A 3-D metrology system for the GMT
A. Rakich*, Lee Dettmanna, S. Leveque, S. Guisardb,
a
GMTO Corporation, 465 N Halstead St, STE 250, Pasadena, Los Angeles, CA,91107, USA;
b
ESO, Karl-Schwarzschild-Str. 2 D-85748 Garching b. Muenchen, Germany
ABSTRACT
The Giant Magellan Telescope (GMT)1 is a 25 m telescope composed of seven 8.4 m “unit telescopes”, on a common
mount. Each primary and conjugated secondary mirror segment will feed a common instrument interface, their focal
planes co-aligned and co-phased. During telescope operation, the alignment of the optical components will deflect due to
variations in thermal environment and gravity induced structural flexure of the mount. The ultimate co-alignment and
co-phasing of the telescope is achieved by a combination of the Acquisition Guiding and Wavefront Sensing system and
two segment edge-sensing systems2. An analysis of the capture range of the wavefront sensing system indicates that it is
unlikely that that system will operate efficiently or reliably with initial mirror positions provided by open-loop
corrections alone3.
The project is developing a Telescope Metrology System (TMS) which incorporates a large number of absolute distance
measuring interferometers. The system will align optical components of the telescope to the instrument interface to
(well) within the capture range of the active optics wavefront sensing systems. The advantages offered by this
technological approach to a TMS, over a network of laser trackers, are discussed. Initial investigations of the Etalon
Absolute Multiline Technology™ by Etalon Ag4 show that a metrology network based on this product is capable of
meeting requirements. A conceptual design of the system is presented and expected performance is discussed.
Keywords: 3-D Metrology, Absolute Distance Meter, Active Optics, Telescope Alignment
1. INTRODUCTION
It is interesting to stand back and consider the evolution of large reflecting telescope technology through the 20th
century. Reflecting telescope technology evolved rapidly in the first half of the 20th century, and reached a peak in the
200 inch Hale telescope. The Hale telescope and its successors of similar ilk, were in general very well engineered but
relied on clever “passive” engineering design, innovations like the truss of Serrurier, and limited open-loop adjustment,
to control image quality.
The transition to the 8-10 m class telescopes of the 1990’s-2000’s, required a “jump” in technology to be feasible. Note
that the 1990’s class of 8-10 m telescopes still retained, and improved on, the excellent engineering that went into the
previous generation’s telescopes. But alone, improved passive engineering was not enough.
This jump came primarily with the ESO development of active optics as embodied in the NTT, and also ESOs’ early
uptake and quick roll-out of the Shack Hartmann wavefront sensor. The Nordic Optical Telescope (NOT) predated the
NTT’s first light by one year, using a thin active mirror controlled by wavefront sensors, but credit still goes to ESO.
Wilson and Noethe of ESO and Schwesinger of Carl Zeiss, building on the basic concepts of Couder and Maksutof, had
laid the technological path that the NOT followed5. The ability to, in “real time”, diagnose and correct a telescope
misalignment or surface deformation led to an immediate jump in the capability of telescopes to reliably deliver seeinglimited image quality to instruments.
This technological jump is generally seen as a prerequisite to the existence of the 8 m class of telescope. Without any
room for argument, it was a prerequisite to their ability to operate efficiently. However, the 8 m class of telescopes as
first delivered still had certain limitations. Not one of that 8–10 m class of telescopes had 3 or more active optics
wavefront sensors built into the basic telescope control (at least not as part of the basic telescope facility, many do now
have more wavefront sensors available on-board science instruments). This meant that the telescopes as delivered were
not able to perform closed-loop corrections for focal plane tilt and field-linear astigmatism that result from M1-M2
Ground-based and Airborne Telescopes VI, edited by Helen J. Hall, Roberto Gilmozzi, Heather K. Marshall,
Proc. of SPIE Vol. 9906, 990614 · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2234301
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misalignment about the coma neutral point. There was some debate in the mid-nineties about whether or not
“decentering astigmatism” needed to be controlled6,7, and the prevailing view was that as the large telescope fields were
narrow angle, the impact of decentering astigmatism on image quality would be negligible.
A mistake was made here. When a secondary mirror of a two-mirror classical or aplanatic telescope is decentred almost
no astigmatism is introduced. However, when the resultant coma from this decentre is corrected with mirror tilt, still no
aberration is introduced on-axis, but astigmatism that is linear with field is introduced, together with a focal plane tilt,
and this aberration can and has occurred in non-negligible quantities at the typical patrol radius of the single off-axis
wavefront sensor. While this mistake was eventually recognized, and at least in some cases has been brought under
control by calibration and open loop correction, it is clear that the image quality of some if not all of the 90’s generation
of telescopes has been effected to some degree by not having an adequate complement of wavefront sensors to provide
closed loop control of this aberration.
The GMT, as one of the new generation of most-ambitious-optical-telescopes-ever-attempted, presents a formidable task
from the point of view of operational optical alignment. For a start there will be the basic complication of collimating,
co-focusing, co-scaling, co-tilting and, most significantly, co-phasing 7 unit telescopes focal planes. All of these can be
regarded as a scale-up of existing technology. The requirement for multiple wavefront sensors for active optics is now
clearly recognized, GMT will have a complement of four patrolling wavefront sensors. However, the problems of
alignment, that have only been partially addressed on the current generation of 8 m telescopes equipped with one facility
wavefront sensor; routine closed loop control of the coma neutral rotations of M1/M2 optics, or closed loop alignment of
the unit telescope optical axes to an instrument rotator, will be multiplied by seven on the GMT. New analysis shows
that when compared to current 8 m telescopes, the GMT will require ~ several times tighter restrictions on the initial
condition of the relative positions of optical components required for guaranteed AGWS convergence8, initial conditions
that will not be obtainable by open-loop look-up table correction.
It can be seen that in general, when technology jumps to new levels, often new and unanticipated difficulties arise. The
specific example of the missed requirement for several wavefront sensors to measure asymmetric field aberration is a
case in point. Informed by this history (as were our predecessors), we may attempt to find the “unknown unknowns”, but
this is no guarantee that they will be found. Given that the absolute size and relative complexity of GMT is significantly
greater than that of last generation’s telescopes, it seems prudent to investigate new technological approaches that will
reduce the height of the hurdle faced in routinely achieving required standards of image quality and pupil stability. By
purposely and purposefully pre-conditioning our system to success, building in redundancy where this can be achieved
economically, reducing the strain on any one system such as the AGWS, we not only weight the statistics towards
reliable performance for the system that we anticipate, but also in some part mitigate the risk posed by the unknown
unknowns9.
It is as a result of these general types of concern, and specific analysis of the initial states of telescope alignment
required by GMTs AGWS system8, that a telescope metrology system (TMS) is now being proposed as an integral
component of GMT active optics. This paper presents the results of analyses that identify an absolute the requirement for
a TMS, and further show that the requirements for certain modes of GMT operation will not be supportable with the
limited accuracies and measurement cadence obtainable with a TMS based on any practical number of laser trackers. An
alternative technological approach to building a TMS, with numerous advantages over a laser tracker network, is an
“optical truss”, built with a network of Absolute Distance Meters (ADMs). ESO technical optics staff have identified a
particular commercial metrology system which offers multiple channels of ADM. ESO and GMTO staff have evaluated
the performance of the Etalon Absolute Multiline Technology (EAMT, by Etalon Ag, in several laboratory and field
tests10. Results of these evaluations will be summarized below.
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2. REQUIREMENTS ON THE INITIAL ALIGNMENT STATE OF THE TELESCOPE
Sensitivity analyses and performance simulations have been conducted to evaluate AGWS convergence from a range of
initial states of telescope alignment. The aim of the exercise was to establish requirements which guarantee convergence,
and to have a first view of the effect of different levels of initial alignment condition on AGWS convergence rates.
Table 1 AGWS system performance requirements and design compliance.
Requirement
Acquisition FOV
TT7 Capture Range
WFS Dynamic Range
SPS Capture Range
Requirement
∅ 20″ unvignetted
∅ 30″ with ≤ 50% vignetting
∅ 15″
Operable with up to 30 µm P-V static astigmatism
± 50 µm WFE
Design Estimate
∅ 20″
∅ 34″
∅ 15″
Yes
± 38 µm WFE
Table 2 Modeled perturbation ranges for GMT Primary and Secondary mirror segments.
Error
Gravity Deflections
Primary mirror segments
Secondary Mirror segments
Instrument Platform
Modeled range
≤ 300 µm
≈ 3000 µm
≈ 1.0 mm
Gravity Tilt
Primary mirror segments
≤ 70 µrad
Secondary Mirror segments
≤ 80 µrad
Instrument Platform
≈ 36 µrad
Thermal Deflection (-15°C to +20°C)
Primary mirror segments
Secondary Mirror segments
Instrument Platform
3.4 mm
0.4 mm
4.8 mm
2.3 mm
Four possible operating modes of GMT have been analyzed, covering the possible permutations of the telescope
operating with or without M1 and or M2 edge sensing systems.
For a ~ 2 year period from first-light, with three or four M1 segments, the current plan is for the telescope to operate
without edge sensing systems. When the full complement of mirrors is installed together with the Adaptive Secondary
Mirror (ASM), the GMT will operate with edge sensing systems for both M1 and M2. These systems, operating with
regular updates from a phasing wavefront sensor, will maintain relative positions of the segments on M1 and M2
respectively to a level in which the segments can be considered to lie on a common conicoidal surface. Periodically the
ASM will be removed for maintenance and replaced with the Fast Steering Mirror System (FSMS), which does not
currently include an edge sensing system. The edge sensing systems may also fail. The analysis presented below will
shows derived requirements covering the extreme cases: full complement of edge sensors and no edge sensors.
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Table 3. Requirements for initial segment positions and orientations to guarantee AGWS convergence.
Parameter
No Edge Sensors (1σ)
With M1 and M2 Edge Sensors (1σ)
M1 (X, Y)
≤ 75 µm
≤ 50 µm
M1 (Z)
M1 (Rx, Ry)
M1 (Rz)
M2 (X, Y)
M2 (Z)
M2 (Rx, Ry)
M2 (Z)
≤ 170 µm
≤ 0.38 arc seconds
≤ 40 arc seconds
≤ 75 µm
≤ 170 µm
≤ 3.0 arc seconds
≤ 330 arc seconds
≤ 95 µm
≤ 1.44 arc seconds
N.A.
≤ 50 µm
≤ 95 µm
≤ 3 seconds
N.A.
At first glance, and with the notable exception of the M1 Rx, Ry requirements, these limits on initial alignment state may
seem well achievable by open-loop models, to people experienced in the performance of the current generation of 8-10
m telescopes. However, a more suitable existing telescope to consider for comparison to the GMT is the LBT, which is
of a more directly comparable scale. In the case of LBT initial alignment states such as these would already be
considered optimistic on even a good, stable night11.
Nth
11
40
Figure 1. Left: Spot diagrams corresponding to the “no edge sensor system”. Right: A perturbed state for a “monolithic” 25m aperture system as delivered by the edge sensors. The top blue spots come from the centre segment and the three
elongates spots are overlapping images from diametrically opposite pairs of off-axis segments.
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Figure 2. AGWS convergence rates from starting points obtained from Monte Carlo simulations using angular ranges as
indicated in table 2 (but much smaller decentre and segment piston ranges). Clearly the AGWS system will operate much
more efficiently if the starting points are achievable at greater accuracies than the values required to merely guarantee
convergence.
In considering figure 2 it is worth also considering the value of reducing the mean number of measurement cycles,
#cycle, required for the AGWS to converge.
One hour of GMT observing time is valued differently, and the real value is currently undergoing re-evaluation, but a
fair estimate baed on the total project cost and annual operating costs divided by life time hours comes out around
$20000/hour. We can use the following estimates to derive an indicative cost per-#cycle per-annum:
-
Mean number of new target settings per night: 20
Mean cycle time:
20s
365 nights p.a.
From these numbers we obtain ~40 hours per-#cycle p.a., so associated cost is ~ $800 000 p.a. Over the 20-year nominal
lifetime of the project, $16 000 000 represents ~2% of the total cost to build.
Some early conclusions can be drawn from the analyses summarized in this section.
a)
A TMS will be required to guarantee convergence of the AGWS.
b) The benefits of squeezing the highest possible performance out of a given TMS will differentiate
technological solutions that do guarantee AGWS convergence with high confidence, in that better
performing systems will tend to reduce the number of cycles required by the AGWS to converge.
c)
A system with a wider performance margin than seems necessary may in fact become a marginal system
under the impact of some “unknown unknown” as raised in the introduction to this chapter. If a TMS with
a wide apparent performance margin can be acquired for a reasonable cost, on a system as novel and
technologically ground breaking as the GMT, it would be wise to allocate some weight to the intangible
benefits in risk reduction this brings.
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3. TECHNOLOGICAL APPROACHES TO A TMS
Two technological approaches have been considered as the basis of a TMS: a laser tracker network and optical truss
arrangements comprised of ADM measurement arms. The authors between them have more than three decades of
experience with laser tracker metrology, with a large part of that experience gained in the context of telescope alignment
tasks. While the laser tracker is a very versatile and accurate tool for general 3-dimensional metrology over large ranges,
there are a number of factors that render a laser tracker network an undesirable approach to the GMT TMS:
1) As mentioned above, the TMS is required to cover all operational modes of the telescope including the mode in
which the ES systems are not available. A minimum of 7 laser trackers would be required just to measure the
relative orientation of each M1 segment to its conjugated M2 segments within the target measurement cycle
time of 30 seconds. More trackers would be required to tie the M1/M2 in to the instrument rotator. A minimum
number of trackers appears to be ~ 8, giving a system with no margin for error.
2) Each laser tracker is a heat source and a stray light source. Enclosures would have to be designed to control heat
and stray light. The laser tracker cannot be turned on and off; there is a warm up period of many minutes for the
laser to stabilize, so stray light would have to be controlled by shuttering.
3) Laser trackers are “quirky”. They are complex systems with motors, encoders, tracking systems etc., as well as
the differential interferometer (IFM) and ADM systems, with everything having to work in harmony for a good
result. Manufactures recommend annual servicing; a recommendation that is seen as necessary by many longterm users. With a network of more than ten trackers the maintenance would be onerous.
4) Prior to the servicing described in (3), laser trackers exhibit various failure modes such as failing to acquire
targets or acquiring then losing targets. With suitable training a skilled technician could replace a faulty unit
with a spare and have that spare “shot into the geometry” of the system in maybe two hours.
5) Given this quirkiness, it would not be reasonable to have the telescope rely on a laser tracker based 3-D
metrology system without having at least two factory calibrated spare laser tracker available on site, bringing
the total number of trackers required by the system to at least 10. At this point a laser tracker network already
costs ~ $1 000 000.
6) The laser trackers have never to our knowledge been used continuously for sustained periods (months/years) in
environments similar to a telescope environment. If GMT were to baseline a 3-D metrology system on laser
trackers it would be prudent to have gained knowledge on system reliability in a telescope environment through
prolonged field testing.
The technological approach of an ADM network, and specifically one based on the EAMT system to be described in
detail below, suffers none of these faults. Referring to the issues with a laser tracker identified above:
1) The EAMT system provides simultaneous measurements over multiple lines. The system GMT will purchase
will have more than 100 measurement channels. A minimum of 6 channels are required to measure the relative
positions between two objects. Therefore 84 channels will be required to measure M1 segments to
corresponding M2 segments and M1 segments to AGWS/instrument.
2) The EAMT electronics, laser, reference arm etc. can all be kept completely outside of the telescope
environment. Fibres (standard telecom single mode fibres, therefore cheap) running from the control unit to the
launch collimator can be up to several kilometers long. The only hardware in the telescope environment is
passive: fibres terminating in fixed small collimators and 12.5 mm retroreflectors. The metrology bandwidth is
~60 nm, centered on 1532 nm. It is TBD what stray light effects would potentially impinge on out-of-H-Band
observing bands, but already it is clear that the system could in fact be operated during science operations, if
desired, whenever the detector is a CCD. In the first envisaged TMS all the laser light is anyway switched off in
the telescope environment once the AGWS begins operating. This can be achieved in < 1 ms with an optical
switch in the base unit. No heat at all, and no light in the telescope environment during science observations.
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3) The EAMT has no moving parts. No motors encoders etc. that contribute to a lot of the laser tracker quirkiness.
TCS standard electronics and a hardware architecture based on high-reliability telecommunication standard
componentry point to a highly stable system. The system has been in regular high I/O use on the CERN
ATLAS detector (inside the particle accelerator) and has proven to be highly reliable.
4) N.A.
5) Given that the reliable and efficient performance of the GMT requires a TMS, any TMS system faults must be
quickly remediable. The core componentry of the EAMT is modular, and spare modules for any key component
can be rapidly swapped in when fault diagnostic software identifies an issue.
6) See (3) above. Note that as will be discussed below, prior to deployment on GMT in 2022, the intent is to
deploy the multiline on a large telescope for an extended field test.
Added to the various advantageous features of the EAMT listed above, there are several other considerations.
The cost of the EAMT is significantly less than the cost identified for the “minimum” laser tracker network.
The minimum number of channels for a segment-to-segment-to-instrument network. With an additional 42 channels, 3channel redundancy can be built into each truss. With 4 retroreflectors per target this will ensure that the blockage of one
retroreflector will not impede performance. The redundant channels provide the added advantages of over-constraint,
allowing the identification of outliers in the 9 measurement arms, and also increase the achievable accuracy of each
measurement.
Simulations have shown that the absolute accuracy obtainable with the EAMT network is between 2 and 50X better than
that obtainable with the laser tracker network. A laser tracker network could easily achieve some of the listed
requirements, just scrape in on others, and fail on several. And EAMT meets all requirements, and meets them with wide
margins in most cases (see table 4 below).
Referring to points (3) above, these illustrate a fundamental difference between the ADM truss and Laser Tracker
network approach to a TMS: The truss is passive, “sit and stare”. It is ideally suited to the task of monitoring small
displacements from a nominal position. The laser tracker is “active”, obtaining its measurements by moving to multiple
points over a large angular range, and losses accuracy as a result.
In summary the cheaper, more accurate, more robust, less “polluting” EAMT network, with margin both in accuracy and
system redundancy, is a clear choice over a laser tracker network for the GMT TMS.
4. EAMT: DESCRIPTION AND PERFORMANCE EVALUATION
4.1 EAMT description.
The ADM metrology tool has been around for a long time. Within the last ten years however, ADM systems have
dramatically improved in terms of their achievable accuracy, which is now approaching the accuracies achievable with
incremental interferometers11. The big advantage of ADM over incremental interferometers is the recovery from beam
break: an incremental interferometer has no way of recovering on its own from a beam break and needs to be reset; an
absolute distance meter does not suffer from this problem; the absolute distance is measured on each measurement. The
EAMT system is a multi-channel ADM system based on the principle of dynamic frequency-scanning interferometry
(dFSI)12. This product is a commercial realization of a technology developed by a team at Oxford University in support
of the stringent metrology requirements of the ATLAS detector at CERN13.
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c
A
-
WA
f:sr
r
4.1 E.
.
Figure3. dFSI precursor to EAMT deployed on the ATLAS detector at CERN. CERN have subsequently purchased EAMT.
ƒ
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Absolute interferometer
Central unit with up to 100 independent channels
(and extendable)
Measurement uncertainty (95%) : 0.5 µm/m
Maximum measurement frequency > 500 kHz over
up to 2 seconds (not real time).
Measurement length > 20 m
Simple measurement channel consisting only of
telecom fiber, collimator and triple reflector (no
electrical systems at detector)
Almost unlimited fiber length possible (several
kilometers)
Eye safe infrared radiation (1532 nm +/- 30 nm).
Metrological traceability by gas absorption cell
Figure 4. EAMT unit.
Two attractive features of the EAMT system for a TMS are accuracy/stability and cost-effective scalability.
Accuracy and stability
As described in ref (XX), there are a combination of features in the EAMT that lead to enhanced accuracy and stability,
-
An in-line gas calibration cell, capturing laser frequency drift with high accuracy during measurements.
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-
The double-sided scanning lasers, allow the capture and back-out of target motion during the measurement, at
frequencies >> 1 kHz.
-
The reference arm is fibre interferometer, providing a very stable basis for metrology.
-
Manufacture claims for 0.17 ppm ADM performance have been either been verified or shown to be pessimistic
by testing.
Scalability
-
The EAMT uses telecom wavelengths, allowing the direct incorporation of commercial telecom industry
hardware components including multiplexers, fibres, optical switches etc.
-
The advantage here is that these components tend to be high-quality, “industry hardened” and relatively cheap.
-
Multiplexing of the measurement arms and discrete analysis of the return signals allows a very large number of
measurement arms. GMT is considering a system with 100+ arms.
-
“Range”. From the base unit fibres can be run very long distances. As the measurement interferometry takes
place at the fibre tip, the path through the length of the fibre is common path and does not disturb the accuracy
of the measurement. With the possibility of sending fibres large distances from the laser source, it will be
possible to provide “facility metrology lines”, not only supporting the TMS, but also for example providing
metrology capabilities further down the mountain at the GMTO integration and test facility for example. In
principle the same unit could provide facility metrology services to our partner institution at the Magellan
telescopes.
4.2 EAMT performance evaluation by ESO and GMTO staff
ESO staff became aware of EAMT in 2014, and developed an immediate interest. Subsequently GMTO staff have
entered into a loose collaborative effort with ESO colleagues in assessing technology and developing plans for
incorporating it into a telescope alignment system. Following some preliminary investigations and presentations from the
manufacturers a teleconference was arranged with the CERN team of metrologists who were actively using the system.
The impression conveyed at that time was of a system that apparently delivered on the promise offered by the
specifications. Mechanical features of particle detector hardware were being kept in co-alignment to better at the ~1
micron level over long path lengths.
Between the 16th and 19th of December 2014 ESO staff obtained a unit for testing. Tests were conducted in the new
integration hall; a large room allowing path lengths of up to 20 m to be measured. Targets were mounted on the E-ELT
prototype M5 fast tip/tilt mount, a test position actuator capable of very fine and fast positional changes, and the E-ELT
M1 segment test stand. In summary there were some issues with robustness, which were later traced to a newly installed
USB hub. Other than that, the system tests produced metrology results that were consistent with the manufactures
claims, and one result which was both very much better than the manufacturer’s claims and indicative of perhaps
unexplored potentials of the system (see figure 7 below).
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I I
SMR nest
.-cg
PAC
Figure 5. Testing EAMT at the ESO integration hall in 2014
Longterm test 18- 12- 2014.mat
300
--ChB
Ch1=2.9 µm (at d-7m)
Ch3=2.7 µm (at d-7m)
-Chi6
-Ch1
200
Ch10
Ch4=3.8 µm (at d-7m)
Ch5=3.2 µm (at d-7m)
E
Ch6=3.2 µm (at d-7m)
Ch8=80 µm (at d-19.5m)
Ch10=2.8 µm (at d-7m)
Ch11=2.4µm (at d-7m)
> -100
itl¡;.
l
L' -200
jl
1
Ch12=3.4 µm (at d-7m)
Ch13=3.2 µm (at d-7m)
Ch14=3.2 µm (at d-7m)
Ch16=2.5 µm (at d-7m)
300
400
0
4
8
10
14
16
18
Time in Hours
Figure 6. Overnight stability tests confirmed manufacturers claims. One faulty signal (channel 8, was subsequently traced back to a
fault in a newly installed USB hub.
The most startling result was the PACT measurement. Figure 7 shows measured points for a retroreflector set to oscillate
in the light of sight of the measurement beam with an amplitude of 150 nm and a frequency of 80 Hz. The measurement
was made at a distance of 200 mm. At this range the manufacturers 1σ accuracy claim would have errors of magnitude ~
35 nm. As we see on the plot, at 80Hz, the error to fit on the sine curve is <<35 nm. While this gives no information
about the absolute accuracy obtained, there is a clear indication that at least incremental data can be obtained at
accuracies comparable to those of an incremental interferometer. A limitation in the practical application of this highfrequency high-accuracy capability is that there is a time lag of ~ 2 seconds between data acquisition and availability,
due to the processing time required to obtain the absolute distances from fringe counting.
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vibrationdata channel 1- PACT80Hz150mu.mat
23 vibration windows appended
vibrationdata channel 1- PACT80Hz150mu.mat
23 vibration windows appended
0.3
0.2
0.2
g_ 0.1
C
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e
t=
m
V
l6
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y -0.1
ö
-0.3
oom
-0.4
500
0
1000
Index
1500
2000
-0.2
340
360
380
400
Index
420
440
460
Figure 7. High frequency data obtained at ~ 200 mm range, showing at least incremental accuracy of ~5 parts per billion.
Subsequent investigations were conducted by ESO at the Etalon factory in 2015. An indoor long path length test (18 m)
showed a measurement stability of 0.5 ppm to 2-sigma over 30 minutes, as shown in figure 8.
Lengths indoor [mm]
18540.95
18540.945
18540.94
18540.935
18540.93
18540.925
18540.92
18540.915
18540.91
18540.905
18540.9
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t.D
b Standard deviation: s = 5.0 µm
Figure 8. An indoor test. Standard deviation equivalent to 0.16 ppm when corrected for thermal drift.
The gradual ramp observed in the data could be consistent with the 0.5 degree temperature change that occurred over
that period, but this was not subtracted from the result. The standard deviation of the drift corrected data matches the
manufacturers claims.
A similar test was conducted outdoors in windy conditions, the results are shown in figure 9.
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Conditions 2: Outdoor, T= 0.3 -0.7 °C, Standard collimator, Open retro -reflector, 30 min, mild wind
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Lengths outdoor [mm]
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b Standard deviation= s = 4.1 pm
Figure 9. An outdoor test. Standard deviation again equivalent to 0.16 ppm when corrected for thermal drift.
In October 2015 further tests were carried out by a group of ESO and GMTO metrologists at the Etalon factory.
-Further indoor and outdoor stability test results were consistent with earlier results.
-By use of a 3X beam expander the effect of varying the focal length of the collimator was investigated. The longer focal
length collimator had a slightly extend range of acceptance for retroreflector displacement, and the shorter focal length a
slightly reduced range. Optimizing focal length of collimator to maximize range of acceptable beam-walk is one area
where further work needs to be done. The current range of +/- 3 mm can probably be improved upon.
A test was set up in which a CMM was used to hold 3 retroreflectors and a truss was set up on the CMM bed. Here the
goal was to test the truss accuracy with the CMM as a “standard”. Measurements were also taken with a laser tracker.
Unfortunately, the implementation of this test faced several problems and in the end no results were obtained that could
significantly differentiate the results from the CMM, laser tracker or EAMT.
A test on measurement stability when a 100 °C hot-air gun was blown through the beam path confirmed the expectation
that beam stability in any sort of conditions to be expected in a telescope environment will not be effected by turbulence
or temperature differentials in the air.
In April 2016 ESO staff obtained a rental unit and used it to conduct field tests on one of the VLT telescopes (UT4)10.
This was the first test of the technology in telescope conditions. The unit arrived on the mountain damaged. From the log
entry “Functional test of the multiline shows transport damages: 2 lasers are broken (1xNew focus LM8700-GA02
metrology laser and the red beacon laser) as well as some relay fibers (squashed). Signs of strong vibration occurring
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during the transport can be seen”. Fortunately, the Etalon technician was able to affect repairs using some spare parts and
a red diode laser ESO had available.
Six fibres were launched from the primary mirror to three retroreflectors on the secondary mirror. The results from this
test are still being analyzed but initial data indicate that EAMT had no particular problems operating in a 14 m/s wind,
gain delivering accurate results. One interesting result that has come out came from measuring the UT4 M2 relative to
M1 in the “start of the night” conditions where the dome was opened and the temperature rapidly dropped by ~ 2 degrees
C. The EAMT hexapod gave M2 positions consistent with those predicted by steel expansion and temperature telemetry
to within 0.00016% or 20 microns over the measurement range. In this case the estimated mirror position based on CTE
and temperature telemetry is most likely the largest source of error.
Figure 10. Optical hexapod setup on UT4.
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Figure 11. Optical hexapod set up on VLT UT4.
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A laser tracker was setup to cross check measurements obtained with the EAMT, as the M2 mirror was driven by its
positioner through a variety of well-defined changes in position and orientation, as the telescope was driven up and down
in elevation, and as the dome was opened at night through the associated rapid structural temperature change.
The results of these tests were presented at a metrology conference in early June 2016 (REFFF). The broad conclusion is
that the EAMT worked without fault in relatively difficult telescope conditions (10 m/s wind and rapidly changing startof-night temperature.
Results so far point to a system that meets or exceeds the manufacturers claims for accuracy, and appears to be viable for
operating in a telescope environment.
5. AN EAMT BASED TMS FOR GMT
Preliminary simulation work has been carried out to establish expected accuracies obtainable with an EAMT based
TMS14. These simulations defined geometries for a number of “optical hexapods” and then determined achievable
relative positional accuracies between M1 and M2 segments and the Gregorian Instrument Rotator. For each hexapod
defined Monte Carlo simulations were carried out in which normally distributed errors were applied to each
measurement leg using the manufacturers 0.17 ppm 1σ errors. As seen above, various field tests have validated these
error ranges in an “outside air” environment.
The simulations do not take into account disturbances and errors in determining the base geometry of the hexapods. In
the case of the M1/M2 hexapods, the retroreflectors will be mounted on the M2 Zerodur substrates, and once in place
their relative positions around each mirror segment can be characterized to an absolute accuracy of < 50 microns. The
launch collimators are mounted directly on the Borosilicate substrates of the M1 segments. In this case the diameter of
an 8.4 m segment can vary by ~600 microns over a 25 degree C temperature range. This geometric uncertainty can be
shown to have only second order effects on the accuracy of the EAMT hexapod; radial symmetry of the Borosilicate
expansion helps negate the effect, and errors in leg length can be shown to be proportional to the Cosine of the very
small angle given by the lateral shift in the collimator position (<300 microns) divided by the length of the leg (~20 m).
An important use of this simulation is in determining the optimal hexapod geometry. It can be shown that the error in
“bi-lateration” of an SMR vertex position, when two ADM channels are incident on a common SMR, is proportional to
where is the angle between the beams (figure 12)14. This volume of the SMR vertex
uncertainty region is minimized when
degrees, when the beams are orthogonal. Considering the test geometry
in figure 11 for example, it can be seen that the configuration is constrained by the relative size and position of the
primary and secondary mirrors. However, given that we know the primary deflections of the optics will be in the
direction of the gravity field, we can choose geometries that maximize sensitivity to this disturbance.
44Z [Co t(B) + Sin(B)]
Figure 12. At
degrees there is a ~50% increase in bi-lateration uncertainty volume compared to the case where the
beams are orthogonal
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Figure 13 below outlines several of the test geometries so far simulated. A geometrical analysis of hexapod uncertainty
as a function of relative angle of incidence of beams gives volumes of uncertainty in SMR vertex location that are ~ 100
times smaller than equivalent volumes for laser tracker measurements. In a simple translation to linear error, this predicts
that an EAMT hexapod will have ~ 1/5th of the linear and angular error in object location and orientation as that
obtainable with a single laser tracker.
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Figure 13. Various geometries so far simulated for an EAMT TMS for GMT.
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Table 4. Requirements for initial segment positions and orientations to vs TMS estimates for EAMT and Laser Tracker.
Degree of Freedom Requirement (1σ)
EAMT Design Estimate (1σ)
LT Design Estimate (1σ)
M1 x,y
≤ 75 µm
1.4 µm
19 µm
0.87 µm
?
M1 z
≤ 170 µm
M1 Rx, Ry
≤ 0.38 arc seconds
0.068 arcsec
2.4 arcsec
M1 Rz
≤ 40 arc seconds
0.054 arcsec
?
M2 x,y
≤ 75 µm
8.2 µm
14 µm
M2 z
≤ 170 µm
1.5 µm
?
M2 Rx, Ry
≤ 3.0 arc seconds
0.64 arcsec
1.3 arcsec
M2 Rz
≤ 330 arc seconds
3.0 arcsec
?
The Laser Tracker data in table 4 is obtained by simply scaling data from a simulation of a laser tracker network on
LBT. This column of data will be replaced by specific simulations in Spatial Analyzer of a laser tracker network in Q3,
which will anyway be required for the AIV planning that is taking place concurrently. (REFXXX)
The laser tracker only struggles to meet the M1 tilt requirement. Different geometrical arrangements could improve on
that result. However, two considerations weigh strongly in favour of an EAMT based TMS here.
1) The laundry list of items given in a previous section comparing various practical aspects of a laser tracker and
EAMT based TMS deployed in a telescope environment, which is overwhelmingly in favour of an EAMT
solution.
2) Margin. Looking at the two columns of TMS accuracy in table 4, it is clear that the EAMT meets all
requirements with significant margin, and significantly more margin than the Laser Tracker in most cases. In
the introduction the strategic benefits of adding margin to systems are discussed. The AGWS “cycles-toconverge” shown in this figure are between 6 and 8. In section 2 an estimate of the cost-per-cycle in the mean
number of cycles to converge is estimated at $1 000 000 p.a. It should be expected that with the much better
starting points of optics positions offered by the EAMT based TMS, at least one of these cycles will be saved,
and possibly more. This saving comes “for free” with the right selection of base technology for the GMT TMS.
6. NEXT STEPS
The EAMT based TMS was proposed as part of a conceptual design review of the GMT Alignment Plan, held on June
8th 2016. At the time of writing this paper the reviewer’s final report is not yet available, but it was clear from the
executive summary on the day that the review has endorsed an EMAT based TMS and encouraged the GMT team to
continue their development work leading up to a PDR within 9 months.
GMT are currently considering their option to purchase a unit. Immediate tasks an EAMT unit will be put to include (but
are not limited to:
1) Lab work, establishing such things as ideal collimator focal length as a function of measurement path distance.
2) Validating prototype retroreflectors giving 120 degree FOV on a 12.5 mm aperture, that have been developed
by GMTO optical designers.
3) Lab work, providing experimental validation of simulation results; simulation calibration.
4) A proposal to use the Multiline at the University of Arizona College of Optical Sciences workshop to assist in a
large optical elements test setup is being enthusiastically considered by the lab’s technical director. Such a test
could directly compare EAMT performance to laser tracker performance, both measured against the “gold
standard” of the interferometric measurements of the optical test itself.
5) Prototyping activities, including simulating performance of prototype M1 and M2 edge sensors.
6) Ultimately the EMAT should be trialed on a working telescope. There are several potential candidate telescopes
operated by various of the partner institutions of GMTO. It is anticipated that one of these will be interested in a
long-term installation of such a beneficial system, also enabling GMTO to gain extremely valuable information.
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A nice feature of the sort of field test in (5) is that the system can run passively in the background, making data available
to the TCS in some directory. The TCS can be configured to either use this data to pre-position optics or not. It is
expected that the software effort involved in smoothly integrating EAMT capability into an existing telescopes TCS
would be minimal.
A field test on an existing telescope would have little to no “downside”. If unanticipated difficulties arise the telescope
would operate with its pre-existing systems which are already capable of giving acceptable performance (by pre-EAMT
standards). If the system functions as anticipated, it can be expected that any telescope will be able to operate with
heightened efficiency as a result.
Likewise, the hardware integration would consist running and securing the fibres, and of bonding small collimators to
the side of the primary mirror (as shown in figure 10), and attaching small retroreflectors to the sides of secondary
mirrors, rotator flanges etc.
It is hoped that such an arrangement can be concluded with a good candidate telescope before the end of 2016. With
such real world data obtained over a period of one to several years, GMT will be in the excellent position of going on
sky with one of their critical systems thoroughly field tested and optimized and characterized, in 2022.
REFERENCES
[1] McCarthy et al. “An overview and status of the Giant Magellan Telescope project”, Proc. SPIE Astronomical
Telescopes and Instrumentation 2016, publication pending.
[2] Brian McLeod, “The GMT active optics system: design and simulated end-to-end performance”, Proc. SPIE
Astronomical Telescopes and Instrumentation 2016, publication pending.
[3] GMT Internal Document , “AGWS M1 and M2 capture range”, GMT-DOC-01245, Rev 1, 2016
[4] John Dale et al. “Multi-channel absolute distance measurement system with sub-ppm-accuracy and 20 m range
using frequency scanning interferometry and gas absorption cells”, Optics Express,
[5] Private communication, Dr. Raymond N. Wilson, 2016.
[6] Bhatia, TNG Tech Report No. 43, 1995
[7] Wilson, R.N., et al., “Concerning the Alignment of Modern Telescopes: Theory, Practice, and Tolerances
established by the ESO NTT”, PASP, 109, pp 53-60, 1997.
[8] GMT Internal Document, “GMT fine alignment and phasing plan”, GMT-DOC-01244, Rev 0.2, 2016
[9] Sun Tzu, “The Art of War”. Chiron Press.
[10] Leveque, S., “The results of the Absolute Multiline testing on the VLT”, 2nd Pacman workshop, publication
pending.
[11] FARO white paper, “Laser Trackers – IFM vs ADM Technology”,
http://farotechnologies.blogspot.de/2009/09/laser-trackers-ifm-vs-adm-technology.html
[12] Coe, P. “An Investigation of Frequency Scanning Interferometry for the alignment of the ATLAS semiconductor
tracker”, Ph.D. Thesis, University of Oxford, 2001
[13] Gibson, S. M. et al. “First Data from the ATLAS Inner Detector FSI alignment system”, proc. The 10th
International Workshop on Accelerator Alignment, KEK, Tsukuba, 2008
[14] GMT Internal Document, “GMT optical alignment plan”, GMT-DOC-01243, Rev 1, 2016
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