The Observatory will carry a single instrument that will incorporate three classical grating spectrometers. Each spectrometer will detect the intensity of radiation within a very specific narrow band at Near Infrared (NIR) wavelengths. The three spectrometers will share a common structure, a cryogenic cooler, and an input telescope.
The telescope will consist of an 11 cm aperture, as well as a primary and a secondary mirror. The relay optics assembly will include a fold mirrors, dichroic beam splitters, band isolation filters and re-imaging mirrors. Each spectrometer will consist of a slit, a two-lens collimator, a grating, and a two-lens camera. Each of the three spectrometers will have, essentially, an identical layout. Minor differences among the spectrometers, such as the coatings, the lenses and the gratings, will account for the different bandpasses that are characteristic of each channel. The focal ratios of the instrument optics will range from f/1.6 to f/1.9.
To implement an optically fast, high-spectral-resolution measurement system, the OCO-2 instrument will combine refractive and reflective optical techniques. Since the light in the common telescope and relay optics assembly will not separate into the three distinct wavelength bands, these instrument subsystems will primarily use reflective optics. On the other hand, the extremely narrow channel bandpasses will make potential chromatic aberrations in the spectrometers negligible, which will enable the use of refractive optics.
