The Sun (and other stars) are very hot and emit energy in the form of radiation. A spectrometer splits the radiation according to wavelength (color), and directs the different colors to different positions on a detector. Our eyes are one example of detectors that can sense different colors.
The STFS spectrometer uses a video camera as the sensor, allowing us to measure the light intensity for 1000 different colors.
Can you think of other naturally occurring color (wavelength) splitters?
The Sun is very bright sending such high radiation that it would burn out the sensors in a video camera. To decrease the energy arriving at the STFS camera we gave the spectrometer “sunglasses” that cut the light intensity down by a factor of 10,000. Those “sunglasses” are the filter stack.