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ATST Scattered Light Issues

ATST Scattered Light Issues. How will mirror microroughness likely impact the coronagraphic performance of ATST? How do these limitations compare to what we can expect from dust and other particulate contamination on the mirror surface?

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ATST Scattered Light Issues

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  1. ATST Scattered Light Issues • How will mirror microroughness likely impact the coronagraphic performance of ATST? • How do these limitations compare to what we can expect from dust and other particulate contamination on the mirror surface? • How frequently will the ATST primary mirror need to be cleaned to maintain acceptable coronagraphic performance?

  2. The ASAP Model • Define a set of parallel rays representing a point source at the position of the sun’s center. • Introduce these rays onto a “scatter” surface just in front of the primary mirror (M1). Scatter the parent rays into a half-degree cone centered on the specular direction. • Add a scatter function to M1 that represents a clean, polished surface, or a surface contaminated by dust.

  3. Sample Positions 2.0 1.5 1.1

  4. Mirror Signature from Microroughness Typical scatter versus angle for a clean, polished glass surface

  5. …In Direction Cosine Space Plotting log10 | sin  – sin 0 | versus log10 BSDF

  6. The Harvey Model b Figure courtesy of Gary Peterson, Breault Research Organization.

  7. RMS Microroughness and Harvey The single RMS roughness parameter () contains insufficient information to completely characterize the BSDF of the polished surface, even assuming a power-law relationship.

  8. Ranges of Slopes All four curves integrate to yield the same total integrated scatter predicted for a 20 Ångstrom RMS surface.

  9. Results for 20 Ångstrom Microroughness: S = – 1.5  = 1.0 Microns

  10. Results for 12 Ångstrom Microroughness: S = – 1.5  = 1.0 Microns

  11. Scatter due to Contamination (dust) Figure courtesy of Gary Peterson, Breault Research Organization.

  12. MIL-STD 1246C The number of particles per square foot with diameters greater than s microns is given by: log(n) = 0.926 [ (log(c))2 - (log(s))2 ] s = particle diameter (m) c = cleanliness level n = number of particles per square-foot with diameters greater than s Courtesy of Gary Peterson, Breault Research Organization.

  13. The Mie Model for 0.01% Coverage(Level ~230)

  14. UKIRT Emissivity data

  15. Scatter Versus Time

  16. Scatter Versus Time: Apache Point Rate of change ≈ 0.04% per hour!

  17. Power Spectral Density Figure courtesy of Gary Peterson, Breault Research Organization.

  18. Profile of a Star

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