Optical Performance of OTA:
Wavefront Error Measurements
Y. Suematsu (NAOJ) and OTA team

SOT#17 Extended, 17 Apr. 2006

1OTA Structure
Aplanatic Gregorian with heat dump mirror and collimator lens unit

Secondarymirror
Heat dump
mirrorUltra-lowthermalexpansion
truss structure

2ndary fieldstop mirror
Primarymirror– Low-expansion CFRP honeycomb
panels and pipes (0.1ppm/K) areused for mirror cell, center section,
ring plate, top ring, upper truss andlower truss.
– High thermal conductive CFRP isused to alleviate hot temperaturespot caused by oblique incidence ofsolar light.
– Panels and pipes are adhesively
bonded instead of connection byflange for weight saving and toavoid differential CTE.

Tip-tiltmirror

Inside view of OTA

SOT#17 Extended, 17 Apr. 2006

CFRP= Carbon Fiber Reinforced Plastics
2

rotation
60cm flat mirror

OTA upward configuration (+1G)
for interferometer measurements
WFE in off the center field (~130”) can bemeasured by tilting the 60 cm folding flat
Measurements for OTA upside-down
configuration (-1G) are also performed
ND (pericle)
or polarizerinterferometer6-degree-of-freedom
active stage
SOT#17 Extended, 17 Apr. 2006

3tilt

FM-OTA Interferometer Measurements
Method・usage of high-speed interferometer and phasemap construction by spatial heterodyning
method tolerant of vibration and seeing effect・ selection of good fringe map and averageover 30-40 phase maps (FLIP bitmap mode)・Consideration of air-flow system to make airtemperature in optical pass uniform
・Zero-G optical performance is evaluated byan average of OTA upward and upside-down
WFE measurements
・Usage of polarizer or ND in front ofinterferometer or off-axis measurement for
rejection of ghost fringe from PMU
・Calibration of polarizer
and ND WFE・Calibration of 60cm
folding flat mirror; nonaxially symmetriccomponent of WFE wasremoved by an average of90 degs step rotation
Null fringe map. Tri-angular astigmatism
due to gravity is prominent.
Measurements in upward (+1G) configuration
SOT#17 Extended, 17 Apr. 20064

Sample of fringe for phase map derivation

SOT#17 Extended, 17 Apr. 2006

5

OTA 0-G Test Configuration
Vertical meterclinometer

Target mirror

Reference flat
M2OTA(Optical telescope assembly)

Interferometer
MiniFiz

M1

Telescope Up

Telescope Down

Six axis stageOTA Optical Test Configuration
SOT#17 Extended, 17 Apr. 2006

6Gravity deforms OTA primary mirror and CFRP truss!

OTA上向き（平面鏡4回転平均) − キャリブレーション

OTA下向き（平面鏡4回転平均) − キャリブレーション

WFE for OTA upward configuration (+1G)(average of 4 data from 90 degs step
rotation of 60 cm flat after calibration)

WFE for OTA downward configuration (1G) (average of 4 data from 90 degs step
rotation of 60 cm flat after calibration)

SOT#17 Extended, 17 Apr. 2006

7

Optical Performance of OTA in 0-G
OTA WFE map for zerogravity condition. This was

Average WFE of upward anddownward OTA

tilt and focus term removed.
SOT#17 Extended, 17 Apr. 2006

8

obtained by averaging upwardand downward OTA WFE. Then,rms WFE gives 0.0288 waves(18.1nm, λ/35) when focus
error which can be corrected
on-orbit are removed. This
indicates that the rms WFE inthe shortest observable
wavelength 388 nm is 0.0469
waves and OTA WFE satisfies
diffraction-limited condition(less than 0.071 waves, λ/14）
in all the SOT observable
wavelengths.

OTA Opto-Thermal Test for On-Orbit Performance
OTA WFE’s were measured with aninterferometer for cases that OTA had
temperature distribution (−30℃〜＋40℃)
predicted for on-orbit condition.
The test demonstrated that OTA on orbit
maintains diffraction limited performance
although it was slightly deformed (14.7 nm rms)
by the temperature change.

Uppershroud
OTA

Duty cycle of operational heaters for M2 and CLU
does not affect the optical performance of OTA.

Dummy OBU

interferometer
SupportLowershroud

OTA alignmentcubeflat mirror

shroudTilt/shift stage

Flat mirror
referenceSOT#17 Extended, 17 Apr. 2006

9

System-level optical test
FPPOTAOTAtop door

Test tower No.2
Φ60cmflat
mirrorOTA Pupil image
FPP

BS,injection port
Injection
port

FPP
XRT

interfero
meter

6-axes stage

SOT#17 Extended, 17 Apr. 2006

10

System-level OTA optical test
Foldingflat

50%ND

ZYGO
referenceflat
lambda/50

D60cm flat

interferometer
Calibration
OTA

Calibration of 5 cm folding flat WFE// Tilt// Focus

11-0.005948
0.017461

0.053928// Coma0.026772-0.010023-0.003328// Astig Tri

0.011442

0.001463// Astig 5-0.008874-0.008480// Coma5-0.012886

0.008832// Sphere 5
SOT#17 Extended, 17 Apr. 2006

-0.002077// Astig// Sphere 3

OTA WFEmeasurement-0.004972

0.011047

History of measurements of FM-OTA WFE
Focus in0G: A20(lambda,double path)

Focus in 1G:
A20(lambda,double path)

Focus in -1G:
A20 (lambda,double path)

test

date

0-G WFE (nmrms, single path)

2004.Jul.5-7

18.2-1.29

-0.98

-1.63

FM-OTA initialintegration

2004.Jul.14-17d(5.2); hot mode–reference-1.60(pre-test)-1.25(post-test)

Opto-thermal test III

2004.Aug.
2004.Sep-Dec

Note: WFE and focus (Zernike A20 term) given in thetable are values measured at field center. WFE for OTA
upward or downward configuration is given by adifference from a reference WFE obtained before (d( )).

2005.Jan.12-14d（7.9）; change
by replacement

2005.Mar.11-20

d(14.7); cold obs.
mode – reference-1.78-2.27(pre-test)-1.43(post-test)

System EIC/MIC
test

Opto-thermal test IV
with thermal cycling
test OTA downward
Acoustic test

18.1

-1.74

-1.43

2005.Jun
2005.Sep.23

Sunlight test I

Replacement ofOTA upward astig. Corrector

2005.Apr
2005.Apr.21-25

OTA downward-2.06
Sunlight test IId（7.3）; change
from Apr.

SOT#17 Extended, 17 Apr. 2006

-2.02

12

System Performance
Test OTA upward

CFRP truss dehydration Focus Change
Truss contraction due to dehydration in vacuum.
Truss expansion due to moisture absorption in air.

A month afterlaunch, the truss isexpected to reaches
steady state*: A20 in +1G+: A20 in –1G: A20 in 0G

Trusscontraction

Focuschange
Trussexpansion

Estimates from+1G measurement
SOT#17 Extended, 17 Apr. 2006

13Summary
OTA WFE at a room temperature (20 degC) for zero-gravity condition was
18 nm rms when removed tilt and focus term and it did not change after aacoustic test and a thermal cycling test in which OTA experienced thecoldest and hottest thermal environment on orbit; the diffraction limited
WFE at wavelength of 500 nm is less than 35.7 nm rms.
Change in WFE from the room temperature in vacuum to temperature on
orbit for cold case observational condition was 14.5 nm rms (BOL) andthis indicates total WFE on orbit is 23.1 nm rms, combined with WFE of18 nm rms for zero-gravity and room temperature condition.
The difference in WFE between 130 arcsec off-center and the field center
was 13.7 nm rms and hence WFE within 130 arcsec field radius isestimated to be less than 26.9 nm rms including the thermal effect; (maxFOV: 160×320 arcsec^2）.
Focus change shown here is due to vapor absorption (in air) ordehydration (in vacuum) effect of CFRP truss and stays in a range
corrected on orbit by re-imaging lens of FPP.
OTA has a perfect optics!
SOT#17 Extended, 17 Apr. 2006

14