Optical Analysis

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• Optical Analysis
• and
• Stray Light Evaluation
• Dennis Charles Evans
• 16 November 2001

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• ZEMAX Optical Layout
• ISAL prescription from TMA62 write-up
• Checked against customer supplied ZEMAX
• Optical Sensitivities
• Basic Sensitivity
• Active Secondary (5-dof)
• Stray Light Baffle Layout
• Spectrograph Enclosure and Insertion
• Views from the Detector
• Solar Flux
• Open Items

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• ZEMAX Optical Layout
• ISAL prescription from TMA62 write-up
• Checked against customer supplied ZEMAX

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Optical Sensitivity

• Basic Sensitivity
• Individual Element Increments of 0.001 mm or
  degree to 0.016
• MF Limit (RMS Spot Radius)
• (0.0028370.005000)2 0.32194276
• Active Secondary
• Five degree-of-freedom
• Dx, Dy, Dz, Tx, Ty
• Limits on Secondary
• 1 mm or degree
• Limits on Elements
• 0.256 mm or degree

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Key Tolerance Comparison
Basic Limit Active Secondary Limit
Primary Tilt 0.0003 deg 0.185 deg
Primary-Secondary Space 0.0008 mm 0.033 mm
Tertiary Tilt 0.016 deg 0.055 deg
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• Basic Sensitivity (limited to 0.005 RMS Radius)

TETX TETY Tilt about X Y 5Primary
0.000335 deg 3600 arc-sec/deg 1.206
arc-sec TTHI Thickness 4-6
0.000802 mm TRAD Radius of
Curvature 5 0.001604 mm ( same
effect as thickness change) TIRX TIRY Sag
(Tilt) Across Lens 5 0.011705
mm TSDX TSDY Surface Decenter 5
0.014518 mm TEDX TEDY Element Decenter
5 0.014518 mm TETX TETY Tilt
about X Y 8Secondary 0.001515 deg
3600 arc-sec/deg 5.454 arc-sec TTHI
Thickness 7-9 0.000602
mm TRAD Radius of Curvature 8
0.001828 mm TIRX TIRY Sag (Tilt) Across
Lens 8 0.011261 mm TSDX TSDY
Surface Decenter 8 0.014658
mm TEDX TEDY Element Decenter 8
0.014658 mm TETX Tilt about X Y
17Fold Flat 0.008217 deg 3600
arc-sec/deg 5.454 arc-sec TETY Tilt
about X Y 17 0.011621 deg
3600 arc-sec/deg 5.454 arc-sec TTHI
Thickness 16-18 0.018010
mm TRAD Radius of Curvature 17
insensitive at 0.256 change TIRX
Sag (Tilt) Across Lens 17 0.150454
mm TIRY Sag (Tilt) Across Lens 17
0.106387 mm TSDX TSDY Surface Decenter
17 insensitive at 0.256 change
(0.002884) TEDX TEDY Element Decenter 17
insensitive at 0.256 change
(0.002884) TETX TETY Tilt about X Y
22Tertiary 0.016927 deg 3600 arc-sec/deg
60.9372 arc-sec TTHI Thickness
21-23 0.016919 mm TRAD
Radius of Curvature 22 0.031079
mm TIRX TIRY Sag (Tilt) Across Lens 22
0.199252 mm TSDX TSDY Surface Decenter
22 insensitive at 0.256 change
(0.003732) TEDX TEDY Element Decenter 22
insensitive at 0.256 change
(0.003732) TETX TETY Tilt about X Y
32-33Filter insensitive at 0.256 change
(0.002886) TTHI Thickness
21-23 0.016919 mm TRAD Radius
of Curvature 22 0.031079 mm
TIRX TIRY Sag (Tilt) Across Lens 22
0.199252 mm TSDX TSDY Surface Decenter
22 insensitive at 0.256 change
(0.003732) TEDX TEDY Element Decenter 22
insensitive at 0.256 change
(0.003732) TTHI Filter to Image
36-37 0.136618 mm (Image Plane is
compensator for fast analysis)
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Active Secondary Sensitivity
------------ Minimum
------------ ------------ Maximum
------------ Type Value
MF Change Value MF
Change (Primary Mirror) Thickness tolerance on
surface 4 TTHI 4 6 -0.033610
0.005000 0.002163 0.033112 0.005000
0.002163 Thickness 8
1.00001E000
-1.00001E000 Radius tolerance on surface 5 TRAD
5 -0.066773 0.004566 0.001730
0.066242 0.005000 0.002163 Thickness 8
1.00001E000
-1.00001E000 Tilt X tolerance on surfaces 5
through 5 (degrees) TETX 5 5 -0.185670
0.005000 0.002163 0.185670 0.005000
0.002163 Parameter 2 on Surface 7
9.12416E-002 -9.12419E-002
Parameter 3 on Surface 7 -1.87703E-001
1.87703E-001 Tilt Y
tolerance on surfaces 5 through 5 (degrees) TETY
5 5 -0.185720 0.005000 0.002163
0.185720 0.005000 0.002163 Parameter 1 on
Surface 7 -9.12903E-002
9.12680E-002 Parameter 4 on Surface 7
-1.87754E-001
1.87754E-001 (Fold Flat) Thickness tolerance on
surface 16 TTHI 16 18 -0.256000
0.004764 0.001927 0.256000 0.004764
0.001927 Thickness 8
1.74322E-001
-1.87717E-001 TIRX 17 -0.256000
0.004388 0.001551 0.256000 0.004388
0.001551 Parameter 1 on Surface 7
-5.13796E-001
5.13786E-001 TIR Y tolerance on surface 17 TIRY
17 -0.222637 0.005000 0.002163
0.222637 0.005000 0.002163 Parameter 2 on
Surface 7 6.31916E-001
-6.31897E-001 Tilt X tolerance on surfaces
17 through 17 (degrees) TETX 17 17
-0.017196 0.005000 0.002163 0.017196
0.005000 0.002163 Parameter 2 on Surface 7
6.31902E-001
-6.31891E-001 Tilt Y tolerance on surfaces 17
through 17 (degrees) TETY 17 17 -0.024319
0.005000 0.002163 0.024319 0.005000
0.002163 (Tertiary Mirror) Tilt X tolerance on
surfaces 22 through 22 (degrees) TETX 22 22
-0.055298 0.005000 0.002163 0.055298
0.005000 0.002163 Parameter 2 on Surface
7 -1.00001E000
1.00001E000 Tilt Y tolerance on surfaces 22
through 22 (degrees) TETY 22 22 -0.055298
0.005000 0.002163 0.055298 0.005000
0.002163 Parameter 1 on Surface 7
1.00001E000 -1.00001E000
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• Stray Light Baffle Layout

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• Spectrograph Enclosure and Insertion

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• Views from the Detector

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• Solar Flux Concentrations inside Instrument
• Solar Flux 1 SC .1367 watt.cm-2 1367
  watt.m-2

Location Image Radius (mm) Flux (SC) Flux (watt.cm-2) Flux (watts)
Entrance Aperture 1000 1 .1367 4295
Secondary Mirror 213 22 3.1 4295
Prime Focus Ring 10.7 4367 597 2147
Focal Plane 1 64 244 33 4295
Shutter Pupil 48 437 60 4295
FP 2 (CCD/Image) 96 109 15 4295
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Open ItemsConcerns
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• Open Items
• Shade angle (lt30o?) and Barrel Baffle length
• Baffle edge finish (Forward Ring)
• Cleanliness, especially of Primary Mirror
• Aft Optics Baffles (Mostly black paint)
• MLI outer layer vs. mission length
• Reverse views
• AutoCAD Photo Ray Trace
• reverse ray trace, TMA62
• Focal plane
• parfocalization (filter thickness)
• S/G placement
• behind focal plane?
• center mounted
• ASAP diffuse scatter model
• (Friday PM Next Week)

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• Mission Unique Concerns
• No mission unique concerns have been observed.
• Stray light issues are manageable using proven
  baffling techniques that have been successfully
  demonstrated on flight missions.
• Analysis done too early in the design cycle is
  likely to give false confidence (analytical
  design is better than real design).

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• Central Obscuration
• Minimum possible - about 5 (Shadow of Secondary)
• Current minimum design to keep direct light from
  Focal Surface 1 - about 15
• Maximum practical limit to keep direct light from
  inner baffle interior and front lip - slightly
  less than 25

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• Stray Light Issues
• Similarity to flight instruments shows successful
  design is possible
• HST, FUSE, COBE-DIRBE, IUE, OAO
• Similarity to flight instruments shows design
  flaws are possible that will not be detected by
  stray light analysis, but by careful design,
  inspection, and testing.
• HST - Door reflections
• DIRBE - Barrel Baffle side wall single bounces.
• IUE/OAO - Pinhole leaks into aft optics
  assemblies bolt head locations.
• Design Analysis Similarity
• SIRTF - IRAC
• WIRE (minor changes make orders of magnitude
  stray light differences)
• GOES Earth Limb Sensor (light leaks around
  filters and side wall reflections)
• MODIS - Stray light in calibrator designs, not
  instrument designs

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• Focus Drive Mechanism
• Definition of acceptable image size needs to be
  reviewed.
• RMS radius of 0.005 mm may be to conservative for
  a pixel limited performance instrument.
• Range of motions used were /- 1 mm and degree.
• The Displacement motions may be effectively
  increased to 5 or 10 mm for some corrections.
• Tolerance analysis so far is only with two
  elements at a time and has not included Image
  Plane tilts.

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• View from the detector inspection is important.
• Fold Flat throat can be illuminated by sky.
• Fold flat throat design as in this study may not
  allow AFT star tracker
• AFT optics baffles and black surfaces
• Baffles not designed yet
• Black paint has been known to cause CTE changes
  in GFRP structures.