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