ATST Scattered Light Issues

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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?

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The ASAP Model

• Define a set of parallel rays representing a
  point source at the position of the suns 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.

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Sample Positions
1.1
1.5
2.0
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Mirror Signature from Microroughness
Typical scatter versus angle for a clean,
polished glass surface
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In Direction Cosine Space
Plotting log10 sin ? sin ?0 versus log10
BSDF
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The Harvey Model
b
Figure courtesy of Gary Peterson, Breault
Research Organization.
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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.
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Ranges of Slopes
All four curves integrate to yield the same total
integrated scatter predicted for a 20 Ångstrom
RMS surface.
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Results for 20 Ångstrom Microroughness S 1.5
? 1.0 Microns
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Results for 12 Ångstrom Microroughness S 1.5
? 1.0 Microns
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Scatter due to Contamination (dust)
Figure courtesy of Gary Peterson, Breault
Research Organization.
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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.
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The Mie Model for 0.01 Coverage(Level 230)
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UKIRT Emissivity data
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Scatter Versus Time
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Scatter Versus Time Apache Point
Rate of change 0.04 per hour!
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Power Spectral Density
Figure courtesy of Gary Peterson, Breault
Research Organization.
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Profile of a Star