Freeform Catoptric Design of Wide-Field-of-View Spectral Imagers for UV Spaceborne Observation of Mars
Vrije Universiteit Brussel
2026
Wide-field-of-view (WFOV) imaging systems are highly valued for space-based
observation. They are particularly challenging to design due to the presence of
off-axis aberrations. This thesis presents three novel wide-field-of-view (WFOV)
ultraviolet (UV) imaging architectures for the Mars Imager for Dust and Ice
clouds Monitoring (MIDIM): a 2D non-scanning multispectral imager, a 2D
scanning multispectral imager, and a 1D pushbroom multispectral imager. The UV
spectral range is intended and optimized for climate monitoring of Mars, including
atmospheric circulation and dynamics, dust cycles and storms, and water cycles,
clouds, and atmospheric chemistry.
Our research work tackles the optical design of WFOV UV imagers, exploring
different configurations and benefiting from freeform optical design. The key
challenges we faced were dealing with the off-axis aberrations due to the wide
fields, vignetting, and angle-of-incidence (AOI) reduction, while still keeping the
designs manufacturable and within the packaging requirements. Furthermore, the
UV operation range further increased the complexity and difficulty of the design as
the optical throughput needs to be increased due to low radiometric signals paired
with the small Airy disk compared to larger wavelength ranges. Finally, to ensure a
manufacturable design, a design-for-manufacturing approach was adopted, taking
system tolerances into account.
The 2D scanning multispectral imager showed the most optimal performance
despite its increased system complexity, such as the inclusion of a scan mirror,
while the 2D non-scanning multispectral imager showed the worst nominal
and radiometric performance. The 1D pushbroom multispectral imager showed
a comparable nominal performance to the 2D scanning system. However,
it lacked the tolerance robustness. The 2D scanning multispectral imager
shows an unprecedented performance featuring a WFOV of 48° × 10°, spatial
resolution of 2.50 km, F/# of 4.63, 80% of the encircled energy confined within
a radius of 2.15 µm, distortion of 7.30%, MTF ≥ 0.91 at 50 cycles/mm, volume of
170 × 150× 135 mm³, and smeared spatial resolutions between 5.96 km and
12.06 km for an SNR of 100 at 220 nm on Mars.
This work presents the full optical design methodology and results, adopting a
design for manufacturing approach, taking into account the path towards further
prototyping of the design. Consequently, with our research, we hope to pave
the way towards a novel UV space-based imaging telescope, enhancing space
exploration.
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