Conceptual Design Review

Stray LightRob Hubbard

Systems Engineering

Definition of Terms

• Stray light is unwanted light• Most problematic when observing faint objects

near the sun

•Possible causes include

–Scatter from optical or mechanical surfaces

–Ghost reflections–Edge scatter or “glints”–Diffraction around edges

The Science Requirement

…The total instrumental scattered light (dust plus mirror roughness) shall be 25 millionths or less at 1000 nm and at 1.1 radii. Values larger than these levels require longer integration times to achieve the desired signal to noise levels.

Relevant Studies Performed

• Some Stray-Light Reduction Design Considerations for ATST (Andrew Buffington and Bernard V. Jackson, UCSD)

• M1 Microroughness and Dust Contamination (Rob Hubbard, ATST Systems Engineering)

• Further Stray-Light Reduction Design Considerations for ATST (Andrew Buffington, UCSD)

• Advanced Technology Solar Telescope (ATST) Stray and Scattered Light Analysis (Scott Ellis, Richard N. Pfisterer, Photon Engineering, LLC)

• “Sunlight reflected from the heat shield/coronagraph occulter does not need to be absorbed nearby, and can be safely dumped into the interior of the building; and…”

• “Except maybe close to the M1 mirror mount, the building interior can be typical black or even gray paint, without generating significant stray light in the FOV.”

• “Specifying, manufacturing, testing and certifying M1 could prove a significant challenge for ATST.”

• “ATST’s success as a coronagraph probably requires aggressive contamination control, even if a low-dust site is found…”

Some Stray-Light-Reduction Design Considerations for ATST(July 2002)

Buffington and Jackson

Conclusions:

Extending the Analysis

• Can we make additional assumptions that will allow us to better quantify the scattering due to M1 microroughness?

• Can we refine the dust contamination predictions so that they can be compared to scatter due to M1 microroughness?

• How frequently will the ATST primary mirror need to be cleaned to maintain acceptable coronagraphic performance?

ATST Technical Note No. 0013

1.11.52.0

Sample Positions

Typical scatter versus angle for a clean, polished glass surface

The Scatter Model

…In direction-cosine space

Plotting log10 | sin – sin 0 | versus log10 BSDF

Figure courtesy of Gary Peterson, Breault Research Organization; measurement by James Harvey.

S

bBSDF

01.0

)sin()sin( 0

2

1002

SbTIS

S

Harvey Model

= 0.57º

Even and small angles?

1.11.52.0

1.6 arcmin

sin 0.0005

Power Spectral Density

Church, Eugene L.,” Fractal Surface Finish,” (Applied Optics 27, No. 8, 15 April 1998.)

~40 arcsec (from grating equation)

Profile of a Star

The Profilometer and Roughness

22

2

1L

LL

Lim z x z dxL

!

2

1002

42

SbTIS

S

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.

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

Slope Ranges

Angle (Degrees)

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

= 1.0 Micrometer

The 20 Ångstrom Finish

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

= 1.0 Micrometer

The Likely Finish

R/Rsun S = –1.5 = 12 Å

S = –1.5 = 20 Å

S = –1.8 = 12 Å

S = –1.8 = 20 Å

1.1 3.93×10-6 10.9×10-6 10.5×10-6 29.3×10-6

1.2 3.20×10-6 8.90×10-6 8.03×10-6 22.3×10-6

1.3 2.72×10-6 7.55×10-6 6.51×10-6 18.1×10-6

1.4 2.36×10-6 6.57×10-6 5.45×10-6 15.2×10-6

1.5 2.08×10-6 5.77×10-6 4.63×10-6 12.9×10-6

1.6 1.85×10-6 5.14×10-6 4.01×10-6 11.1×10-6

1.7 1.67×10-6 4.64×10-6 3.53×10-6 9.82×10-6

1.8 1.52×10-6 4.21×10-6 3.13×10-6 8.71×10-6

1.9 1.38×10-6 3.84×10-6 2.80×10-6 7.79×10-6

2.0 1.27×10-6 3.53×10-6 2.52×10-6 7.02×10-6

Range of values

Figure courtesy of Gary Peterson, Breault Research Organization.

Dust Contamination

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.

MIL-STD 1246C

0.001 0.01 0.1 1 10 100Scattering A ngle, D egrees

1E -006

1E -005

1E -004

1E -003

1E -002

1E -001

1E +000

1E +001

1E +002

1E +003

1E +004

1E +005

1E +006

1E +007

1E +008

BR

DF

(p

er s

tera

dia

n)

AB g = 2.5 e-6, 1 e-13, 3

AB g = 2.5 e-6, 1 e-9, 3

AB g = 2.5 e-6, 1 e-5, 3

AB g = 4 e-5, 1 e-8, 2

AB g = 4 e-5, 1 e-4, 2

S pyak an d W olfe (1992), 0.6 n m(3 m on th s exposu re)

Buffington and Jackson

• Measurements are only available to within a degree of the specular direction.

• We know the linear relationship cannot go on indefinitely and retain a finite TIS.

• The roll-off will likely occur right in our angular domain, so knowledge of the position of the “knee” is critical to dust analysis.

Roll-off in the IR (10 microns)

From Spyak and Wolfe, Scatter from particulate-contaminated mirrors, Part 3

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

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

Mie at 0.01% Compared to Microroughness

0.0E+00

1.0E-06

2.0E-06

3.0E-06

4.0E-06

5.0E-06

6.0E-06

7.0E-06

8.0E-06

9.0E-06

1.0E-05

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Distance from Sun Center (solar radii)

Ra

tio

to

On

-Dis

k I

rra

dia

nc

e

0.01% coverage

12 A Microroughness

Mie at 0.01% Compared to Microroughness

0.0E+00

1.0E-06

2.0E-06

3.0E-06

4.0E-06

5.0E-06

6.0E-06

7.0E-06

8.0E-06

9.0E-06

1.0E-05

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Distance from Sun Center (solar radii)

Ra

tio

to

On

-Dis

k I

rra

dia

nc

e

0.01% coverage

12 A Microroughness

Dust results at 1 Micron

Dust accumulation

UKIRT 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

UKIRT 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 with Accumulation

0.0E+00

2.0E-05

4.0E-05

6.0E-05

8.0E-05

1.0E-04

1.2E-04

1.4E-04

1.6E-04

1.8E-04

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 (0.063%)

1 Week (0.38%)

Accumulation with time

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!

40 Times faster at Apache Point

Kitt Peak Dust Experiment

• At what rate does dust accumulate in the McMath-Pierce tunnel?

• What is the distribution of particle sizes?

• What affect does an air knife have on dust accumulation rates?

A Large Compressor!

The Experiment

The Air Knife and Samples

Super Air Knife by Exair

24-Hour Accumulation

330 m

200 Magnification

10 m

The Need for Clean Air

Dust Scatter vs. Wavelength

Dust BSDF at 1, 3, and 10 microns

-6

-5

-4

-3

-2

-1

0

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

Log(Beta)

Lo

g(B

SD

F)

1 Micron

3 Microns

10 Microns

Other Stray Light SourcesREFLECTED LIGHT FROM THE SUN (YELLOW)

WHAT THE GREGORIAN FOCUS SEES (BLUE)

REFLECTED LIGHT FROM THE SUN (YELLOW)

WHAT THE GREGORIAN FOCUS SEES (BLUE)

Relative Contributions

Scattered Power - Gregorian Focus

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

Primary Mirror(M1)

Secondary Mirror(M2)

Everything Else

Po

we

r @

Co

ud

e Im

ag

e

With Dome

No Dome

Rel

ati

ve

Co

ntr

ibu

tio

n

Conclusions from the Reports

“Scattering due to dust contamination of the primary mirror would appear to be the most serious stray-light concern for coronagraphic observations. The accumulation of dust on the primary quickly overwhelms the effects of surface microroughness from the polishing process.”

Mie at 0.01% Compared to Microroughness

0.0E+00

1.0E-06

2.0E-06

3.0E-06

4.0E-06

5.0E-06

6.0E-06

7.0E-06

8.0E-06

9.0E-06

1.0E-05

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Distance from Sun Center (solar radii)

Rat

io t

o O

n-D

isk

Irra

dia

nce

0.01% coverage

12 A Microroughness

M1 Microroughness and Dust Contamination:

Dust Dominates

• In situ washing is already part of the baseline plan.

• Operational procedures will have to be developed (as with any telescope) that establish criteria for “safe exposure” of the telescope to high winds in high-dust situations.