The Earth atmosphere is opaque over large regions of infrared light due to absorption by water and carbon dioxide. There are however certain windows with minimal absorption that allow the detection of infrared emission from the Earth surface in space. FUEGO will use observations in two of these windows.

The FUEGO instrument is a telescope operating in the infrared and recording images at 3.8-3.99 µm band (MWIR) for the detection of fires, and in the 8.95-9.2 µm band (LWIR) serving as an emissivity reference for the distinction between fires and reflected sunlight. Additionally, a standard visible camera (RGB) will support the geolocation of fires and the identification of clouds. The important design specifications are shown in the table.

Parameter	Value
Wavelengths	3.8-3.99 µm (MWIR), 8.95-9.125 µm (LWIR), 400-700 nm (RGB)
Field of view	2.5° x 2.5°
Number of pixels	4000 x 4000 MWIR, ~6000 x 6000 RGB
Spatial scale on ground	460 x 750 m (E-W and N-S)
Imaging cadence	One MWIR, LWIR, and RGB image every 10 seconds
Sensitivity	Relative changes <0.5° K

A point design for the FUEGO instrument fulfils most of the requirements. The system is diffraction limited for all relevant IR wavelengths. It covers the desired 2.56×2.56 degree FOR by staring with a 4K FPA. A 10 um pixel subtends 2.25 arcsec. A solar VIS blocking filter upstream of the first mirror is not shown. This is a three mirror anastigmat with focal length of 917 mm and, as drawn, f/4.1 optics. The FOR (and FOV) is 2.56×2.56 degrees. The beam splitter and MWIR & LWIR FPAs are located in a cold enclosure that is separated from the system by a window. The steering mirror shifts the scene to avoid bad pixels.