## 2 Materials and Methods
### Characterizing FIREFLY with GLAMR  
Goddard's laser for absolute measurement of radiance (GLAMR, manufacturer, city, state, country) is a high-quality calibration facility, utilized by this study for several purposes: to characterize detector response and dark current; to estimate FWHM and signal to noise ratio (SNR) for each of the 266 spatial pixels of FIREFLY; and to assess spectral stray light. GLAMR is NASA Goddard Space Flight Center's portable version of the National Institute of Standards and Technology (NIST) Spectral Irradiance and Radiance responsivity Calibrations using Universal Sources (SIRCUS) [37]. It consists of a wavelength-tunable laser coupled to an integrating sphere to produce a near-monochromatic source of uniform radiance. It facilitates a full aperture absolute calibration approach and a spectral scan of an instrument's operating spectrum allows determination of the absolute spectral responsivity (ASR) of an instrument for all of its spectral channels [38] (Figure 2). GLAMR is designed to provide uncertainty of a sensor's radiance responsivity to around 0.5 to 5% depending on spectral region. Radiance provided by the GLAMR system is traceable to the primary standards at NIST. The international system of units (SI) traceability to the NIST's primary optical watt radiometer (POWR) is established using transfer radiometers [39; 40]. The GLAMR facility was used to characterize the response of FIREFLY in wavelengths ranging from 690 nm to 1000 nm (Table 2). Between the wavelengths of 690 nm and 764 nm, four wavelength ranges were sampled at 0.05-nm steps, and six ranges were sampled at a lower resolution of 0.15 nm. The full wavelength range of 690 nm to 1000 nm was sampled at 1-nm steps to investigate spectral stray light from within and beyond the spectral range of the sensor. Background (laser source is shuttered closed) data were collected for about 10 seconds after every 10 seconds of open shutter. The sensor response digital numbers (DN) were then matched to corresponding data from GLAMR using synchronized time stamps. GLAMR integrating sphere radiance measurements were combined with the background adjusted DNs, to provide absolute spectral response (ASR) for each spectral pixel [41] (Figure 4). GLAMR radiance measurements with high wavelength variance (standard deviation > 0.03) were excluded from the study. All of the ASRs were visually inspected, and those with insufficiently sampled central peaks were excluded. The ASRs were then interpolated to a wavelength step of 0.001 nm to facilitate characterization of reported bandwidths of <0.2 nm. A Gaussian fit was used to interpolate the central peak area. After the described filtering had been implemented, 104 bands were sufficiently characterized. After interpolation of ASR to a uniform wavelength step, the full-width at half maximum (FWHM), was estimated using the following two equations:  
\[\textit{Band Responsivity}\ =\ \sum_{\lambda=690}^{\lambda=1000}ASR(\lambda)\cdot \Delta\lambda \tag{1}\]  
\[\textit{FWHM}\ =\ \frac{1}{ASR_{MAX}}\ \sum_{\lambda=690}^{\lambda=1000}ASR( \lambda)\cdot\Delta\lambda=\frac{\textit{Band Responsivity}}{ASR_{MAX}} \tag{2}\]  
where \(\Delta\lambda\) is wavelength spacing (0.001 nm) between interpolated measurements. There are two kinds of stray light considered here, spectral and spatial. Spectral stray light is described by the spectral stray light signal distribution function (SDF) and spatial stray light is described by the point spread function (PSF). Here we have focused on characterizing spectral stray light, since spectral stray light is potentially highly influential on SIF retrievals. Stray light is mostly caused by non-ideal behavior of the optical components (mirrors, slits, and grating etc. ).