Blake, Peter

Crow, John

Howard, Joe

Livas, Jeff

Mueller, Guido

Preston, Alix

Sanjuan, Pep

Sankar, Shannon

Seals, Len

Shiri, Ron

West, Garrett

NASA Technical Reports Server

eLISA Consortium Meeting Paris 12-14 Oct 2015 
Telescope Development for a 
Space-based Gravitational Wave 
Observatory 
Prepared by Jeff Livas 
Shannon Sankar, Peter Blake, John Crow, Joe Howard, Len Seals, 
Ron Shiri, Garrett West 
NASA Goddard Space Flight Center 
Guido Mueller, Alix Preston, Pep Sanjuan 
University of Florida 
1 
eLISA Consortium Meeting 
12-14 Oct 1 2015 
https://ntrs.nasa.gov/search.jsp?R=20150021898 2019-08-31T05:19:41+00:00Z
eLISA Consortium Meeting Paris 12-14 Oct 2015 
Project Objective and Approach 
•  Objective: 
•  Key challenging requirements 
–  Optical pathlength stability 
–  Scattered light performance 
–  Manufacturable design 
•  Approach 
–  Develop a telescope design that 
o  Meets eLISA technical requirements 
o  Can be manufactured (need multiple (~ 10) copies) 
o  TRL-5 by CY2018 (nominally for EM model) 
–  Commission a study with a commercial optics/telescope vendor for 
advice on manufacturability 
–  Demonstrate we can implement the design 
2 
To design, fabricate and test a telescope to verify that it meets 
the requirements for precision interferometric metrology for 
space-based gravitational-wave observatories.  
eLISA Consortium Meeting Paris 12-14 Oct 2015 
Telescope Requirements 
3 
challenging 
challenging 
 Parameter Derived From eLISA/NGO 
1 Wavelength  1064 nm 
2 
Net Wave front quality departure 
from a collimated beam of as built 
telescope subs system over Science 
field of regard under flight-like 
conditions 
Pointing ≦  λ/30 RMS 
3 Field-of-Regard (Acquisition) Acquisition +/- 200 µrad (large aperture) 
4 Field-of-Regard (Science) Orbits +/- 20 µrad (large aperture)  
5 Field-of-View (Science) Stray light +/- 8 µrad (large aperture) 
6 Science boresight FOV, pointing +/- 1 µrad  (large aperture)
 
7 
Telescope subsystem optical path 
length1 stability under flight-like 
conditions 
 
 
Path length Noise/ 
Pointing 
€ 
≤ 1 pm / Hz × 1+ 0.003f
$ 
% 
& 
' 
( 
) 
4$ 
% 
& & 
' 
( 
) ) 
 
 
where 0.0001 < f < 1 Hz 
1 pm = 10-12 m 
8 Afocal magnification short arm 
interferometer 
200/5 = 40x (+/-0.4) 
9 Mechanical length  < 350 mm TBR 
10 Optical efficiency (throughput) Shot noise >0.85 
11 Scattered Light Displacement 
noise 
< 10-10 of transmitted power 
into +/- 8 µrad Science FOV 
 Interfaces: Received beam (large aperture, or sky-facing) 
12 Stop Diameter (D) (large aperture) Noise/ pointing 200 mm (+/- 2 mm) 
13 Stop location (large aperture) Pointing Entrance of beam tube or 
primary mirror 
Interfaces: Telescope exit pupil (small aperture, or optical bench-facing) 
14 Exit pupil location Pointing 13.5 +/- 2 cm (on axis) 
behind primary mirror 
15 Exit pupil diameter optical bench 5 mm (+/- 0.05 mm) 
16 Exit pupil distortion SNR < 10% 
17 Exit pupil chief ray angle error   +/- 10 µrad 
 
SGO-Mid = 250 mm 
From U of Glasgow 
bench design, courtesy 
of Ewan Fitzsimons 
and Harry Ward 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
  Spacer Activity Objective 
–  Develop and test a design for the main 
spacer element between the primary 
and secondary mirrors 
–  M1 - M2 spacing identified as critical by 
tolerance analysis 
–  SiC limited by lab thermal fluctuations 
–  Would meet requirements on orbit 
Previous Work: On-axis Telescope Spacer Design 
ΔT=1.5º 
ΔT=~ 0º 
-71
º 
Thermal Model to Determine Test Conditions 
SiC Spacer Design: QuadPod
Can Meet Requirements at -65C 
Requirements 
SiC Spacer  Design
SiC Spacer Thermal Environment
J. Sanjuan, et al.; Rev Sci Instrum. 83(11), 116107 (2012). 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
Commercial Vendor: Designs considered 
•  Both designs have the same nominal requirements 
•  Exclusion zone (in red) is for bench optics 
5 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
Commercial Vendor: Manufacturability 
6 
Off-axis mirror difficulty On-axis mirror difficulty 
•  On- vs off-axis mirrors similar in complexity 
•  On- vs off-axis system alignment similar in complexity 
–  Compensation techniques are similar 
•  Schedule is 16 months for first copy 
–  Driver is material availability for SiC (study contractor makes material!) 
–  Once material is cast, then machining is the bottleneck 
–  “pipeline” approach is possible and reduces recurring schedule to ~ 10-12 months/copy 
eLISA Consortium Meeting Paris 12-14 Oct 2015 7 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
Scattered Light Analysis 
8 
•  Source power = 1W 
•  Total power on the detector = 
6.6x10-11 W ! (barely) meets 
specification of less than 10-10 
Exit pupil 
Primary (M1) 
Secondary (M2) 
M3 
M4 
Intermediate 
focus 
Pupil Plane Scatter Irradiance 
Mirror RMS surface roughness (Å) MIL-STD 1246D CL 
M1 15 300 
M2 15 200 
M3 5 200 
M4 5 200 
Conflicting 
accounts of 
on-orbit 
levels 
M3 and M4 contribute most of the scattered light on the detector 
mirror 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
Prototype Telescope Design 
9 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
10 
•  Validate scattered light model 
–  Determine surface roughness 
o  needed to meet requirements 
o  Where particulates become 
important 
–  Components get dirty while making 
measurements 
•  M3/M4 dominate budget 
–  Test M3/M4 separately 
o  Faster cycle-time than full telescope 
–  Use mirrors with different properties 
o  Surface roughness 
o  Reflective coatings 
o  Surface contamination levels 
–  Mirrors need not have telescope 
prescription for some tests 
–  Practice alignment techniques 
•  Develop analysis pipeline 
–  BRDF (component level) to predict 
system level 
M3/M4 Scattered Light Test Bed 
Scattered Light Test Bed 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
Optical Test Setup 
•  Telescope tested double-pass from 
the small aperture side 
•  Currently aligned to better than λ/34 
•  stable under normal lab conditions 
•  Room temperature operation only 
Interferometer 
M1 
M2 
M3 
M4 Flat 
mirror 
200 mm 
5 mm 
Flat mirror Telescope 
Interferometer 
Optical Layout 
As installed in the cleanroom Measured WFE performance 
λ/34, center field, 632.8 nm 
eLISA Consortium Meeting Paris 12-14 Oct 2015 
SUMMARY/NEXT STEPS 
•  Prototype installed and aligned 
• Delivered to GSFC 6/5/15 (originally 3/20/15) 
• Reassembled and realigned by 7/27/15 
•  Tested double-pass with an interferometer (LUPI) 
•  Residual wavefront error is λ/34 (λ/30 spec) at 632.8 nm 
•  Alignment is stable under laboratory conditions 
•  Next steps: 
•  verify wavefront error at 1064 nm 
•  beam dump for transmitted light needed 
•  use carbon nanotubes (R < 0.5%) 
•  verify scattered light model 
•  Concern: mirrors are dirty 
•  Vendor packaged poorly for shipping 
•  May have to try cleaning M1, M2 (no spares) 
•  Have clean spares for M3, M4 
Particulates on Primary  
Telescope 


Telescope Development for a Space-Based Gravitational Wave Observatory

Telescope Development for a Space-Based Gravitational Wave Observatory

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