The calibration setup used for IAS calibration is described in detail in a companion paper5. A previous configuration of this setup was used to calibrate the Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem (SIMBIO/SYS) instrument13. This setup was upgraded to meet all MAJIS calibration requirements. In particular, specific sources were implemented to fulfill the spectral calibration objectives previously mentioned. The OH and the main electronics were integrated into a thermal vacuum chamber. The OH itself was mounted on a hexapod housed in the thermovacuum chamber to allow the angular exploration of the FOV and the slit by the fixed beam from the calibration bench. The full entrance baffle of MAJIS was illuminated at IAS, while only the part of the first mirror corresponding to the entrance pupil was illuminated at LDO. The complete ground calibration setup has five optical paths, each having its own illumination source.
They are referred to as OP# with numbering from 1 to 5. Channels OP1, OP3, and OP5 were used for spectral calibration. OP1 is the dedicated spectral source equipped with a monochromator and two sources to cover the spectral range of MAJIS (Table II). While OP3 was initially dedicated to the radiometric calibration of the VISNIR channel, this optical path was used after having stopped the nitrogen flushing to introduce atmospheric contamination inside the calibration setup, leading to telluric features in the MAJIS spectra. OP5 is the ground calibration setup assigned to the reflectance measurements of samples (minerals and Spectralon). In addition, spectral calibration data were acquired with the two sources of the ICU (also referred to as OP0). Below, we describe these sources in more detail.
The OP1 source provides the monochromatic illumination thanks to a monochromator illuminated by a stabilized Quartz Tungsten Halogen (QTH) source for the 300 nm to 2000 nm range and a thermally stabilized blackbody for the 2000 nm to 6000 nm range. The monochromatic source can be spatially adjusted thanks to the monochromator’s output slit to ensure both the required flux and the spectral resolution. The parameters of the monochromatic source were optimized thanks to the monochromator’s grating choice and output slit width: a constant spectral width of 0.7 and 1 nm for the VISNIR and IR channels were selected, respectively. Then, an optical relay system and a collimator redirect the collimated OP1 beam toward the window of the TVC facing the MAJIS telescope entrance. The optical relay system consists of lenses and mirrors positioned to guide light from the source to the collimator. Given the magnification ratio of the collimator, the monochromator’s exit slit is imaged on about 20 nominal spatial pixels by MAJIS. Three FOV positions were explored to derive the spatial uniformity of the spectral response: nominal pixels 20, 200, and 380 out of the 400 introduced in Section I. These values correspond to the center of the FOV (position 200, boresight), bottom of the FOV (position 20, ~ -1,5°), and top of the FOV (position 380, ~ +1.5°).

The OP3 optical path was initially designed for the VISNIR radiometric calibration, but relevant data for assessing the spectral performances were also acquired with this setup. It consists of an integrating sphere connected to a QTH lamp as a light source to produce a flat field dedicated to the radiometric calibration of the VISNIR channel. An off-axis conical aluminum mirror collimates the sphere output. The output of the integrating sphere is large enough to cover the entire FOV of MAJIS.
The source and the collimating mirror are placed in an N2-flushed extension of the calibration optical tank directly in front of the CaF2 window of the TVC, limiting the 293 K thermal contribution and potential H2O signatures while offering a wide range