The optical unit of MAJIS is made of two co-aligned spectral channels4. One is a VISible to Near-InfraRed (VISNIR) channel, and one is an InfraRed (IR) channel. The Optical Head (OH) of MAJIS incorporates a Three Mirror Anastigmat (TMA) telescope shared between the two channels. A slit and a collimator, common to both channels, control light input and ensure parallel propagation towards a dichroic filter. This filter splits the light into the two channels. Each channel is equipped with a grating following a quasi-linear dispersion law and a focusing lens for spectral analysis. Both channels collect spectral images of a common slit on their Focal Plane Arrays (FPAs). Due to optical aberrations in the OH of the imaging spectrometer, a spectral shift (smile effect) of the sensor over its entire Field Of View (FOV) is present. (e.g., imaging a homogeneous target over the full FOV can result in misaligned spectra for different FOV positions). The FPAs are 1024×1024 pixels H1RG Teledyne detectors with an 18×18 µm² pitch. The row and column directions correspond to the spectral and spatial dimensions, respectively. Any pixel will be referred to as “spectel” hereafter when its spectral properties are considered. During the nominal scientific operations, these 18 µm pixels are binned by two in the spatial direction or more (spatial and/or spectral direction) depending on the selected spectral or spatial binning commanded by telemetry (see below). Thus, in the following sections of this article, “nominal pixel” will refer to four physical pixels binned in a two-by-two square (i.e., 36 µm wide). The instrument’s FOV is projected onto the two FPAs across 800 contiguous rows of 18 µm pixels; the wavelengths are dispersed across 1016 columns of 18 µm spectels. After nominal binning, this results in a nominal frame acquisition of size 400×508 pixels for each channel.

[Figure: Fig. 1. Schematic view summarizing the nomenclatures related to the FOV and spectel positions of MAJIS. Positions (in pixel) inside of the FOV are exemplified. The positions along the FOV are given in terms of nominal pixels (blue), physical pixels (green), and angular position (red).]

MAJIS has the capability to perform spectral oversampling and binning of its nominal pixels over specific spectral ranges (referred to as spectral bands)4. Oversampled pixels (18 µm) are not binned by two over the spectral dimension. The binning ×2 and binning ×4 modes allow the physical pixels (18 µm) to be grouped by 4 and 8, respectively. Up to 16 spectral bands with different oversampling/binning modes can be defined per channel during one observation. A specific type of oversampling/binning is implemented for each spectral range. Oversampling can increase the number of spectels per frame to a maximum of 640 pixels instead of the nominal 508. Higher spectral sampling rates will provide more detailed spectral information. However, this must be balanced with the collected data quality regarding the Signal-to-Noise Ratio (SNR) and the data volume of the observations.
MAJIS also has an acquisition mode called No Process. This mode transmits the 1024 by 1024 raw images. This mode will be infrequently utilized during science operations due to its large data size and inefficient use of memory space/data bandwidth (> 200 by 1024 pixels provide no usable data). However, during ground calibration, these disadvantages are nullified.

Furthermore, the data provided by this mode can easily be numerically post-processed (e.g., numerical binning) to simulate any acquisition settings used in flight and derive its associated spectral calibration performances. Therefore, the vast majority of ground calibration measurement series was performed using the No Process mode. This article will describe the spectral calibration performances for the nominal, oversampling, binning ×2, and binning ×4 modes using data acquired with the No Process acquisitions.

[Figure: Fig. 2. Example of a MAJIS frame summarizing its oversampling/binning modes capabilities. The resulting frame (up to 640 spectels) can combine up to 16 spectral bands with a specific spectral mode defined by TC (TeleCommand) for each spectral band.]

A scanning mirror at the entrance of the telescope provides the capability to change the instrument’s pointing through a 4° across-slit optical angle centered around the boresight. This scanning mirror can also point to the Internal Calibration Unit (ICU) light sources4 that provide relevant data to check the fly performances of the instrument.