Design and conceptualization of a multiband camera for Outer Solar System objects

Introduction
The exploration of the Outer Solar System offers unique scientific opportunities while posing significant engineering challenges. Missions targeting Outer Solar System bodies, such as icy moons (e.g., JUICE [1]), dwarf planets (e.g., New Horizons [2]), gas giants and their ring systems (e.g., Cassini [3]), require advanced instrumentation capable of operating in harsh environments while acquiring high-resolution data across a broad spectral range.  In particular, multi-spectral imaging systems play a crucial role in characterizing the composition, morphology, and geological history of these distant targets, enabling the identification of ices, minerals, and organic compounds, the detection of surface activity, and the reconstruction of their geologic histories. Instruments, such as JANUS [4], Ralph [5] and OSIRIS NAC and WAC [6], have demonstrated the critical role of such systems in advancing our understanding of planetary processes and the potential habitability of outer Solar System bodies.
Recent progress in detector technology has enabled the design of imaging systems capable of covering a much broader spectral range within a single optical channel, i.e. from visible to short-wave infrared [7],[8]. This innovation significantly enhances compactness and reduces payload mass, which are both crucial for deep-space missions.
This work presents the preliminary design and performance evaluation of a multi-spectral imaging payload covering the 400-2400 nm spectral range, tailored to meet the scientific objectives of Outer Solar System exploration.

Design and optimization
The optical design is based on an unobstructed Three-Mirror Anastigmat (TMA), with an entrance pupil diameter (EPD) of 200 mm, a field of view (FOV) of 1.14° × 1.14°, an instantaneous field of view (IFOV) of 10 µrad, and a broad spectral range from 400 to 2400 nm. A 2k x 2k pixel CHROMA-D detector from Teledyne e2v has been selected for the instrument[9]. The 18 µm pixel size, combined with the effective focal length (EFL) of 1800 mm determines a scale factor of 1 m/pixel at a target distance of 100 km. The resulting F-number is F/# = 9. 

Specification	Value
EPD	200 mm
EFL	1800 mm
F/#	9
FOV	1.14° x 1.14°
Detector format	2k x 2k pixels
Pixel size	18 µm
IFOV	10 µrad
Spectral range	400-2400 nm

Table 1. Optical prescriptions.

Beyond the instrument requirements, the optical system should also comply with several geometrical constraints. Specifically, it must include an accessible intermediate focal plane to accommodate a field stop for stray light mitigation. The exit pupil should likewise be accessible to allow the integration of two filter wheels and a cold stop.
During the optimization design process, multiple configurations met the instrument's constraints. Therefore, a trade-off analysis was conducted in order to identify the most suitable configuration based on the following criteria:

• maximizing optical performance in terms of spot diagram and MTF, ensuring optimal imaging quality;
• minimizing dimensions and bulks to reduce the payload’s mass and overall size;
• maximizing the available space around the intermediate focus and exit pupil, facilitating the design and integration of the field stop, filter wheels and cold stop;
• minimizing the exit pupil diameter to reduce the required filter size, consequently optimizing the filter wheels dimensions.

Figure 1. Raytrace diagram. The colors represent different fields. Key elements, such as the mirrors, field stop, filter, and focal plane, are highlighted. The exit pupil is located at the filter position.

Performances
The system provides diffraction-limited optical performance across the entire field of view. This all-reflective configuration is an optimal solution, given the wide spectral range over which it operates. The only contribution to chromatic aberration comes from the filter. 
Figure (2) presents the spot diagram computed using Zemax – OpticStudio. The square box has the size of 2x2 pixels (36 µm x 36 µm) while the circle represents the Airy disk at 400 nm, the shortest wavelength.

Figure 2. Spot Diagram evaluated in 8 different fields. The circles in the Spot diagram represent the Airy disk at 400 nm (Airy Radius = 4.55 µm). The square box's size is 2x2 pixels (36 µm x 36 µm).

Figure (3) shows the polychromatic diffraction MTF for the whole spectral range 400-2400 nm, in the eight fields considered. The maximum spatial frequency considered is relative to the Nyquist frequency, which is: 

MTF_nyq =(2 × pixel size)^-1=(2 × 18 μm× 10³)^-1= 27.7 cycles/mm

The theoretical diffraction-limited MTF value is 0.603, whereas the designed system achieves MTF values ranging from 0.543 to 0.596, depending on the field.
The MTF is always above 54% over the whole field of view for frequencies smaller than 27.7 cycles/mm.

Figure 3. Polychromatic diffraction MTF.

To assess the expected post-alignment performance, a Monte Carlo analysis with 1000 iterations was performed, accounting for tolerances on all key parameters, including optical element positioning, figure, and surface irregularities. The quality of each perturbed optical configuration was evaluated using a performance criterion based on the RMS wavefront error, with a threshold of 70 nm (λ/14 at 1000 nm), in accordance with the Marechal criterion. The results highlight the need for particularly stringent tolerances in the alignment of the primary mirror. 

The instrument is designed to capture narrowband images across the visible 400-2400 nm spectral range, using 22 narrowband and broadband filters selected based on scientific requirements. A pair of coupled filter wheels, each holding 11 filters plus one empty slot, has been considered.

Conclusion 
This work presents the preliminary design of an unobstructed Three-Mirror Anastigmat telescope, specifically developed for a mission to the outer Solar System. The camera is a multi-band imagery through a single optical channel, covering a broad spectral range from 400 to 2400 nm. The telescope is diffraction-limited across the entire field of view and spectral range.

Acknowledgements: This activity has been developed under the ASI/UniBo-CIRI agreement n. 2024-5-HH.0.

 

References
[1] Grasset, O. et al. (2013),PSS 78,1–21;
[2] Weaver, H. A. et al. (2008),SSR,140,75–91;
[3] Matson, D.L. et al. (2002),SSR,104,1–58;
[4] Palumbo, P., Della Corte V., Noci G., et al. (2025),SSR,221(3),1–25;
[5] Reuter, D. C., et al. (2008),SSR,140(1–4),129–154;
[6] Keller, H. U., et al. (2007),SSR,128(1–4),433–506;
[7] Fox, N. P., et al(2020),RemoteSensing,12(15),2400;
[8] Nieke, J., et al, Proceedings SPIE,12729,1272909.
[9] https://www.teledynespaceimaging.com/en-us/Products_/Pages/infrared-hgcdte-chroma-d.aspx