The LISA Mission proposal was selected by ESA's Science Program Committee (SPC) for the L3 Cosmic Visions launch opportunity, and is currently in a Phase A mission formulation study. An essential component of the instrument is a set of optical telescopes that make precision displacement measurements between pairs of proof masses that are flying drag-free in three separate satellites. We discuss the key requirements on the telescope, the current design, and discuss progress toward solving some of the key challenges for realizing the telescope designs

Keski-Kuha, Ritva

Livas, Jeffrey C.

NASA Technical Reports Server

LISA Telescope Challenges
For the Telescope Team
Presented by Ritva Keski-Kuha/GSFC
Mirror Tech Days
Nov 5, 2018
1
https://ntrs.nasa.gov/search.jsp?R=20190001503 2019-08-30T10:46:17+00:00Z
Telescope Team
• PI: Jeff Livas
• Product design lead (PDL): Ritva Keski-Kuha
• Optics: Hui Li with help from Garrett West, Joe 
Howard, Mark Wilson
• Scattered light: Shannon Sankar, Len Seals
• Mechanical: Michael Hersch, Alex Miller, Andrew 
Weaver, Joe Ivanov
• Thermal: Angel Davis
• Instrument scientists: Ryan DeRosa, Shannon Sankar
• UF: Guido Mueller, Paul Fulda, Joe Gleason, Ada 
Uminska, Alex Weaver
2Mirror Tech Days Nov 5, 20180
Telescope Functional Description
• Efficiently deliver power on-axis to 
the far spacecraft 
• Simultaneous transmit and receive
• Afocal beam expander
• 300 mm dia primary
• 2.24 mm dia on bench
• 134X magnification
• Conjugate pupils to minimize tilt 
to length coupling
• Map angular motion of the 
spacecraft jitter to angular 
motion on the optical bench with 
minimum lateral beam walk or 
piston
3
• Application is precision length 
measurement NOT image formation
• Keep optical pathlength stable to ~ 
1 pm/√Hz
• Minimize coherent transmitter 
backscattered light
Mirror Tech Days Nov 5, 20180
Key Telescope Requirements
challenging
challenging
4
+/- 225 microradians
Mirror Tech Days Nov 5, 20180
Telescope Design Drivers
• Robust optical design
• Adequate build tolerances
• Adequate environmental sensitivityà low CTE glass
• Thermal
• Steady state
• Response to fluctuations
• Vibration, shock
• Adequate interface tolerances
• Acceptable scattered light performance  à off-axis design
• Reasonable particulate contamination requirements
• Robust mechanical design
• Materials choice can handle loads and be thermally and mechanically stable
• Can be manufactured on a small scale
• Acceptable cost, risk
5Mirror Tech Days Nov 5, 20180
Key Telescope Milestones
• Nominal Launch Date is 2034
• 2022 is Adoption (Implementation Phase gate) 
• NASA plans to supply a telescope that meets LISA mission 
requirements
• Procurement initiated Feb 2019 (pending confirmation of baseline 
trades and design at the Mission Consolidation Review (MCR))
• 12 months for a mechanical model (Feb 2020)
• 18 months for first optical model (Aug 2020)
• 24 months for second optical model (Feb 2021)
• Elegant breadboard delivery (Nov 2021)
• Adoption June 2022
6
Not much time for 
testing!
SCHEDULE IS TIGHT
Mirror Tech Days Nov 5, 20180
Telescope Design Summary: ”Bobsled”
• 4-mirror Cassegrain design plus 
Schwarzshild pupil extender
• 345 mm clear aperture F/1 primary
• M1- M2 spacing 725 mm
• External (virtual) pupil located over 
proof mass
• 134X magnification
• All low-CTE material construction
• ULE or Zerodur
7
Component Mass (kg)
Bobsled 14.32
M1 (50% Lightweight) 6.63
M2 with Mount 0.10
M3,M4, mechanism 1.16
Total 22.21
M1
M2
M3
M4 Internal Pupil /STOPIntermediate Image
345 mm External Pupil
Nominal center of test mass
410 mm to 
M1 Vertex
724.5 mm spacing
M1: Parabola
M2: Hyperbola
M3: Elliptical
M4: Sphere
Gravitational 
Reference 
Sensor (GRS)
Optical Bench 
Telescope: all Zerodur?
• Notional: need to agree on interfaces, etc
• Modular to allow for alignment and integration of OB with telescope or GRS
• Lightweighting and structural analysis in progress
Moveable Optical Sub Assembly (MOSA)
Notional CAD Model
Supplied by UK
Supplied by Italy
“keep out zones” 
(to allow for optics 
on the bench)
Materials Challenges: dimensional stability
• Concept: ultra-low expansion (CTE) glass construction plus 
thermally stable environment provides dimensional stability
• CTEs of Zerodur and ULE are both acceptable
• Zerodur also exhibits long term contraction
• ~ Exponential with ~ 11 year time constant (out of measurement band)
• Does not appear to be a direct displacement noise problem
• Over mission lifetime may degrade the WFE (defocus?)
• ULE production may require expensive stack seal or multiple-piece 
construction
9
Measurements of dimensional stability
0 20 40 80months
250
MHz
100
150
50
0
Δ
ν
t
0 2000 4000 6000 s
121.4
121.5
121.6
MHz
Figure 3. The measured variation of the offset frequency
between a suitable resonance frequency of a Fabry–Pe´rot
interferometer and the iodine stabilized laser shows a
shrinking of the Zerodur spacer of 1l/l = 4⇥ 10 7 over
about 76 months. Zero of the abscissa corresponds to
December 1 1986. The inset shows the temporal variation
of the length of the same Fabry–Pe´rot interferometer
measured over two hours on the day indicated by the arrow
in the main plot. It is attributed to temperature fluctuations
of the FPI.
to ageing as taken from figure 3 corresponds to about
3.66 MHz/month = 1.44 Hz s 1 in the beginning and
1.86 MHz/month = 0.71 Hz s 1 after about 75 months.
If we again assume that it is necessary to wait for at
least five times the uncertainty (2.5 MHz), which is here
determined by the temperature instability, about 20 days
should be left between two consecutive measurements to
obtain a meaningful estimate of the ageing.
3.2. Improved techniques
These results, however, can be improved considerably as
can be seen from an analysis of data obtained in another
experiment differing in several aspects from that described
above. An optical frequency standard is utilized where
the laser frequency is stabilized to the intercombination
transition of atomic Ca [12]. The laser frequency (  =
657.46 nm) is stabilized to a suitable resonance frequency
of a FPI comprising of two Zerodur mirrors optically
contacted to a Zerodur M spacer (L = 23 cm). The
Zerodur M sample is from a batch of samples different
to that in section 2. The entrance mirror and the second
mirror have a radius of curvature of R = 10 m and
R = 1 m respectively, and reflectivities that lead to a
measured finesse of more than 11 000. The so-called
Pound–Drever–Hall phase-modulation technique [15] was
used for stabilization (figure 2(b)). In contrast to figure 2(a)
where a second (offset) laser was used, here (figure 2(b)) the
same laser was stabilized to the optical resonator and to the
atomic absorber using an acousto-optic modulator (AOM)
to allow for an adjustable offset [10]. Since the laser
light double passes the AOM the radio frequency driving
the AOM is equivalent to twice the frequency difference
between both references and can be used to monitor the drift
0 200 400 600
-20
-10
0
10
MHz
Δ
ν
days
t
Figure 4. Measured variation of the offset frequency
between a suitable resonance frequency of a Fabry–Pe´rot
interferometer (comprised of Zerodur mirrors and a Zerodur
M spacer) and the centre of the Ca intercombination
transition. Zero of the abscissa corresponds to May 15
1995.
of the FPI. Again the FPI was kept in a vacuum housing
which differed from the previous one in several aspects.
Heater foils were attached to a closed aluminium shield
covered with a thermally insulating material surrounding
the vacuum housing. The dimensions of the housing were
smaller than in the previous case. Again three zones were
temperature controlled independently.
The frequency offset between a suitable resonant
frequency of the FPI and the Ca intercombination line was
measured for more than 600 days (figure 4). Immediately
after placing the FPI into the vacuum a large drift is
observed that gradually decreases before changing sign after
about 80 days and emerging into an almost linear increase
of below 0.8 Hz s 1 corresponding to a relative shrinking
of about 1.7 ⇥ 10 15 s 1. This value is similar to that
observed with the previous FPI made of Zerodur. The
reason for the expansion of the FPI during the first 80
days is not yet clear. The most obvious hypothesis is that
stress inside the material is relieved when the specimen
is put into vacuum. This hypothesis might be checked
by investigations of subsequent cycles between normal
pressure and vacuum. Another possible cause for the
unusual behaviour could be the fact that the vacuum vessel
including the FPI was heated to about 100  C for a few days
after installation. Hils and Hall [2] observed a ‘memory’
effect when they raised the temperature of their Zerodur FPI
by only 2.4 K observing ‘a gentle stress, lasting perhaps
100 days until the original creep rate was resumed’. There
is clearly some interesting physics going on that can be
investigated (but has not yet been) by the methods discussed
here.
The sensitivity of the method can be estimated from a
record of about 1270 s (figure 5), taken at the time indicated
by the arrow in figure 4 with a sample rate of two per
second. A linear fit to the data gives a slope of (0.770 ±
0.001) Hz s 1. When this drift is subtracted from the
experimental data the residual deviations can be described
very well by a Gaussian distribution of   = 20 Hz.
Applying the same assumption as before, i.e. that one has
1045
Measurements of dimensional stability
0 20 40 80months
250
MHz
100
150
50
0
Δ
ν
t
0 2000 4000 6000 s
121.4
121.5
121.6
MHz
Figure 3. The measured variation of the offset frequency
between a suitable resonance frequency of a Fabry–Pe´rot
interferometer and the iodine stabilized laser shows a
shrinking of the Zerodur spacer of 1l/l = 4⇥ 10 7 over
about 76 months. Zero of the abscissa corresponds to
December 1 1986. The inset shows the temporal variation
of the length of the same Fabry–Pe´rot interferometer
measured over two hours on the day indicated by the arrow
in the main plot. It is attributed to temperature fluctuations
of the FPI.
to ageing s taken from figure 3 orresponds to about
3.66 MHz/month = 1.44 Hz s 1 in the beginning and
1.86 MHz/month = 0.71 Hz s 1 after about 75 months.
If we again assume that it is necessary to wait for at
least five times the uncertainty (2.5 MHz), which is here
determined by the temperature instability, about 20 days
should be left between two consecutive measurements to
obtain a meaningful estimate of the ageing.
3.2. Improved techniques
These results, however, can be improved considerably as
can be seen from an analysis of data obtained in another
experiment differing in several aspects from that described
above. An optical frequency standard is utilized where
the laser frequency is stabilized to the intercombination
transition of atomic Ca [12]. The laser frequency (  =
657.46 nm) is stabilized to a suitable resonance frequency
of a FPI co prising of two Z rodur mirrors optically
contacted to a Zerodur M spacer (L = 23 cm). The
Zerodur M sample is from a batch of samples different
to that in section 2. The entrance mirror and the second
mirror have a radius of curvature of R = 10 m and
R = 1 m respectively, and reflectivities that lead to a
measured finesse of more than 11 000. The so-called
Pound–Drever–Hall phase-modulation technique [15] was
used for stabilization (figure 2(b)). In contrast to figure 2(a)
where a second (offset) laser was used, here (figure 2(b)) the
same laser was stabilized to the optical resonator and to the
atomic absorber using an acousto-optic modulator (AOM)
to allow for an adjustable offset [10]. Since the laser
light double passes the AOM the radio frequency driving
the AOM is equivalent to twice the frequency difference
between both references and can be used to monitor the drift
0 200 400 600
-20
-10
0
10
MHz
Δ
ν
days
t
Figure 4. Measured variation of the ffset frequency
between a suitable r s nance frequency of a Fabry–Pe´rot
interferometer (comprised of Zerodur mirrors and a Zerodur
M spacer) and the centre of the Ca intercombination
transition. Zero of the abscissa corresponds to May 15
1995.
of the FPI. Again the FPI was kept in a vacuum housing
which differed from the previous one in several aspects.
Heater foils were attached to a closed aluminium shield
covered with a thermally insulating material surrounding
the vacuum housing. The dimensions of the housing were
smaller than in the previous case. Again three zones were
t mp ratu e c ntrolled independently.
The frequency offset between a suitable resonant
frequency of the FPI and the Ca intercombination line was
measured for more than 600 days (figure 4). Immediately
after placing the FPI into the vacuum a large drift is
observed that gradually decreases before changing sign after
about 80 days and emerging into an almost linear increase
of below 0.8 Hz s 1 corresponding to a relative shrinking
of about 1.7 ⇥ 10 15 s 1. This value is similar to that
observed with the previous FPI made of Zerodur. The
reason for the expansion of the FPI duri g the first 80
days is not yet clear. The most obvious hypothesis is that
stress inside the material is relieved when the specimen
is put into vacuum. This hypothesis might be checked
by investigations of subsequent cycles between normal
pressure and vacuum. Another possible cause for the
unusual behaviour could be the fact that the vacuum vessel
including the FPI was heated to about 100  C for a few days
after installation. Hils and Hall [2] observed a ‘memory’
ffect when they raised the temperature of their Zerodur FPI
by only 2.4 K observing ‘a ge tle stress, lasting perhaps
100 days until the original creep rate was resumed’. There
is clearly some interesting physics going on that can be
investigated (but has not yet been) by the methods discussed
here.
The sensitivity of the method can be estimated from a
record of about 1270 s (figure 5), taken at the time indicated
by the arrow in figure 4 with a sample rate of two per
second. A linear fit to the data gives a slope of (0.770 ±
0.001) Hz s 1. When this drift is subtracted from the
experimental data the residual deviations can be described
very well by a Gaussian distribution of   = 20 Hz.
Applying the same assumption as before, i.e. that one has
1045
F. Riehle 1998 Meas. Sci. Tech.
Mirror Tech Days Nov 5, 20180
Materials Challenges: Stiffness
10
Constraints
Twisting of ”arms”
Lowest frequency mode 
is well over 40 Hz
Preliminary Modal Analysis
Mirror Tech Days Nov 5, 20180
Summary
• Robust 4 mirror design has been developed
• Meets LISA requirements
• Buildable
• Schedule is tight to build and test for 2022 adoption
• Challenges still to be addressed
• All glass construction: joints, yields
• High magnification design requires tight tolerances and careful alignment
• Structural/thermal analysis and materials and joints testing as needed
• Interface definition: Telescope-OB requires precision alignment
• Testing definition: what can realistically be accomplished/needed for 
Adoption
11Mirror Tech Days Nov 5, 20180


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