The monochromatic measurements with IAS and LDO OP1 setups consist of several series of frames captured while receiving a monochromatic beam with a full width at half maximum several times smaller than the expected spectral response FWHM. For each position of the monochromator scan, one or more frames were captured in order to determine the correlation between the selected wavelength and the observed signal as a function of the position along the MAJIS spectral axis. Moreover, each scan across the wavelength range of the detector aims to characterize the spectral response of the illuminated spectels and determine the absolute central wavelength. A tradeoff taking into consideration the signal-to-noise of the data, the capability of setups, and the required number of measurements to derive the spectral response of a given spectel with acceptable accuracy led to performing a spectral scan with a scan step of ~1/5 of the expected spectral sampling of the instrument (i.e., 0.7 nm (VISNIR channel) or 1 nm (IR channel)) for the IAS data and 0.5 nm (VISNIR channel) or 1 nm (IR channel) for LDO data5.
An example of a frame acquired with the IAS OP1 setup is shown in Fig. 3 (A). The accuracy of the measurements depends on the accuracy of the monochromatic sources. The absolute accuracy is 1 nm (resp. 0.05 nm) for both IAS and LDO setups (relative accuracy). The monochromator spots were around 20 physical pixels wide (14 pixels for the IAS setup with the VISNIR channel) in the along-slit direction (which is equivalent to 10 nominal pixels out of the 400 nominal FOV pixels). Fig.
3 (B) displays the spectra derived from sequential measurements during a monochromatic scan. The generation of these spectra can be summarized in two steps. First, the quasi-monochromatic signal from the monochromator is convolved with the spectral response of MAJIS, creating a combined spectrum. This combined spectrum is then sampled by MAJIS to produce the final spectra shown.

[Figure: Fig. 3. Panel A: Illustration of a monochromatic acquisition around 700 nm at the FOV position 200 (nominal). The main central spot (between rows ~38 to ~70) of the monochromator is spectrally and radiometrically uniform for about 20 physical pixels along-slit. Panel B:]

Example of spectra of 10 median images (out of 51) from a monochromatic scan around 900 nm at the FOV position 200.
Due to the setup differences between IAS and LDO (see Section II.A.), the OP1 light source only covered the entrance pupil of the OH at LDO facilities, whereas the full entrance baffle was covered by a light source from the setup at IAS. This was not expected to have an impact on the resulting acquisitions. However, due to issues from either the design or the assembly of the instrument, residual stray light effects can be seen (mainly on the VISNIR channel). Therefore, light originating from outside the entrance pupil can be seen on the acquisitions. These imperfections hint at the possibility that the actual performances of the instrument will differ from the performances when only the entrance pupil receives light, especially for the VISNIR channel.
Table IV and Table V describe the datasets acquired during IAS calibration and LDO characterization. The spectral and spatial coverage is also illustrated in Fig. 4. As seen on this figure, the number of spectral intervals and positions inside the FOV that were explored was restricted because of the planning constraint. Achieving a good signal-to-noise for wavelengths beyond 3.6 µm was also very challenging due to the thermal background and the low level of the source flux. The mitigation of these limitations was to complement monochromatic measurements with polychromatic measurements over solid samples and gas, as described in the following section.

Table IV. Monochromator scan measurements series from IAS OP1