Wide-field off-axis telescope for the Mesospheric Airglow/Aerosol Tomography Spectroscopy satellite

Abstract

We present the development of a compact f ∕7.3 (D 35 mm) three-mirror reflective telescope for the atmospheric-research microsatellite Mesospheric Airglow/Aerosol Tomography Spectroscopy (MATS). The telescope design was driven by the end users' need for a reflective wide-field (5.67°× 0.91°) optic with high stray light rejection and six detection channels with separate image sensors, operating at wavelengths 270-772 nm. For the first time, a design method for wide-field off-axis telescopes-in which linear astigmatism is eliminatedwas applied and tested in practice. Single-point diamond turning was used to produce two sets of 37-110 mm large free-form aluminum mirrors with surface figure errors and roughness values of 34-62 nm (RMS)/193-497 nm (PV) and 2.8-3.5 nm (RMS), respectively. A method that combines precise machining and geometry measurements (using a coordinate measuring machine) was employed to fabricate an aluminum structure to accurately position the mirrors without the need for manual alignment. The telescope was tested with a network of plate beamsplitters and filters, which define the spectral selection for the six detection channels. Imaging performance measurements were carried out using a reflective off-axis collimator, which projects imaging targets at infinite focus. A modulation transfer function (MTF) value of 0.45 at 20 lp/mm was measured at ∼760 nm (diffraction limit: 0.85) using a slanted edge target. By modeling the measured mirror surfaces in optical design software, a reoptimization of the mirror positions could be performed and an improved MTF of ∼0.75 at 20 lp/mm was predicted. The results demonstrate design-and building methods that can be utilized to make off-axis telescopes for a vast range of applications.

The MATS satellite mission – gravity wave studies by Mesospheric Airglow/Aerosol Tomography and Spectroscopy

“…All gravity wave analysis from these nadir imagers is largely restricted to information about horizontal wavelengths in the observed layer. For NLCs, however, information about vertical structures has recently been obtained by applying tomographic methods to the different viewing angles available from the AIM/CIPS observations (Hart et al, 2018).…”

Development of Linear Astigmatism Free—Three Mirror System (LAF-TMS)

“…The resulting mirror shapes are freeform (Chang, 2019). diffraction limited performance in 0.532 µm wavelength because we perform conservative performance tests in visible wavelength ( Figure 2).…”

Flight model characterization of the wide-field off-axis telescope for the MATS satellite

“…probability. From the sensitivity analysis of the surface RMS errors, we expect that ∼0.03 MTF could be additionally degraded when taking count the fabricated surface RMS errors, which are 0.049, 0.034, and 0.062 µm for M1, M2, and M3, respectively (Hammar et al, 2019). Tilt and decenter errors can be compensated thanks to shims and L-brackets that are used to precisely position the mirrors and can be chosen in different thicknesses for relocations of the optical components.…”

“…Mesospheric Airglow/Aerosol Tomography Spectroscopy (MATS) is a Swedish microsatellite mission that observes noctilucent clouds (NLC) (80 -86 km altitudes) and O 2 atmospheric band dayglow/nightglow (75 -110 km altitudes) over wide field of view (5.67°× 0.91°) in two ultraviolet (UV) channels, and four infrared (IR) channels within wavelength range between 270 -772 nm (Gumbel et al, 2020). The main optical system of the MATS satellite is the limb-viewing telescope which is designed with a 35 mm entrance pupil diameter and a focal ratio of 7.4 (Hammar et al, 2018(Hammar et al, , 2019). The whole system includes three off-axis mirrors, beam splitters, broad/narrow bandpass filters, and six Charge-Coupled Devices (CCDs) in the same compact limb housing (see Figure 1).…”