| 2.991 | 4.093e-4 | -3.138e-7 | 1.502e-10 |
| Option 2 | VISNIR | 491 – 2358 | 0.7 | 4.902e2 | 1.768 | 3.639e-4 | -5.518e-7 | 2.604e-10 |

IR 2271 – 5555 2 2.270e3 2.991 3.801e-4 -2.536e-7 1.170e-10

[Figure: Fig. 11. VISNIR absolute spectral calibration (left panels: option 1; righ panels: option 2) for the FOV position 200 at the nominal temperature. These panels compare spectel numbers to their associated CWL. (A) Fourth-degree polynomial fit of the central positions of the spectels for the VISNIR channel with all calibration data considered (“option 1”, see text for details). (B) Comparison between the derived CWL and the measurements. One box corresponds to one series of measurements (i.e., multiple data points). The box size in the X axis indicates the spectral range of the measurement series, and the box size in the Y direction shows the error in the spectral position of the data points. The position of the middle line represents the fitted model considered. As the wavelength increases, so do the spectel numbers. (C) and (D) A & B respectively, but for a selection of calibration data only (“option 2”; unused calibration data are in light grey).]

[Figure: Fig. 12. IR absolute spectral calibration (both options) for the FOV position 200 at the nominal temperature. All panels (A, B, C, and D) are the same as Fig. 11 but for the IR channel.]

The resulting spectral samplings of both channels (i.e., the difference in CWL between two successive spectels) are well in-line with the pre-calibration OH models (Fig. 13). For the VISNIR channel, option 1 has the closest overall match. The fitting only deviates from the model at the edges of the spectral domain (within a 2% deviation for the rest). Option 2 is an excellent match for lower wavelengths (<1750 nm) but differs significantly at higher wavelengths. It should be noted that the last data point for this option is around 1950 nm; therefore, the deviations observed at higher wavelengths can be attributed not only to the data but also, to some extent, to the choice of a fourth-degree polynomial fitting. The third-degree alternative resulted in an opposite trend in the sampling, leading to significant differences at the edges (up to 3 physical spectels of difference on the absolute calibration at the end of the wavelength range). The third-degree alternative has the drawback of further deviating from the OP1-LDO data points (discarded for this option), whereas the fourth-degree alternative provides satisfactory fits of all data. For the IR channel, the sampling is virtually identical for both options, remaining within a 2% deviation from the precalibration model. Note that the third-degree polynomial fitting gives results virtually identical to the current calibration (<1 physical spectral of variation across the whole spectral range).

[Figure: Fig. 13. Spectral sampling of nominal spectels for the two considered dataset options at nominal temperature and in the center of the FOV of both channels. (A) Spectral sampling of the VISNIR channel based on the polynomial fitting results of the absolute calibration provided by option 1. The grey dashed lines indicate the requirements, and the colored dashed line shows the sampling expected based on the theoretical model of the OH performances developed during the design phase. (B) Same as (A) but for the IR channel. (C) Same as (A) but for option 2. (D) Same as (C) but for the IR channel.]