Instrument

The OCO spectrometers measure sunlight reflected off the Earth's surface. The rays of sunlight that enter the spectrometers pass through the atmosphere twice, once as they travel from the Sun to the Earth, and then again as they travel from the Earth's surface to the OCO instrument. Carbon dioxide and molecular oxygen molecules in the atmosphere absorb light energy at very specific colors or wavelengths. Thus, the light that reaches the OCO instrument will display diminished amounts of energy at those characteristic wavelengths. The OCO instrument employs a diffraction grating to separate the inbound light energy into a spectrum of multiple component colors. The reflection gratings used in the OCO spectrometers consist of a very regularly spaced series of grooves that lie on a very flat surface. The back of a compact disc is an everyday example of a diffraction grating.

The characteristic spectral pattern for CO₂ can alternate from transparent to opaque over very small variations in wavelength. The OCO instrument must be able to detect these dramatic changes, and specify the wavelengths where these variations take place. Thus, the grooves in the instrument diffraction grating are very finely tuned to spread the light spectrum into a large number of very narrow wavelength bands or colors. Indeed, the OCO instrument design incorporates 17,500 different colors to cover the entire wavelength range that can be seen by the human eye. A digital camera covers the same wavelength range using just three colors.

The OCO experiment requires the measurement of three relatively small bands of electromagnetic radiation. The spectral wavelength ranges of these three critical bands are widely separated. To accomplish this task economically, OCO uses three spectrometers instead of one. Each spectrometer measures light in one specific region of the spectrum. The focal plane associated with each spectrometer is designed to detect very fine differences in wavelength within each of these spectral ranges.

OCO measurements must be very accurate. To eliminate energy from other sources that would generate measurement errors, the light detectors for each camera must remain very cold. To ensure that the detectors remain sufficiently cold, the OCO instrument design includes a cryocooler, which is a refrigeration device. The cryocooler keeps the detector temperature at or near -150° C (-240° F).

For details about the OCO instrument design, proceed to the next page.