The absolute calibration is compared using the data sets acquired with the ICU as well as the OP1 and OP3 setups. Due to planning constraints, the other optical paths were not used in the hot case. The OP1-LDO dataset acquired in the lower and upper operative temperature cases is also used to estimate the relative shift in the central wavelength of the explored spectels across a larger OH thermal range. Each thermal case has a dataset that covers different spectels. For this reason, the trend shown by the comparison between the lower and upper operative temperature cases is used as a relative reference. No significant difference in the absolute calibration of the VISNIR channel between the two thermal configurations is found. The OP1-LDO dataset confirms that no relative change is observed between both cases for the VIS-NIR channel. Conversely, the four data sets show a shift in the absolute calibration of the IR channel. The spectral shift to be applied to the hot case data for correcting the spectral calibration was thus derived (Fig. 21). A constant shift of about 2.0 nm is observed at wavelength < 4200 nm between the two temperatures (126 K and 137 K). For the ICU signature around 5.2 µm (physical spectel N°910) and the atmospheric signature around 5.4 µm (physical spectel N°970), an inversion in the shift is found (Fig. 21). However, associated error bars are very large and cover largely the shift derived at shorter wavelength. Actually, because of the increasing thermal background of the instrument for the hot case, these signatures are poorly resolved, leading to a more significant error in the CWL calculation at higher wavelengths (Fig. 22). These points are thus excluded in the final estimation of the spectral shift. If we assume a linear trend with temperature to interpolate and extrapolate these two measurements, the spectral response is drifting with temperature by + 2.0 nm ± 0.3 nm per 11 K, leading to the relationship 𝐶𝑊𝐿(126 𝐾+ Δ𝑇) = 𝐶𝑊𝐿(126 𝐾) + Δ𝑇∗0.18 𝑛𝑚 / 𝐾). In addition, the OP1-LDO data confirmed that a shift can be observed in the CWLs of the spectels of the IR channel between the lower and upper operative temperature cases (II.A.3). The relative shift computed between both cases is in line with the IAS dataset (the shift is equal to + 8.5 nm ± 0.3 nm per 40 K or + 0.21 nm per K).

[Figure: Fig. 21. Shift of the spectral signatures (opposite to the shift of the CWLs) observed during the hot case relative to the nominal case for the IR channel]

[Figure: Fig. 22. Illustration of the degradation and the loss in resolution of the polystyrene signature around 5.2 µm between the nominal and hot case resulting from the increasing thermal background. (A) Example of polystyrene signature fitting for the IR channel in the nominal case.]

(B) Same as (A) but for the hot case. The fitting algorithm performance is significantly less reliable in this case.

[Figure: Fig. 23. CWL average shift measured across the tested thermal cases. The shift corresponds to the average difference between the data points at a given temperature and the absolute spectral calibration. Each data point summarizes several measurements. The mean spectel of each dataset is displayed in the legend. It must be noted that the LDO data is used as a relative reference, which is why its spectral position is shifted below the actual calibration (dashed line).]