while TNO is developing the instruments. The TANGO space segment consists of two agile 16U CubeSat satellites flying closely in tandem, each equipped with an imaging spectrometer. TANGO Carbon measures the emission of CH₄ and CO₂ in the SWIR1 spectral band (1590-1675 nm at 0.45-nm spectral resolution), while TANGO Nitro measures the emission of NO₂ in the visible spectral range (405- 490 nm at 0.6-nm spectral resolution). Both instruments are reflective pushbroom spectrometers, made almost entirely from aluminum, and will cover a 30-km swath from a 500-km altitude with a spatial resolution of 300 m. The instruments share a similar architecture, using freeform mirrors to achieve high optical performance in a compact 8U envelope. In this paper, we will present the design and performance of the Carbon instrument, where a key engineering challenge is to achieve the desired spatial resolution and SNR from the limited instrument volume (8U). A tight integration of optical and mechanical design, coupled to detailed tolerance, alignment, straylight and STOP (structural thermal optical performance) analyses, allow us to reach that goal.
Spatial heterodyne spectroscopy has become increasingly attractive for remote sensing of the atmosphere from microsatellites. Its outstanding light gathering power makes this technology particularly suitable for the detection of faint signals with minimal volume requirements. This paper is about an instrument, which was designed to measure the spectral shape of an atmospheric oxygen emission. The near infrared emission is observed in limb viewing geometry from space. The optical setup and specific characteristics of the design are presented. A focus is on the straylight behaviour of the system. In-field and out-of-field contributions are discussed. Straylight kernels are applied to expected background radiation fields with regard to performance-limiting factors of the system.
Jena-Optronik GmbH is developing the Sentinel-5 NIR spectrometer optics operating in the spectral range from 685 nm to 773 nm. It is purely based on refractive optics and makes use of a grating as dispersive element and a prism for aberration correction. One of the design drivers for the optics design is straylight as small absorption effects in the atmosphere need to be detected. To minimize false light on the detector the straylight characteristics have been analyzed and optimized together with Hembach Photonik GmbH. Special attention needs to be paid to the dispersive nature of the optics as compared to a pure imaging system. Sources of ghosts and scattered light such as surface roughness, particle contamination and scattering at mechanical surfaces are quantified. Main contributors to straylight are identified and strategies for straylight reduction are discussed. Among others this comprises effective means to suppress out-of-band light.
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