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?
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? - PowerPoint PPT Presentation
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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?
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.
Sample Positions
1.11.52.0
Mirror Signature from Microroughness
Typical scatter versus angle for a clean, polished glass surface
…In Direction Cosine Space
Plotting log10 | sin – sin 0 | versus log10 BSDF
The Harvey Model
S
bBSDF
01.0
)sin()sin( 0
2
1002
SbTIS
S
b
Figure courtesy of Gary Peterson, Breault Research Organization.
RMS Microroughness and Harvey
2
1002
42
SbTIS
S
The single RMS roughness parameter () contains insufficient information to completely characterize the BSDF of the polished surface, even assuming a power-law relationship.
Ranges of Slopes
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
0.0001 0.0010 0.0100 0.1000 1.0000
S = -1.5
S = -1.6
S = -1.7
S = -1.8
All four curves integrate to yield the same total integrated scatter predicted for a 20 Ångstrom RMS surface.
Microroughness – 20 Ångstrom s=-1.5
0.0E+00
2.0E-06
4.0E-06
6.0E-06
8.0E-06
1.0E-05
1.2E-05
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Distance from Sun Center (solar radii)
Rat
io t
o O
n-D
isk
Irra
dia
nce
20 A -1.5
Results for 20 Ångstrom Microroughness: S = – 1.5
= 1.0 Microns
Two Sample Polishes
0.0E+00
2.0E-06
4.0E-06
6.0E-06
8.0E-06
1.0E-05
1.2E-05
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Distance from Sun Center (solar radii)
Rat
io t
o O
n-D
isk
Irra
dia
nce
20 A -1.5
12 A -1.5
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 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 leveln = number of particles per square-foot with diameters greater than s
n 500 s( )
n 300 s( )
n 100 s( )
s1 10 100 1 10
31
10
100
1 103
1 104
1 105
1 106
1 107
# of Particles Over a Given Diameter
Particle Diameter
Num
ber
of P
arti
cles
Courtesy of Gary Peterson, Breault Research Organization.
The Mie Model for 0.01% Coverage(Level ~230)
Harvey Fit to Mie Data
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0.0001 0.0010 0.0100 0.1000 1.0000
Sine Theta
BS
DF
Harvey 1 Harvey 2b b0 l s %TIS 1 b b0 l s % TIS 2 % Sum