Conceptual Design Review

1
Conceptual Design Review

• Stray Light
• Rob Hubbard
• Systems Engineering

2
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

3
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.
4
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)

5
Buffington and Jackson
Some Stray-Light-Reduction Design Considerations
for ATST (July 2002)
Conclusions

• 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.
• ATSTs success as a coronagraph probably
  requires aggressive contamination control, even
  if a low-dust site is found

6
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?

7
ATST Technical Note No. 0013
8
The ASAP Model

• Define a set of parallel rays representing a
  point source at the position of the suns 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.
• Normalize flux so disk center 1.
• Add a scatter function to M1 that represents a
  clean, polished surface, or a surface
  contaminated by dust.

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The Source Model at 1.1 Rsun
10
Scatter Towards
10,000 out of 10,000 delivered
1 out of 10,000 delivered
11
Sample Positions
12
The Scatter Model
Typical scatter versus angle for a clean,
polished glass surface
13
In direction-cosine space
Plotting log10 sin ? sin ?0 versus log10
BSDF
14
Harvey Model
Figure courtesy of Gary Peterson, Breault
Research Organization measurement by James
Harvey.
15
Even and small angles?
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Power Spectral Density
40 arcsec (from grating equation)
Church, Eugene L., Fractal Surface Finish,
(Applied Optics 27, No. 8, 15 April 1998.)
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Profile of a Star
18
The Profilometer and Roughness
19
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.
20
Slope Ranges
Angle (Degrees)
21
The 20 Ångstrom Finish
? 1.0 Micrometer
22
The Likely Finish
? 1.0 Micrometer
23
Range of values
24
Dust Contamination
Figure courtesy of Gary Peterson, Breault
Research Organization.
25
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
level n number of particles per
square-foot with diameters greater than s
Courtesy of Gary Peterson, Breault Research
Organization.
26
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.

27
Roll-off in the IR (10 microns)
From Spyak and Wolfe, Scatter from
particulate-contaminated mirrors, Part 3
28
The Mie Model for 0.01 Coverage (Level 230)
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Dust results at 1 Micron
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Dust accumulation
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Accumulation with time
32
40 Times faster at Apache Point
33
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?

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A Large Compressor!
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The Experiment
36
The Air Knife and Samples
37
Super Air Knife by Exair
38
24-Hour Accumulation
200 ? Magnification
10 ?m
330 ?m
39
The Need for Clean Air
40
Dust Scatter vs. Wavelength
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Other Stray Light Sources
42
Relative Contributions
Relative Contribution
43
Conclusions from the Reports
M1 Microroughness and Dust Contamination

• 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.

44
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.