ATST Scattered Light Issues How will mirror microroughness

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?

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 (M 1). Scatter the parent rays into a half-degree cone centered on the specular direction. • Add a scatter function to M 1 that represents a clean, polished surface, or a surface contaminated by dust.

Sample Positions 2. 0 1. 5 1. 1

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

…In Direction Cosine Space Plotting log 10 | sin – sin 0 | versus log 10 BSDF

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

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

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

Results for 20 Ångstrom Microroughness: S = – 1. 5 = 1. 0 Microns

Results for 12 Ångstrom Microroughness: S = – 1. 5 = 1. 0 Microns

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

MIL-STD 1246 C 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.

The Mie Model for 0. 01% Coverage (Level ~230)

UKIRT Emissivity data

Scatter Versus Time

Scatter Versus Time: Apache Point Rate of change ≈ 0. 04% per hour!

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

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