# Presentation: 551498

## Optical Analysis
and
Stray Light Evaluation

Dennis Charles Evans
16 November 2001 

- and
- Stray Light Evaluation
- Dennis Charles Evans
- 16 November 2001

## 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

- 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

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

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

## Slide 4

## Slide 5

## Slide 6

## Optical Sensitivity

- Basic Sensitivity
- Individual Element Increments of 0.001 mm or degree to 0.016
- MF Limit (RMS Spot Radius)
- (0.002837÷0.005000)*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

## 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 |

## Basic Sensitivity (limited to 0.005 RMS Radius)

- TETX TETY   Tilt about X & Y        5=Primary       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        8=Secondary     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        17=Fold 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        22=Tertiary     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        32-33=Filter    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)

## Active Secondary  Sensitivity 

- 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:                     1.00001E+000                       -1.00001E+000
- Radius tolerance on surface 5
- TRAD   5           -0.066773    0.004566    0.001730    0.066242    0.005000    0.002163
-  Thickness   8:                     1.00001E+000                       -1.00001E+000
- 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:       -1.00001E+000                        1.00001E+000
- 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:        1.00001E+000                       -1.00001E+000

- Active Secondary  Sensitivity

## Stray Light Baffle Layout

## Slide 12

## Slide 13

## Slide 14

## Slide 15

## Slide 16

## Spectrograph Enclosure and Insertion

## Slide 18

## Slide 19

## Slide 20

## Slide 21

## Views from the Detector

## Slide 23

## Slide 24

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

- 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 |

## Open Items&Concerns

## Open Items

Shade angle (<30o?) 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)

- Shade angle (<30o?) 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)

## 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).

- 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).

## 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%

- 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%

## 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

- 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

## 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.  

- 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.

## 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.

- 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.