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INVITED
PAPER
Performance Improvement of a
Flexible Telescope Through
Metrology and Active Control
Extremely accurate measurement methods, and measuring and sensing devices,
are needed to meet new requirements for radio telescope pointing accuracy and
control of structural deformations.
By Albert Greve and Hans Jürgen Kärcher
ABSTRACT | A radio telescope is a flexible structure under the
influence of gravity, temperature, and wind. Even after all
passive means of telescope construction have been applied,
the residual structural deformations of a high precision telescope may still lead to focus, pointing, and path-length errors
and a loss in gain that exceed the performance specifications.
Gravity-induced deformations can be calculated and corrected
with high precision. While the time-variable nature of temperature- and wind-induced deformations can only partially be
explored in numerical simulations, their control requires the
input from metrology. Corrections can be made through the
telescope control system but also, to some extent, through
deformable mirror surfaces. The progress in metrology and the
correction of certain telescope errors are described.
KEYWORDS
|
Metrology; performance improvement; radio
telescopes
I. INTRODUCTION
The design, and operation, of a radio telescope is based on
a finite-element analysis (FEA) that which investigates the
structural deformations and associated focus, pointing,
path length, and gain errors under the influence of gravity,
temperature, and wind. Structural deformations can be
calculated from elastic deformation theory applied to steel
and aluminum [1]–[4] and modern carbon fiber materials
Manuscript received October 29, 2008; revised December 17, 2008. First published
June 23, 2009; current version published July 15, 2009.
A. Greve was with IRAM, 38406 St. Martin d’Heres, France (e-mail: [email protected]).
H. J. Kärcher is with MT Mechatronics GmbH, 55130 Mainz, Germany
(e-mail: [email protected]).
Digital Object Identifier: 10.1109/JPROC.2009.2013566
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(CFRPs) [4], [5]. Gravity is a constant force, though
changing its direction and magnitude in a predictable
way with the tilt of the telescope, the gravity-induced
deformations can be derived from a classical FEA today
applied to telescope structures with tens of thousands of
elements. Temperature and wind are transient phenomena
with a vast spectrum of variability, either due to changes in
the thermal and aerodynamic environment in which a
telescope operates or due to a variable direction of interaction because of changing positions of the telescope. The
influence of temperature and wind is derived from an
exploratory flexible body analysis (FBA) that which predicts
the dynamic behavior of a telescope. The FBA uses a model
of the telescope structure (finite-element model) combined
with its mechanics of drives and servo systems, i.e.,
analyzing the telescope as a mechatronics system [6]; the
dynamic variability of temperature and wind is considered in
the input of the analysis. In summary, a radio telescope is a
flexible structure in a variable environment.
The performance evaluation of a radio telescope is
based on electromagnetic diffraction theory (see
Section III). The theory specifies the tolerances of
structural flexibility in order to operate with acceptable
degradation compared to a perfect telescope, in a perfect
environment. If a telescope design does not meet the
performance specifications even after application of all
passive construction means of homology, paint, insulation,
ventilation, low thermal expansion materials, and a
radome/astrodome, then a metrology system may be
envisaged to monitor the transient structural deformations
with the aim of correcting by means of the measured values
the focus, and/or pointing, and/or path length errors, and/
or the reflector surface deformations in order to fulfill the
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Greve and Kärcher: Performance Improvement of a Flexible Telescope
specifications. However, in many telescope projects, this
aim of metrology has not passed the proposal phase.
Gravity produces repeatable flexibility. The repeatable
effect of gravity on the pointing due to deformations of the
reflector and the quadripod are incorporated in the
pointing model. The repeatable focus variation is available
as a lookup table (LUT). The gravity-induced reflector
surface deformations are available as LUTs and a gainelevation curve. The repeatable pointing and focus
corrections are applied, in an open loop, through the
telescope drive program commanding the corresponding
actuators; the gain-elevation correction is applied later to
the observation. Today several telescopes use active
surface control that eliminates, to a large extent, the
application of a postobservation gain-elevation correction.
The transient influence of temperature and wind produces nonrepeatable flexibility. The variability of the
ambient air temperature and of the wind speed and wind
direction hasVin generalVa fast and a slow component.
The fast variations of the ambient air temperature have no
influence on the telescope performance because of their
small amplitude and the long thermal time constants of
telescope components. The large amplitude variations
occur on time scales of an hour, hours, days, and seasons.
Temperature monitoring of the ambient air and of telescope components for possible telescope control should be
made on a time scale of, say, 1/10 to 1/4 hour. The slow and
fast component of air flow are the steady wind and gusts.
Both components may contain sufficient power to affect a
telescope, with, in the extreme situation, serious effects on
telescope stability, requiring a stow of the telescope. The
influence of wind on telescope structures is explained in
[7] and [8]. Wind monitoring and monitoring of the effect
of wind on the telescope structure for possible telescope
control should be made on a time scale of, say, 1/2 to 1 min,
in order to register at least the steady wind.
stability is determined by the stability of the entire
telescope structure.
c) The random reflector surface errors should reduce the gain by less than, say, 30% [10%]; hence
the root mean square value of the surface errors
must be smaller than =20 [=40] (derived from
the Ruze relation [9]). The random reflector
surface errors are due to random deformations of
the backup structure.
d) The path length difference between telescopes of
an array [either connected array or very long
baseline interferometry (VLBI) array] should be
stable within =10. The path length changes with
a uniform temperature change of the telescope.
These criteria show i) that the tolerances scale with
wavelength so that millimeter-wavelength telescopes must
be more stable than centimeter-wavelength telescopes and
ii) that a metrology system must provide direct or indirect
information on a level of precision to evaluate these
criteria. If required, temperature measurements should be
made with a precision of 0.1–0.5 C, wind speed with
0.5 m/s precision, distances and angles with 0.01 to
0.1 mm, and 1–3 arcsec precision.
Observations of a radio source provide quasi-real-time
information (though not all) of the integrated telescope
performance; radio observations are the only way to
evaluate the performance of a telescope which does not use
metrology. The focus and pointing of a telescope is easily
measured in a radio observation of a point source, though
this facility may not eliminate the necessity of metrology
(see Table 2). It is relatively easy to measure in a radio
observation under good atmospheric condition the loss in
gain resulting as the cumulative effect of reflector surface
deformations; however, it is rather laborious, if not impossible, to map the transient reflector surface deformations
either due to temperature or wind. Snapshot mapping at
low spatial resolution can be done by in-and-out-of-focus
radio (OF) holography [10], [11], which is, however, a
significant interruption of normal telescope operation.
Metrology may here provide a considerable help.
II I. STABILITY CRI TERIA AND
OBSERVABILITY
IV. FL EXI BL E TE LE SCOPE STRUCTURE
AND FL E XI BL E B O D Y CON T R O L
The transient flexibility of a telescope produces a tolerable
change of telescope performance in case the focus, the
pointing, the surface precision, and the path length are stable
over a considerable length of time (between repeated
calibrations) within the following limits:
a) The focus should not change by more than
f =10 (with the wavelength of observation). The focus stability is determined by the
stability of the reflector and the quadripod.
b) The pointing should not change by more than 1/10
of the beam width, i.e., =10 ð=10Þ=D
(where D ¼ reflector diameter). The pointing
A telescope is designed in an iterative way, applying in the
design according to experience and necessity the passive
means of reflector homology [12], thermal protection of
paint, insulation, ventilation, and/or climatization (heating and cooling of the ventilating air), low thermal expansion materials (CFRP, invar), or even a radome/astrodome.
Gravity-induced structural deformations are investigated
in an FEA for different tilt angles of the telescope. Thermal
deformations are investigated in an FBA in an exploratory
way by assuming temperature jumps, temperature gradients, and random temperature distributions to occur in the
telescope structure. In order to base the exploratory FBA
II . REPEATAB LE AND
NONREPEATABLE FLEXIBILITY
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Greve and Kärcher: Performance Improvement of a Flexible Telescope
on a more realistic situation, the thermal and associated
mechanical behavior of the telescope structure can be
studied from thermal model calculations, which use as
input a realistic thermal state of the environment [13]. The
influence of wind on a telescope is investigated in a similar
exploratory FBA using for the wind force distribution
across the telescope, for instance, the JPL tables [3]. The
response of the telescope to the variability of wind is
obtained by combining the wind attack tables with a wind
power spectrum. For realistic operation conditions of
gravity, temperature, and wind, the results of the FBA are
summarized, among others, in the error budget tables.
These tables, which consider the telescope as a mechatronics system, are indispensible to decide whether a classical telescope construction will provide satisfactory
performance or whether metrology must be applied.
In case metrology is not required, the optimized
telescope has two main axis drives with motors and gears
as actuators and encoders as sensors. The classical position
control uses an open-loop1 pointing model and refraction
model, and LUTs for open-loop subreflector positioning to
control the elevation-dependent deformations of the
quadripod with respect to the backup structure (BUS).
Examples of telescopes constructed with passive techniques are the Effelsberg 100-m telescope (homology [14]),
the IRAM 15-m telescope, and the ALMA/APEX 12-m
telescopes (CFRP, invar, [15]–[18]), and the IRAM 30-m
telescope (insulation, climatization [19]).
For high-precision telescopes, usually of large diameter, the performance tolerances are extremely tight (for
instance, because of the small beam width of 10 arcsec),
and the FBA may indicate that a purely passive constructive solution is not possible and that metrology is required.
As an example, Tables 1 and 2 summarize [4], as a result of
an FBA, the reflector surface error budget and the pointing
error budget of the LMT 50-m millimeter-wavelength
telescope [20]. Table 1 indicates that an active reflector
surface is required. Table 2 indicates that metrology is
required to control the influence of wind on the pointing.
The FBA provides information on the location and
magnitude (observability) of structural deformations,
under the influence of gravity, temperature, and wind; it
provides information on where to place sensors, how
many, of what precision, and the algorithm for interpretation (FBA, FEM) of the metrology data for control; it
provides information on the improvement in telescope
performance when corrections are made using metrology
data. The sensors are inclinometers, transducers, temperature sensors, etc. The control necessity and feasibility, the
control concepts, and the use of sensors are explained, for
instance, in [21]–[25].
Table 1 Error Budget of Reflector Surface Precision (LMT Telescope).
Uncorrected (Uncorr.) and Corrected Error (Corr.), Correction Technique:
LUTVLookup Table, FBCVFlexible Body Control. The Entries in the Table
Are in Micrometers (m)
Table 2 Pointing Error Budget (LMT Telescope). Uncomp: Uncorrected
Error, Comp: Corrected Error. Correction Technique: LUTVLookup Table,
FBCVFlexible Body Control. The Entries in the Table Are in Arcsecond
1
Open-loop control uses predictions of a model [for instance, finiteelement method (FEM) and gravity forces, temperature measurements,
and FBA calculation]; the correction is as good as the model. Closed-loop
control measures the result of a correction as feedback for further
correction (for instance, a guide star of optical telescopes).
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Greve and Kärcher: Performance Improvement of a Flexible Telescope
Table 3 Instrumentation for Metrology
The method to find from the FBA the location of temperature sensors for optimal prediction of temperatureinduced reflector surface deformations, the temperature
interpolation procedure to obtain from the temperature
measurements the temperatures for the large number of
elements of the FEM and the formulation of the influence
matrix for use of control is described for the IRAM 30-m
telescope in [26] and [27]. Similar information from a
study on the influence of wind is not (yet) available.
V. THE CONTROL OF GRAVITY-INDUCED
DEFORMAT IONS
The FEA of a telescope under gravity load provides analytic
relations for focus and pointing control and LUTs of
gravity-induced main reflector surface deformations. The
elevation-dependent reflector surface deformations can be
corrected using the LUTs in case the panels, or panel
frames, are supported by actuators, or in case a deformable
subreflector or deformable tertiary mirror is available. The
method of main reflector actuator control is used with
success on the NOTO 32-m telescope [28] and the GBT
100-m telescope [29], and will be used on the LMT 50-m
telescope2 and the SRT 64-m telescope.3 On the Effelsberg
100-m telescope, the correction of reflector deformations is
made with a deformable subreflector [30]; a deformable
flat mirror in the optics path is proposed for correction of
reflector surface deformations of the 70-m Deep Space
2
See www.lmtgmt.org.
See www.ca.astro.it/srt.
3
Network antennas [31]. The deformable mirror can be very
close to the receiver while still providing a good phase
correction [32, and references of other applications of
active correction therein]. The effect, and the fine-tuning,
of the correction can be derived from efficiency measurements (gain-elevation curve) and low-spatial-resolution
high-speed OF holography [10]. The low-resolution OF
holography measures only large-scale deformations. The
Zernike polynomial representation allows a unique solution of the OF-holography data [11]. The small-scale
deformations follow from the FEA and LUTs and the initial
high-resolution holography for reflector adjustment. The
installation of a laser triangulation system on the GBT 100-m
telescope to measure in real time the reflector surface
deformations (and the feed leg deformation) has been
abandoned [33]. Shearing interferometry has been successfully applied for surface mapping of the CSO 10-m
telescope [34] and is proposed for regular control of the
surface of the Caltech Cornell Atacama (CCAT) 25-m telescope [35]. The method requires, however, a bright source
of small angular diameter, like Mars, Uranus, and Neptune.
VI . TEL ESCOPE METROL O G Y
This section reports several applications, or proposed
applications, of monitoring instrumentation as mentioned
in the open literature; many applications and test results
are, however, difficult to access in internal observatory
reports. Table 3 lists instrumentation that can and has
been used for telescope metrology.
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Greve and Kärcher: Performance Improvement of a Flexible Telescope
IRAM 30-m telescope. Measurements of P4 , P5 are
shown in Fig. 1. In other measurements, the electronic
drifts can perhaps be reduced/eliminated when using the
correlated output of parallel inclinometers.
Fig. 1. IRAM 30-m telescope. Pointing model corrections P4 (black)
and P5 (gray), defining the change in tilt of the azimuth axis,
derived from inclinometer measurements. [Data from J. Peñalver
(IRAM, Spain).]
1) Inclinometer: Inclinometer measurements have
been made on many telescopes in order to understand
temperature- and wind-induced pointing changes; however, the measurements were often of little success. The
inclinometer cannot be used on telescope components that
tilt (reflector, backup structure, quadripod); they can be
used on telescope components that are fixed in space or
that rotate around the vertical axis. If not installed on-axis
(for instance, on fork support arms and alidade towers),
however, the inclinometer is sensitive to off-axis accelerations. Very disturbing are go-and-stop movements when
changing radio source and when performing scanning observations. While the tilt of the telescope azimuth axis can
be measured easily with an on-axis inclinometer, the
important measurement of elevation axis tilt (along and
perpendicular to the elevation axis) remains difficult and
not yet proven in an operating system. In addition, an
inclinometer is often prone to electronic drifts and drifts
due to changes of the inclinometer or environmental
temperature (for instance, 1.3 arcsec= C (x-axis) and
0.3 arcsec= C (y-axis), inclinometer Nivel 20, Leica, reported by Ukita et al. [37]). The problematic use of inclinometers, and temperature sensors, on the alidade of the
Medicina 32-m telescope for pointing predictions is
reported by Ambrosini et al. [38].
An x, y-axis inclinometer is used on-axis on the IRAM
30-m telescope in a way that eliminates a linear electronic
drift [39]. Whenever the telescope slews by more than
100 in AZ-direction, the constant slew velocity and the
linear drift of the inclinometer produce a linear slope of
the recording. Elimination of this slope and cosine/sine fit
of the data gives the instantaneous inclination of the
azimuth axis (amplitude and tilt direction, P4 and P5
parameter of the pointing model [40]). The cosine/sine
analysis gives the upgrades P4 , P5 , which are
introduced in real time into the pointing model. This
method has considerably reduced the frequency and the
measurement time for upgrading the pointing model of the
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2) Quadrant/Position Detectors: These detectors measure
relative deviations of a laser beam from straightness, i.e.,
deviations x, y perpendicular to the beam. The measurements
do not discriminate between a displacement of the laser
emitter or the target, or both, unless perhaps when using
systems that measure in both directions. The measurements
are sensitive to atmospheric turbulence, in particular on
horizontal paths and paths over warm surfaces. The
turbulence can be reduced by propagation of the laser beam
in a tube, for instance, an alidade beam, a quadripod leg, or a
fork support arm. As an example, successful short-term
monitoring of the lateral position (off-sets in azimuth and
elevation direction) of the subreflector on the GBT 100-m
telescope is reported in [41]. Such measurements can be
routinely made. Similar monitoring is planned on the 64-m
SRT [42] and the 50-m LMT telescope.
3) Transducer/Edge Sensor/Strain Gauge: The ALMA
(prototype)/APEX 12-m telescopes are equipped with an
internal CFRP reference structure [43], [44], leading from
the fork traverse through the fork arms to the elevation
bearings. This reference structure is insensitive to
temperature changes. The position of the base of the
EL-bearings is measured with transducers fixed to the
reference structure; these measurements are intended to
provide in particular in real time the cross-elevation
pointing error. The important novelty of this and similar
systems is the idea to transfer the base of measurement by
a stable structure to a higher location in the telescope.
According to present plans, the proposed CCAT 25-m
telescope4 may have a reflector surface consisting of
actuator supported panels of relatively small size. The
position of adjacent panels is measured with edge sensors,
in a similar and proven way as applied on the 10-m Keck
optical telescope [45], [46]. While the edge sensors
measure and control the position of adjacent panels, the
large-scale gravity and temperature-induced deformations
must be measured with a metrology system, still to be
designed. Optical telescopes have the advantage of using
the image of a star for wavefront analysis.
4) 4-D Measurements: Position Detector and Tilt Detector:
The position detector of the API 5D-System measures
deviations x, y perpendicular to straightness; the tilt
detector (in the same detector unit) measures pitch and yaw . The measured displacements x, y and
tilts , depend on the pivot position of the detector.
The position and tilt measurements are affected by
atmospheric turbulence when used in the open air. A
4
See www.caltech.edu and www.submm.org.
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Greve and Kärcher: Performance Improvement of a Flexible Telescope
ments and an FEA, illustrating the difficulty in using such
measurements directly for telescope control. A useful
relation between such and similar measurements and
pointing errors may be obtained from special Btraining
measurements,[ for instance, using measurements of a
radio source close to the pole [41].
Fig. 2. Accuracy of 5-D measurements (API instrumentation) on a
closed path of 3.5 m length (fork arm). LE: laser emitter, D: detector
unit. x, y: displacement, ; : pitch and yaw, z: distance. The linear drift
of the z recording is due to a temperature change of the fork arm;
the recordings of and are displaced by 0.5 arcsec.
measurement made inside the 3.5-m-long fork arm of the
ALMA VertexRSI prototype telescope is shown in Fig. 2.
Measurements of straightness and pitch and yaw with the
emitter of the API 5D system installed at the base of the
fork and the detector at the top of the fork (elevation
bearing) are shown in Fig. 3. Regular and daily repeatable
displacements and tilts are noticed and assumed to be
temperature effects (for more data, see [44]); however, the
data could not be interpreted from temperature measure-
Fig. 3. 24-hour measurements with the API 5D (two consecutive days)
of the change x, y (black points) and , (gray points),
with the laser emitter at the base of the fork, the detector at the
position of the elevation bearing. Data obtained during tests of the
ALMA VertexRSI antenna.
5) Temperature Sensors: The thermal state of a telescope
can easily be monitored with electric temperature sensors,
placed at strategic thermal locations. The thermal control of a
telescope depends on the number of sensors. A small number
of sensors, say, 10 to 20, allows temperature monitoring of
basic structural components, like the alidade, support fork,
yoke, BUS, or quadripod, which may however be sufficient
(when supported by FEM calculations) to control the focus
and the pointing from empirical relations. Temperature
monitoring with a small number of sensors for open-loop
focus control is, for instance, made on the IRAM 30-m
telescope [47] using the temperature difference between the
BUS and the yoke as control parameter. On the GBT 100-m
telescope [41], the temperature difference between the BUS
(measured at five locations) and the temperature of the feed
leg measured at the position of the subreflector is used for
similar control, using a control matrix established from
dedicated temperature and focus measurements (Btraining
measurements[). On the Cambridge MERLIN 32-m telescope, the measured temperature difference of the alidade
front side and rear side (eight sensors in total) is used to
correct the temperature-induced cross-elevation pointing
error [48]. Similar temperature measurements of the alidade
front and rear side (eight sensors) are used on the astrodome
enclosed JCMT 15-m telescope.5 However, the difficulty of
using temperature and inclinometer measurements of the
alidade of the Medicina 32-m telescope for pointing prediction
is reported in [38]. It is worthwhile to mention that from longterm temperature monitoring of the ASTE 10-m telescope
pedestal and fork support and simultaneous inclinometer
measurements, a useful correlation between the temperature
gradients and the tilts (in S–N and E–W direction) is derived
[37]. The importance of these measurements lies in the
demonstration that dedicated Btraining measurements[ can
lead to useful control relations, in this case using temperature
measurements to predict the inclination of the fork support,
and avoiding inclinometer measurements in the later
operation.
The most realistic picture of a telescope’s thermal
state is obtained from temperature measurements with
many sensors, as, for instance, used on the JCMT 15-m
telescope5 [220 sensors], the IRAM 30-m telescope
[27; 150 sensors], the ALMA-J 12-m prototype telescope
[18; 270 sensors], and other telescopes. A large number of
sensors allows the prediction of temperature-induced
reflector surface deformations (BUS deformations) for use
on a telescope with active reflector surface. Fundamental
5
See www.jach.hawaii.edu/JCMT/telescope.
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Greve and Kärcher: Performance Improvement of a Flexible Telescope
Fig. 4. IRAM 30-m telescope, BUS and yoke. Small dots: elements of
the FEM and big dots: location of temperature sensors for optimal
prediction of temperature-induced reflector surface deformations.
M: membrane, SRS: subreflector supports, SR: subreflector,
PFC: primary focus cabin.
exploratory work was made on the JCMT telescope and
the IRAM 30-m telescope [27].6
The search for where to place temperature sensors and
the temperature interpolation algorithm to obtain the
temperature distribution for all FEM elements from the
limited number of sensors is explained for this telescope in
[27]; the installed sensors are shown in Fig. 4. On this
telescope, of which the employed FEM has only 2376 elements, the thermal deformations of the panel frame
support points are predicted with a precision of 0.005 mm,
as derived from comparison of a holography surface map
and a map based on the prediction from an FEA using the
measured and interpolated temperatures. The comparison
is shown in Fig. 5. This method of prediction of
temperature-induced surface deformations can be incorporated in an open-loop control of actuator settings. On
the IRAM 30-m telescope, the temperature dependence of
the focus position is predicted with 0.1 mm accuracy in
prime focus and secondary focus operation [27]; the
prediction of the temperature dependence of the pointing
correction was not fully successful because the available
FEM does not contain the whole steel structure. For
correction of temperature-nduced pointing errors, an FEM
of the entire telescope steel structure must be available.
It is shown in [44] that temperature-induced pathlength changes can easily be corrected from temperature
6
The installation of temperature sensors on this telescope was made
to determine the origin of a transient reflector surface astigmatism. As an
earlier generation telescope construction, this telescope has, unfortunately, no panel frame actuators.
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Fig. 5. IRAM 30-m telescope. (a) Reflector surface deformations
derived from holography measurements, and (b) calculated from
the FEM using the temperatures of the BUS recorded during the
holography. Contours between 0.060 and 0.060 mm, in steps of
0.015 mm.
measurements and an FEM calculation. Similar investigations are in progress for temperature-induced path-length
differences in VLBI observations [49].
6) Wind Monitoring, Pressure Sensors: While temperature
monitoring and open-loop temperature control is well
advanced for application, this is not at all the case for windinduced effects. A high expectation is placed on inclinometer measurements for pointing control, perhaps using
new low-drift inclinometer or measurements from correlated parallel inclinometers, in combination with accelerometers [50]. The idea to derive pointing errors and
reflector surface deformations from measurements with
pressure sensors as input in an FBA may fail because of the
large number of sensors to be placed on a telescope, and
especially on the reflector surface. In this context, it is
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Greve and Kärcher: Performance Improvement of a Flexible Telescope
Table 4 Present State of Telescope Control
worthwhile to mention that an optimized (material and
costs) calotte astrodome is in discussion for the CCAT 25-m
telescope, in particular to reduce the influence of wind.
VII. SUMMARY
The progress in telescope modeling with the concept of a
mechatronics system (dynamic behavior of structure and
drives), the progress in the engineering of surface actuators and reflector surface control, and the availability of
cheap temperature sensors and data acquisition devices
have led to the implementation on several telescopes of
open-loop reflector surface control and to fundamental
steps of open-loop temperature control, with accuracies
in the micrometer region as required for millimeter-wavelength telescopes. The algorithms for selection of temperature sensor locations and temperature interpolation for
all elements of a FEM for FBA are developed and proven so
that the implementation of open-loop temperature control
of focus, pointing, and reflector surface shape is today
possible. This step will probably in the near future be taken
on the LMT 50-m telescope and the SRT 64-m telescope.
Although the control strategy and control algorithms
are well developed, and although there are well-advanced
studies of the wind-induced dynamic behavior of telescopes
(vibrations, eigenfrquencies), there is only moderate
progress in the measurement of wind-induced deformations of telescope structures and their eventual control. It is
important to arrive at a metrology system using inclinREFERENCES
[1] J. W. Mar and H. Liebowitz, Eds., Structures
Technology for Large Radio and Radar
Telescope Systems. Cambridge, MA:
MIT Press, 1969.
[2] V. S. Polyak and E. Y. Bervalds, Precision
Construction of Reflector Radiotelescopes
(in Russian). Riga, Latvia: Zinatne, 1990.
ometers, and inclinometers in combination with accelerometers and perhaps strain gauges. While this is certainly
the field of telescope engineering, nevertheless, important
exploratory experimental work can still be done on existing
telescopes by observatory staff. While for smaller telescopes, recourse can be taken in the use of an astrodome
(for instance, CCAT), this approach fails in the case of large
millimeter-wavelength telesopes, like the LMT. As shown,
metrology is required on this telescope to achieve optimal
operation under wind load.
The influence of the atmosphere on wave propagation
(amplitude and direction) can degrade the performance in a
comparable way as the flexible structure of a telescope. Next
to the progress in telescope control by passive engineering
means and metrology, there is considerable progress in the
monitoring of atmospheric phase effects and their correction
in interferometer and VLBI observations. There is, however,
so far little progress in the phase correction of anomalous
refraction [51] in single-dish observations.
Table 4 summarizes the present state of telescope control
in view of an estimated global loss in observing time. h
Acknowledgment
The authors are grateful for information from D. Balser
and J. Mangum (NRAO), J. Peñalver (IRAM), D. Woody and
S. Radford (Clatech), A. Lundgren (APEX), and A. Kraus
(MPIfR). They thank the referees for their comments.
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