1 April 1996 Measurement of near-specular visible-wavelength scattered light from two superpolished "coronagraph quality" mirrors
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Optical Engineering, 35(4), (1996). doi:10.1117/1.600949
Abstract
Several coronagraphs with mirror objectives are planned or have been constructed for terrestrial and space-based observatories. To justify the nontraditional choice of an internally occulted, reflective design for the inner coronal channel of the large-angle spectrometric coronagraph (LASCO) instrument constructed for the Solar Heliospheric Observatory satellite, we measured the near-specular (0.09 to 1.25 deg) scatter of two superpolished, spherical coronagraph mirrors with a unique geometry scatterometer. The visible stray light emanating from defectfree portions of the best mirror was measured to be 3.5 X 10-7B/B? (coronal brightness relative to the mean solar brightness) at 2R? (heliocentric distance in solar radii) and 2 X 10-7B/B? at 3R? . These levels are well below typical sky brightnesses of ~10-5 B/B? present at mountaintop observatories with clear skies. This stray light level would allow spectroscopic differencing observations of visible coronal emission lines (~4 X 10-8 B/B? at 2R? for coronal green line features) well into the inner corona from a space-based platform. An aspheric mirror with better performance than this stray light has been installed in the LASCO flight instrument. We describe the scatterometer apparatus, detail the experimental results, and present a comparison between predicted coronagraph stray light levels and nominal coronal signal levels.
Clarence M. Korendyke, Dennis George Socker, Guenter E. Brueckner, Rainer Schwenn, "Measurement of near-specular visible-wavelength scattered light from two superpolished "coronagraph quality" mirrors," Optical Engineering 35(4), (1 April 1996). https://doi.org/10.1117/1.600949
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