Measurement of near‐specular visible‐wavelength scattered light from two superpolished ‘‘coronagraph quality’’ mirrors

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 3.5ϫ10 Ϫ7 B/B ᭪ (coronal brightness relative to the mean solar brightness) at 2R ᭪ (heliocentric distance in solar radii) and 2ϫ10 Ϫ7 B/B ᭪ at 3R ᭪ . These levels are well below typical sky brightnesses of ϳ10 Ϫ5 B/B ᭪ present at mountaintop observatories with clear skies. This stray light level would allow spectroscopic differencing observations of visible coronal emission lines (ϳ4ϫ10 Ϫ8 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. © 1996 Society of Photo-Optical Instrumentation Engineers.