The Orbiting Carbon Observatory instrument optical design

Abstract

The Orbiting Carbon Observatory (OCO) will measure the distribution of total column carbon dioxide in the Earth's atmosphere from an Earth-orbiting satellite. Three high-resolution grating spectrometers measure two CO 2 bands centered at 1.61 and 2.06 µm and the oxygen A-band centered at 0.76 µm in the near infrared region of the spectrum. This paper presents the optical design and highlights the critical optical requirements flowed down from the scientific requirements. These requirements necessitate a focal ratio of f/1.9, a spectral resolution of 20,000, and precedencesetting requirements for polarization stability and the instrument line shape function. The solution encompasses three grating spectrometers that are patterned after a simple refractive spectrometer approach consisting of an entrance slit, a two-element collimator, a planar reflection grating, and a two-element camera lens. Each spectrometer shares a common field of view through a single all-reflective telescope. The light is then re-collimated and passed through a relay system, separating the three bands before re-imaging the scene onto each of the spectrometer entrance slits using an allreflective inverse Newtonian re-imager.

The on-orbit performance of the Orbiting Carbon Observatory-2 (OCO-2) instrument and its radiometrically calibrated products

“…The OCO-2 spacecraft carries and points a single instrument that incorporates three co-boresighted, long-slit imaging grating spectrometers optimized for observing the O 2 Aband at 0.765 µm and the "weak" and "strong" CO 2 bands at 1.61 and 2.06 µm Haring et al, 2004;Pollock et al, 2010). These three channels are designated ABO2, WCO2, and SCO2, respectively.…”

“…The light then traverses a narrowband pre-disperser filter designed to transmit wavelengths within ±1 % of the central wavelength of the channel and reject the rest (out-of-band transmission < 10 −4 of peak transmission). The light is then refocused onto each spectrometer slit by a reverse Newtonian telescope (Haring et al, 2004). Each slit is about 3 mm long and 28 µm wide.…”

“…2a). For routine science operations, a 220 (spatial rows) by 1016 (spectral columns) pixel window on each FPA is continuously scanned using a "rolling readout" method for recording and resetting each pixel on the FPAs to their bias levels (Haring et al, 2004). This readout approach precludes the need for a physical shutter and eliminates spatial gaps between the exposures.…”

High-resolution methods for fluorescence retrieval from space

“…By way of a demonstration, we combined forward modeling at the maximum spectral resolution of 0.1 cm −1 of the MODTRAN5 program (beta version) with spectral fitting for a VIS-NIR interferometer/spectrometer resolutions of 0.38 cm −1 [4,5] and 0.77 cm −1 [6]. These represent the spectral resolutions of existing instruments for space observations in the O 2 A atmospheric absorption band.…”

“…Two cases were treated which we named same and bare depending on the type of surroundings of the same or different soil type as the (vegetated) target, respectively. In this paper the comparative study at the three different resolutions and the analysis in the single A band at the OCO [6] and TANSO-FTS [4] instrument spectral resolutions was performed only for this case. Furthermore, the fluorescence retrieval in both bands was also performed at the TANSO-FTS spectral resolution by using the results obtained in the single A band: a two-step retrieval which greatly enhanced accuracy for high noise values.…”