## Appendix A
### Spectrometer Fabrication, Integration, and Key Ground-based Performances Tests  
The flight-like (brass-board) MISTiC spectrometer optical assembly was fabricated and was integrated with the detector/dewar assembly at BAE Systems. In addition to the spectrometer opto-mechanical assembly and the FPA itself, the onboard radiometric calibration sources and scan-mirror assembly are nearly identical to those in the planned space instrument implementation. Integration of the ground/airborne version of the instrument is shown at right in Figure 19.  
Several aspects of the instrument design, alignment, and spectrometric performance were assessed during ground testing. The precise geometrical alignment between the entrance slit, grating, and FPA was performed during instrument integration. Once aligned, the geometric distortion, including spectrometer optical field geometric distortions referred to as \"smile\" and \"keystone\" distortions as well as residual angular misalignment of slit, grating, and the photodiode array. The residual distortions were found to be low, providing common spectral channel sampling (within 5% of predicted spectral response function (SRF) full-width-half-maximum (FWHM) across the slit field), and a suitably low level of spatial/spectral miss-registration (5% of GSD). The spectral resolving power of the instrument was assessed in ground testing, with the spectrometer assembly cooled to its planned operating temperature. The observed and modeled outputs of the monochrometer test for spectral channels near 4.2 and 5.3 \(\upmu\)m are shown in Figure 20. The observed FWHM of the SRF was within 2% of the designed value. The different portions of the spectral range employ different diffractive orders of the grating-with order 2 being used for the wavelengths longer than 4.65 \(\upmu\)m, and grating order 3 used for shorter wavelengths. The ratio of the SRF FWHM-to-spectral sampling interval was greater than 2:1 (oversampling) for all spectral channels. In addition to spectral resolving power, critical performance attributes of moderate-resolution dispersive spectrometers employed for infrared sounding include the knowledge and stability of the spectral calibration. Following AIRS, spectral calibration knowledge-to a few parts in 100,000 of the channel wavelength is needed. Again following the practice of AIRS, the absolute spectral calibration will be established for selected cloud-free scenes using a vicarious calibration approach that uses the observed wavelength of a selection of known, stable atmospheric spectral features. In this way, the spectral calibration knowledge requirement flows directly to a spectral calibration stability requirement which, in turn, flows directly to a spectrometer temperature stability requirement, since thermally induced distortions of the spectrometer are the dominant driver of spectral calibration shift. For MISTiC, a spectral calibration stability requirement of 5% of the SRF led to the establishment of an initial temperature stability requirement of \(<\)100 mK over the orbit. This drove the overall instrument thermal requirements and design of the space instrument. Detailed thermal modeling of the space-borne spectrometer shows a spectrometer temperature stability estimate of 30 mK, 6\(\sigma\) over the orbit. The assumed orbit is sun-synchronous, and for most nodal crossing times, the spacecraft move in and out of solar illumination with a \(\sim\)100-min period. Airborne tests of the spectrometer imagery, (described in more detail below) also provided an experimental opportunity to evaluate the temperature sensitivity of the spectral calibration for the MISTiC spectrometer optical assembly. Figure A4 shows spectra taken over a particular (cloud-free) ocean scene off the California coast near Vandenberg AFB, at two different times, but for the same cross-track position.