FGS Tunable Filter Imager:
Updates From PDR
Alex Fullerton
STScI / UVic
TIPS/JIM
May 19, 2005
FGS PDR: May 4/5, 2005
01-1
Design Updates to TF Imager at PDR
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Maturing Instrumental Design
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•
•
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•
Coronagraph moved
Revised distortion map
Progress on coatings for etalons and dichroic beamsplitter
Much more work on ghosts
Mechanisms and Etc.
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•
•
Dual filter wheel design is well in hand
Some elaboration of onboard calibration units
Preferred material for etalon launch lock identified
FGS PDR: May 4/5, 2005
01-2
FGS-TFI Solid Model
FGS PDR: May 4/5, 2005
01-3
Current JWST FOV Layout
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•
The FGS Optical Assembly supports two
pickoff mirrors at or near the OTE focal plane
Packaging is tight but current design meets
the allocation
FGS-TF Coronagraphic
Mask locations
80”
20”
7λ/D
(1’’)
FGS PDR: May 4/5, 2005
10λ/D
(1.4’’)
15λ/D
(2.2’’)
20λ/D
(2.9’’)
01-4
Coronagraphic slide design
7λ/D (1’’)
10λ/D (1.4’’)
15λ/D (2.2’’)
20λ/D (2.9’’)
RMS WFE versus Coronagraph Slide Thickness
40
LW Full Field Performance
LW Coronagraph
SW Full Field Performance
RMS nanometers
35
SW Coronagraph Performance
30
25
20
•
Occulting spots have a graded intensity profile
(Bessel^2, Sinc^2)
•
•
Best performance at long wavelengths (~4 µm)
•
Bars optimized for short wavelengths (~2 µm)
Circular spots optimized for long wavelength
operation
15
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Slide Thickness (mm)
FGS PDR: May 4/5, 2005
01-5
FGS-Tunable Filter Imager Distortion
as Plate Scale Variations
Plate Scale vs Field
Variation over image
Pos 1
Plate Scale vs Field
MAS
Variation over image
Pos 2
MAS
65.558
65.520
64.913
64.914
64.307
64.267
Plate scale units = mas/pixel (FOV each pixel sees)
Longwave
Shortwave
Average (x/y)
64.96
64.95
Minimum (x/y)
64.27
64.31
Maximum (x/y)
65.56
65.52
FGS PDR: May 4/5, 2005
01-6
FGS TFI Plate Scale Anisotropy
X and Y Values
Longwave
Shortwave
X
Y
X
Y
Average
64.59
64.96
65.27
65.77
Minimum
63.91
64.63
63.94
64.66
Maximum
65.30
65.82
65.27
65.77
FGS PDR: May 4/5, 2005
01-7
Preliminary Dichroic Design
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Germanium Substrate
72 layer a-Si / SiO2
FGS PDR: May 4/5, 2005
01-8
Fabry-Perot Etalons
Optical path difference = 2µl cosθ
Phase difference = (2π/λ) 2µl cosθ + δr
mλ = 2µl cosθ + δr /2π
Tuning: For fixed m,
on-axis λ(θ=0) ∝ l
µ=index of refraction
FGS PDR: May 4/5, 2005
Detuning: For fixed m, l,
off-axis λ(θ) ∝ cos θ ≈ 1 − θ2/2
01-9
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
Substrate (SiO2)
FGS PDR: May 4/5, 2005
18.5
72.6
231.9
79.5
382.8
18.3
90.7
26.9
437.3
18.2
34.7
166.6
75.4
18.4
18.1
74.4
242.3
79.3
381.6
16.9
93.4
26.5
459.0
16.6
38.2
173.9
71.9
20.1
5.0
0.875
4.5
0.850
4.0
0.825
3.5
0.800
3.0
2.5
0.775
Reflectance
Phase
0.750
800
1000
1200
1400
1600
1800
2000
2.0
2200
Wavelength (nm)
Design Update
130
4000
120
3500
110
3000
100
2500
90
2000
80
1500
Gap (nm)
Material
Air / Vacuum
SiO2
"In Process"
Original Design Design Update
Thickness (nm)
0.900
Reflectance
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•
Deposit first 8-layers of design &
measure layer properties
Re-optimize design of remaining layers
Deposit remaining 6 layers
5.5
Resolution
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0.925
Phase (radian)
SW Etalon Mirror Coating
Deposition Process
Resolution
Gap
70
Measured
800
1000
1200
1400
1600
1800
2000
1000
2200
Wavelength (nm)
01-10
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Material
Air / Vacuum
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
a-Si
SiO2
Substrate (Si)
4.5
0.875
4.0
0.850
3.5
0.825
3.0
0.800
2.5
Thickness (nm)
20.92
201.34
484.53
210.5
562.99
30.47
240.21
400.57
62.14
120.2
91.47
521.62
102.62
139.75
96.24
536.97
86.27
140.4
109.85
150.08
46.22
0.775
2.0
Reflectance
0.750
2000
Phase
2500
3000
3500
4000
4500
1.5
5000
Wavelength (nm)
112
9000
110
8500
108
8000
106
7500
104
7000
102
6500
100
6000
98
5500
96
5000
94
4500
92
4000
3500
90
88
86
2000
Resolution
Gap
2500
3000
3500
4000
4500
3000
2500
5000
Wavelength (nm)
FGS PDR: May 4/5, 2005
( di
)
0.900
Ph
Design updated to use optical
parameters from INO’s high
temperature deposition process
21 layer design on
silicon substrate
Candidate silicon
substrate from
Lattice Materials
Polished by BMV
Technologies
Discussed in
Etalon Design
section
5.0
Resolution
•
0.925
Reflectance
LW Etalon Status
01-11
Ghost Analysis FGS-TFI Pupil Region
Coronagraph
Plane
Dichroic
BeamSplitter
SW Pupil
Region
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3 elements create 15 ghost images in each
channel
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3 intra-element ghosts
12 inter-element ghosts
LW Pupil
Region
FGS PDR: May 4/5, 2005
From document 843183 FGS TFI Ghost Image Analysis
01-12
FGS-TFI Ghost image analysis
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FGS PDR: May 4/5, 2005
Example: Tilting pupil mask one degree
translates ghost image location
01-13
FGS-TFI Ghost Image Elimination
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Tilt Pupil Mask and Blocking Filter by 3 degrees across plane of instrument
Prevent redirected light from propagating along unwanted paths along TMA components.
One ghost image from bottom edge of detector to Etalon presents potential ghost of
relative strength of 0.0004, using conservative reflectance values
FGS PDR: May 4/5, 2005
01-14
Dual Wheel Configuration
Filter Wheel
8 position Filter Wheel allows for 6
blocking filters, an open for
calibrations and one spare
Open
8 position Pupil Wheel allows for a Lyot
mask, up to 4 apodization masks,
2 calibration source positions and a neutral
density position for target acquisition.
B6
Spare
B5
B1
B4
B2
B3
Pupil Wheel
Lyot
Apo4
FFCal
Apo3
Note: Calibration source positions on Pupil
Wheel also serve as closed
positions for dark calibrations
FGS PDR: May 4/5, 2005
Apo1
WCal
Apo2
ND
01-15
Calibration Optics (PFlat & LFlat)
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Diffuser & Cavity mounted in Pupil Wheel
Light sources located adjacent to pupil wheel
on main optical bench
Multiple sources feeding a small (<6 cm
diameter) sphere can provide some
redundancy
Pupil Wheel:
FFCal position
Note: Difference between PFlat & LFlat is in
blocking filter & etalon settings
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PFlat: Filter wheel set to open position, etalon
scanned during integration
LFlat: Blocking filter selected & etalon set during
integration
FGS PDR: May 4/5, 2005
01-16
Calibration Optics (Etalon Monitor)
Pupil Wheel: WCal position
Option 1
Option 2
Side view:
LW WCal
Si
Cut Away Side View
Front View
Pupil
10.00
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MM
Two options for WCal
Relative Intensity of images on
detector will allow etalon set point
and parallelism to be monitored
FGS PDR: May 4/5, 2005
01-17
Flight Tie-Down Design
The Question: How to provide 890 N preload (sufficient by analysis) under
launch conditions, and only 20 N under operational conditions?
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High-Modulus / High-Strength / Negative CTE tie-down using Zylon HM.
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Low elastic strain @ 890 N, < 40 µm.
Creep is an issue that can be designed for,
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Since stress level is only 2.4% of the UTS.
Three-week creep test performed,
needs to be confirmed with flight tie-down.
Fiber grows as it cools to operational temperature.
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> 100 µm growth.
Positive or 0 CTEs of other materials in the load path
will aid this mechanism.
This mechanism has been tested more than 30 times at various preload
levels, by tightening a representative loaded assembly with a Zylon tiedown, subjecting it to LN2 and feeling it go slack when cold, and
measurably recovering the preload after the return to room temperature.
Loop of fiber fed through a hole in the screw and around a steel dowel,
then the nut is tightened with a torque wrench, preloading the assembly.
FGS PDR: May 4/5, 2005
01-18