Measuring sunlight reflected off the Earth's surface by employing a diffraction grating to separate the inbound light energy into a spectrum of multiple component colors (Orbiting Carbon Observatory).

Aura instruments enable daily global observations of Earth's atmospheric ozone layer, air quality, and key climate parameters (Tropospheric Emission Spectrometer - TES).

To the naked eye, our atmosphere looks clear much of the time, or filled with clouds, or somewhere in between. When not obstructed by clouds, sunlight and starlight appear to pass through our atmosphere pretty much unimpeded in the blur of colors we call "light."

With instruments that divide this "light" very finely into its individual components of color and shade, the picture begins to look much more complex. Some wavelengths are strongly absorbed, so not much light gets through, while wavelengths nearby pass through the atmosphere with little attenuation. This fine spectral structure of absorption, throughout the ultraviolet, visible and infrared portions of the electromagnetic spectrum, is diagnostic of the many chemical species present in our atmosphere, or in any other atmosphere, such as at Mars, Jupiter, or Titan.

Different spectrometers are used to divide light finely into its spectral components, and to accurately "read" absorption and transmission "lines" in the electromagnetic spectrum of transmitted or reflected sunlight. As examples, the Orbiting Carbon Observatory (OCO) Spectrometer is being built to very accurately measure the heat-absorbing gas carbon dioxide in Earth's atmosphere. The Tropospheric Emission Spectrometer (TES), launched in 2004 aboard NASA's Aura Earth Observing System spacecraft, measures ozone and a variety of trace molecules down to very low concentrations.

People at JPL and JPL's subcontractors have built a variety of optical spectrometers for atmospheric measurements, based on which a large body of scientific work has been published on an astonishing variety of topics.

Orbiting Carbon Observatory (OCO)

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 -1500 C (-2400 F).

The Observatory carries a single instrument that incorporates three classical grating spectrometers. Each spectrometer detects the intensity of radiation within a very specific narrow band at near infrared (NIR) wavelengths. The three spectrometers share a common structure, a cryogenic cooler, and an input telescope.

The telescope consists of an 11 cm aperture, as well as a primary and a secondary mirror. The relay optics assembly includes a fold mirrors, dichroic beam splitters, band isolation filters and re-imaging mirrors. Each spectrometer consists of a slit, a two-lens collimator, a grating, and a two-lens camera. Each of the three spectrometers has an essentially identical layout. Minor differences among the spectrometers, such as the coatings, the lenses and the gratings, account for the different bandpasses that are characteristic of each channel. The focal ratios of the instrument optics range from f/1.6 to f/1.9.

To implement an optically fast, high-spectral-resolution measurement system, the OCO instrument combines refractive and reflective optical techniques. Since the light in the common telescope and relay optics assembly has not yet been separated into the three distinct wavelength bands, these instrument subsystems primarily use reflective optics. On the other hand, the extremely narrow channel bandpasses make potential chromatic aberrations in the spectrometers negligible, which enables the use of refractive optics.