Additional MAJIS calibration measurements useful to the spectral calibration are described in this section. They are separated into three subsections corresponding to three different OPs and are described in Table VI.
Table VI. Atmospheric measurements series main parameters

| Measurement type | Channel | OH Temperature (K) | Position |
| --- | --- | --- | --- |
| Atmospheric | VISNIR / IR | 126, 137 | Full FOV |
| Internal Calibration Unit | VISNIR / IR | 126, 137 | Full FOV |
| Solid Samples | VISNIR / IR | 126, 137 | 20, 200, 380 |

a. Atmospheric absorption OP35 was purposely used after the flushing was stopped so that atmospheric gases such as water vapor and carbon dioxide could be identified in the MAJIS frames. In addition, CO2 gas was flushed in OP3 to increase its concentration along its 1-m long optical path so as to increase the atmospheric signatures beyond 3 µm. The positions of the telluric signatures provide unique reference points for the absolute calibration of MAJIS. The High Resolution Transmission (HITRAN)15 model was used to calibrate the absolute wavelength positions of the various signatures observed with the measurements. Note, however, that the integrating sphere of OP3 was adapted to characterize the instrument in the VISNIR range (see Section II.A.2). This limits the capability to identify atmospheric absorption bands in the longest IR wavelengths where the flux is dominated by the 14 thermal contribution of the optical elements that are at ambient temperatures.

Conversely, these acquisitions have the advantage of covering the full FOV in one acquisition.
b. Internal Calibration Unit The ICU acquisitions (OP0) provided additional reference measurements for the absolute calibration thanks to the didymium and polystyrene signatures, and in this respect, they were used for the on-ground spectral calibration. Samples of polystyrene similar to the ICU material were spectrally characterized at the Istituto di Astrofisica e Planetologia Spaziali (IAPS, Rome) facilities in the same temperature range to allow comparison with MAJIS measurements16. A series of specific measurements as part of the Full Function Tests (FFT) provides a reference benchmark for the ICU signals. These tests were performed during the on-ground calibration of MAJIS at the two thermal configurations (126 K,137 K). As stated before, the ICU measurements are also used as an absolute reference for monitoring the in-flight performances. The in-flight tests can be thus compared to the on-ground calibration to monitor any potential change between pre-flight and after-launch over the entire FOV (Section V).
c. Solid Samples The other data set of interest for the spectral calibration was acquired with OP5 illuminating different solid samples with a QTH lamp. The samples were spectrally characterized before and after the calibration campaign using a a PerkinElmer FTIR spectrometer. Hyperspectral images of the samples were acquired by scanning them with the hexapod along the direction orthogonal to the MAJIS slit or by commanding the internal scan mirror. Samples’ spectra were then extracted and compared to the FTIR spectra (Section III.A.C.3). A companion paper details these measurements and their analyses17. Here we focus on the use of this data set for the purposes of the spectral calibration. The spectral calibration analyses rely primarily on two types of images. For the VISNIR channel, acquisitions with the WCS18 from Labsphere provide the most relevant observation since it exhibits many reference spectral signatures that can be used for the spectral calibration. Most signatures are present at wavelengths < 2.1 µm (see Fig. 5), making the WCS suitable for calibrating the VISNIR channel of MAJIS. Calcite acquisitions were relevant to cover a part of the IR channel. Note that windowing has been routinely applied during these acquisitions.

[Figure: Fig. 5. FTIR spectra of the two solid samples used for the spectral calibration (See text for details).]