Preliminary alignment, characterization, and comparison of next generation carbon mapping imaging spectrometers

Through an innovative public-private partnership, a new generation of high-fidelity imaging spectrometers has been designed for the detection and measurement of methane (CH4) and carbon dioxide (CO2) plumes from super-emitters to help improve accounting and enable reduction of greenhouse gases in the Earth’s atmosphere. Two identical instruments, built concurrently at NASA Jet Propulsion Laboratory (referred to by JPL as the Carbon Plume Mapper project “CPM”) and Planet Labs as part of the Carbon Mapper Coalition, will operate over the spectral range of 400 – 2500 nm with a spectral sampling of 5.0 nm. The identical optical design comprises a three-mirror anastigmat (TMA) telescope and Dyson form spectrometer which reduces volume and mass for a fast (F/1.8) optical system. The instruments will be integrated into Planet-built Tanager spacecrafts and launched into low-Earth orbit (LEO). This work describes the assembly and alignment of the two identical instruments. At the subsystem level, both instruments follow the same procedure. For telescope alignment, the mirrors are first coarsely aligned with a coordinate measuring machine (CMM) and then finely aligned in a double-pass interferometer setup. The spectrometer subsystem is aligned on-axis using a commercial lens alignment instrument for precise, non-contact measurements. The telescope and spectrometer alignment results and performance are presented and compared. At the system level, the procedures deviate due to the separate and unique optical ground support equipment (OGSE) configurations utilized by JPL and Planet to interrogate the same instrument design. Both end-to-end optical alignment configurations are discussed, and the final CPM performance is shown with a focus on the five key and driving imaging spectrometer performance requirements.