11 Jul 2022
11 Jul 2022
Status: this preprint is currently under review for the journal AMT.

Performance and polarization response of Slit Homogenizers for the GeoCarb Mission

Sean Crowell1, Tobias Haist2, Michael Tscherpel2, Jérôme Caron3, Eric Burgh4, and Berrien Moore III1 Sean Crowell et al.
  • 1University of Oklahoma, Norman, Oklahoma, USA
  • 2Institut für Technische Optik, University of Stuttgart, Germany
  • 3TNO, Optics Department, Stieltjesweg 1, 2628 CK Delft, The Netherlands
  • 4Lockheed Martin Advanced Technology Center, Palo Alto, California, USA

Abstract. The observing strategy of the Geostationary Carbon Observatory (GeoCarb), which is a "step and stare" approach, can lead to distortions in the instrument spectral response function (ISRF) when there are gradients in brightness across instru- ment field of view. These distortions induce errors in the retrieved trace gases. In order to minimize these errors, the GeoCarb instrument design was modified to include a "slit homogenizer" whose purpose is to scramble the pattern of the incoming light and effectively remove the ISRF distortions causing by the variations in illumination across the slit. As a risk reduction, Geo- Carb procured six different homogenizers and had them tested for performance in a bench-top optical system. The major finding is that the homogenizer performance depends strongly on the polarization of the incoming light, with the sensitivity growing as a function of wavelength. The width of the ISRF is substantially smaller when the light is vertically polarized (orthogonal to the slit length) compared to horizontally polarized (parallel to the slit length), and the throughput is accordingly reduced. These effects are due to the effects of the gold coating and high incidence angles present in the GeoCarb homogenizer design, which was verified using a polarization-dependent model generalized from previous homogenizer modeling work. The results strongly recommend controlling the polarization of the light entering a similar implementation for other instruments attempting to mitigate scene illumination non-uniformity effects, as well as a robust characterization of the polarization sensitivity of all key subsystems.

Sean Crowell et al.

Status: open (until 25 Aug 2022)

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Sean Crowell et al.

Sean Crowell et al.


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Short summary
Variations in brightness in radiance measurements cause errors that can be mitigated with hardware that scrambles the pattern of the incoming light. GeoCarb took this route to minimize this source of errors, but lab testing determined that the solution chosen was too sensitive to the the polarization of the incoming light. Modeling found this was a predictable result of using gold coatings in the design, which is typical of spaceflight optical instruments.