# Presentation: 180027

## MSFC Ball SBMD Cryogenic Optical Testing Data Review

- James Hadaway, Joseph Geary, Patrick Reardon, Bruce Peters, Phil Stahl, Ron Eng, John Keidel, & Jeff Kegley
- The University of Alabama in Huntsville
- & Marshall Space Flight Center
- May 9, 2001

## Outline

- SBMD Requirements
- Optical Testing System
- Results of first test
- Results of second test
- Results of third test
- PSD analysis of mid-spatial errors
- Gradient & 100 K experiment results
- Conclusions

## Ball SBMD CryogenicOptical Specifications

## General System Layout

## The Ball SBMD at MSFC

## Optical Testing System Goals

- Figure:  λ/10 P-V at 633 nm or better over full aperture with a spatial resolution of 10 cm or better.
- Mid-spatial Figure: λ/20 P-V at 633 nm or better over a 10 cm diameter circle with spatial resolution of 1 cm or better.
- ROC:  10 mm or better.

## SBMD OTS Design

- Wavescope (with 150x150 lenslets, 1Kx1K, 12-bit CCD, & 0.1 msec exposure time) located atop 5-DOF stage on stabilized pier in guide tube at 1 ATM pressure.
- 300 mm diameter extension installed from chamber gate valve to within 600 mm of pier center with 150 mm diameter clear aperture window (16 mm thick BK-7) with 1° tilt (horiz).
- Fiber source located 12 mm off-axis (horiz) with achromat to collimate return beam.
- PDI and PSF camera installed on Wavescope platform as back-up measurements.
- Leica laser distance meter used to measure ROC.

## SBMD OTS Schematic

## SBMD OTS Performance

- Wavescope accuracy is λ/50 P-V per lenslet.  Surface measurement error determined experimentally to be <λ/20 P-V (ideal environment).
- Window tilt & source off-set introduce <λ/500 P-V astigmatism, coma, & spherical.
- Overall figure error estimated at **<λ/13 P-V** (w/o window correction) or **<λ/20 P-V** (w/ window correction) .
- Spatial wavelength resolution is 7 mm for this mirror.  Requires no subaperture testing for mid-spatial.
- ROC error dominated by Leica error (±3 mm) & focus error (±4 mm) for total error of **±5 mm**.
- All OTS goals met.

## SBMD Testing History

- 1st cryo test carried out during October & November of 1999. Several issues identified & addressed:
- Distortion in Wavescope relay lens prevented use of 150x150 lenslet array
- “Bumpiness” seen in 3-D plots
- “Spurious” astigmatism seen in data
- Difficult to accurately register results after the fact
- 2nd cryo test, to confirm findings of 1st test & improve accuracy, carried out during April & May of 2000.
- 3rd cryo test, following cryo-figuring, carried out during August & September of 2000.

## Definitions

- Total or Raw Results - basic Wavescope output that has piston, tilt, & focus removed.
- Zernike Fit - fit of first 42 terms.
- Residual - difference of Total & Zernike Fit.

## SBMD 1 Temperature Profile

## SBMD 1 Ambient/Cryo Figure Results

- Note:  These results are OPD, not surface error.

## SBMD 1 Cryo Deformation

## SBMD 1 ROC

## SBMD 2 Ambient/Cryo Figure Results

## SBMD 2 Cryo Deformation

- Cryo Change (287 K to 34 K):  Total = 0.571 μm PV & 0.081 μm rms
-                                              Residual = 0.134 μm PV & 0.016 μm rms

## SBMD 3 Ambient/Cryo Figure Results

## SBMD 3 Cryo Deformation

## SBMD 2 Row PSDs

**SBMD 2 Row PSDs**

## SBMD 2 Column PSDs

**SBMD 2 Column PSDs**

## SBMD 3 Row PSDs

**SBMD 3 Row PSDs**

## SBMD 3 Column PSDs

**SBMD 3 Column PSDs**

## SBMD 2 & 3 Cryo Row PSDs

**SBMD 2 & 3 Cryo Row PSDs**

## Summary of Numerical Results(units are μm)

| 
CASE | RESIDUAL |  | MID-SPATIAL |  |
| --- | --- | --- | --- | --- |
|  | PV | rms | PV | rms |
| S2 AMB | 0.128 | 0.016 | 0.036 | 0.005 |
| S2 34 K | 0.169 | 0.022 | 0.094 | 0.015 |
| S2 CHG | +0.041 | +0.006 | +0.058 | +0.010 |
| S3 AMB | 0.197 | 0.026 | 0.090 | 0.014 |
| S3 38 K | 0.145 | 0.019 | 0.057 | 0.009 |
| S3 CHG | -0.052 | -0.007 | -0.033 | -0.005 |

## Axial Gradient Test

- Small heater installed between facesheet & backsheet and facing facesheet to reverse axial gradient during SBMD 3.
- Gradient (facesheet – backsheet) changed from –3.69 K (i.e. facesheet colder) to +1.39 K (i.e. facesheet hotter).
- Almost no change seen in either low or high-order figure with the 5.08 K axial gradient change.

## 38 K vs. 99 K

- Performed stable 99 K measurement during SBMD 3 for comparison to min temperature of 38 K.
- Low-order difference of 0.249 μm PV & 0.051 μm rms (some residual tilt) and high-order difference of 0.067 μm PV & 0.009 μm rms.
- See changes in trefoil, dimple, & depressions at titanium bipod attachment points (no change in rib print-thru).

## Conclusions

- Good agreement between MSFC Wavescope & Ball/Tinsley interferometer measurements.
- Dominant cryo changes are trefoil, 0° astigmatism, dimple near center, and rib print-thru (total of about 1 HeNe wave P-V).
- Cryo deformation repeatable between first two tests.  ROC change also repeatable & matches prediction.
- Cryo-figuring achieved surface error requirements.
- Mirror assembly not sensitive to 5 K axial gradient change.
- Most cryo change seen before 100 K, but significant amount still seen from 100 K to 35 K.  LN2 testing probably not sufficient for this mirror.

## Contacts

**hadawayj@email.uah.edu**
**phil.stahl@msfc.nasa.gov**