Advanced hyperspectral imagery, as MAJIS does, implies a complex instrumental design. This results in several instrumental effects within the datasets to be characterized. To properly extract and derive the relevant scientific information, an in-depth ground characterization of the instrumental effects of MAJIS was performed from subsystems to integrated system4,5.
Imaging spectrometers are susceptible to optical distortions caused by optical aberrations and misalignments among optical elements. Even if these can be reduced by optimizing the optical layout and utilizing a mechanical mount to keep the optical elements in the correct positions, it is required to characterize the central wavelength position and the spectral response characterized by the Full Width at Half Maximum (FWHM) of each detector pixel after the mechanical and optical integration and the final alignment.

The optical distortions can also result in a smile effect6 (i.e., a wavelength drift along a detector row), and calibration variations can be sensitive to OH temperature (110-150K operative range for MAJIS). Therefore, the spectral calibration aims to assess these items with dedicated ground setups. The requirements for the MAJIS spectral calibration were driven by the MAJIS scientific performances summarized in Table I. However, this article provides a detailed analysis that is not limited to the validation of the requirements. As MAJIS oversampling capabilities can be used on any part of the spectral range (within the limitations mentioned in Section I.A), the strategy for the analysis was to focus on the instrumental performances of oversampled spectels. Then, the effects of the different types of binning on the performances are characterized for the different parameters. This allows us to deduce the performance of MAJIS in its various operational modes.
Table I. MAJIS spectral requirements for a nominal pixel (36-µm) in both directions (spectral and spatial, i.e., columns and rows on the detectors). These requirements shall be met across the entire operational thermal range of the instrument (110-150 K).

| Item | Metrics | Channel | MAJIS requirement |
| --- | --- | --- | --- |
| Absolute Calibration | Range | VISNIR | 500 - 2350 nm |
|  |  | IR | 2250 - 5540 nm |
|  | Sampling | VISNIR | 3.65 ± 0.25 nm |
|  |  | IR | 6.50 ± 0.60 nm |
| Spectral response | FWHM | VISNIR | ≤ 5.5 nm |
|  |  | IR | ≤ 10 nm |
| Spectral distortion along the FOV | Smile | VISNIR | ≤ 3.90 nm |
|  |  | IR | ≤ 7.10 nm |

The ground calibration measurements dedicated to MAJIS were based on the legacy of several previous hyperspectral space imaging spectrometers6,7,8,9. The overall ground calibration strategy at sub-systems and instrument levels is described in Poulet et al.4. Specifically, the ground calibration at the instrument level was performed during two main campaigns. The first campaign occurred at LeonarDO Company (LDO) facilities (Firenze, Italy) to characterize the performances of the OH at three 5 thermal configurations (nominal, hot, and cold cases)

in using the focal plane acquisition pipeline. The following main campaign, held at the Institut d’Astrophysique Spatiale (IAS) in Orsay, was designed to provide the measurements required for the spectral (this article), spatial10, and radiometric11 calibration in using the nominal acquisition pipeline with the flight spare unit of the main electronic5. The launch of the JUICE mission was followed by the Near-Earth Commissioning Phase (NECP).
During this commissioning, the MAJIS instrument acquired its first in-flight frames that could be used to monitor the consistency of the performances (including spectral ones) with the ground characterization.
Section II briefly describes the ground calibration setups and the datasets acquired with the different calibration sources (monochromatic scans, atmospheric measurements, internal calibration unit, sample, and mineral images). Section III introduces the methodology applied to the reduction of data before their analyses to retrieve the spectral performances of the instruments. Section IV presents the results of the calibration data analysis for each spectral requirement listed in Table 1, namely the spectral response, the spectral sampling, the spectral range, the smile, and the impact of the OH temperature on the spectral calibration. Using the internal calibration unit, a comparison between these performances and the first post-launch measurements is described in Section V. Finally, the spectral performances are summarized and discussed in Section VI.