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
7.000E-02 0.156739 0.007 -2.26 0.0135 0.00754 2.5014E-05 0.59 -1.4 0.004569 0.0180
UKIRT Emissivity dataUKIRT Emissivity versus Time
y = 0.0011x + 0.2
0.0000
0.5000
1.0000
1.5000
2.0000
2.5000
3.0000
3.5000
0 500 1000 1500 2000 2500
Hours
Rel
ativ
e E
mis
sivi
ty
Scatter Versus Time
Scatter with Accumulation
0.0E+00
1.0E-05
2.0E-05
3.0E-05
4.0E-05
5.0E-05
6.0E-05
7.0E-05
8.0E-05
9.0E-05
1.0 1.2 1.4 1.6 1.8 2.0
Distance from Sun Center (solar radii)
Ra
tio
to
On
-Dis
k I
rra
dia
nc
e
Microroughness
0.01% Coverage
1 Day (0.036%)
1 Week (0.19%)
Apache Point Accumulation Rates
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0 20 40 60 80 100 120 140 160 180 200
Hours
Fra
ctio
nal
Co
vera
ge
Rate of change ≈ 0.04% per hour!
Scatter Versus Time: Apache Point
Power Spectral Density
Figure courtesy of Gary Peterson, Breault Research Organization.
Profile of a Star
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