The Large Millimeter Telescope (LMT), located in central Mexico, saw completion of the final construction phase in 2017 with the installation of the full 50-meter primary reflector, following three years of operation as a 32-meter facility. The task was accomplished by adding two more concentric rings of surface segments to the existing three inner rings. Various techniques have been used previously to measure and align the 32-meter surface, including multiple laser trackers and far-field phased holography. Whilst the former method is time-consuming, requiring a full night to obtain a single surface map, holography provides low spatial resolution and requires removal of the secondary reflector for installation of the twin-horn receiver at the primary focus.
Photogrammetry has been used as an alternative measurement technique for the 32-m primary since 20151, and has gradually replaced our use of holography and laser trackers for this task2 during recent years. Once the object has been targeted, photogrammetry maps may be obtained in around one hour. The technique does not require the installation of special equipment on the antenna, and has the advantage of allowing surface maps to be taken at any chosen elevation. The main drawbacks for the LMT application are environmental, since the antenna operates without an enclosure; strong winds may prevent use of the site tower crane for image taking, while the formation of condensation and frost on the reflector surface will "switch off" the reflective targets.
In this paper we discuss comparative measurements taken as the first outer segments were installed, and the use of photogrammetry to carry out the alignment of the fully installed 50-meter surface. At the time of writing this activity is still in progress, however full-surface alignment to the order of just over 100 microns was achieved quite quickly, with multiple elevation maps allowing the development of a usable 50-m active surface model for compensation of gravitational distortions.
The Large Millimeter Telescope (LMT), located in Central Mexico, saw completion of the final construction phase in 2017 with the installation of the full 50-meter primary reflector, following three years of operation as a 32-meter facility. The task required the assembly and alignment of 96 primary surface segments, each comprising 8 laminated Nickel subpanels. These segments are installed on the antenna back structure in two concentric rings, expanding the existing 3- ring 32-meter configuration.
Prior to assembly of the new segments, a review of the original surface support system was carried out. Based on previous experience with the alignment and stability of the inner rings, it was decided to upgrade a large number of the early subpanel support and adjustment components. A key modification was the inclusion of lateral adjustment for subpanel support points, helping to minimize bending moments on the panels both during and after integration. Improvements in the ease of surface setting and greater surface stability were immediately observed following component overhaul. Form setting of individual segments was carried out at the LMT facilities in Puebla and again at the telescope site, using the iterative process developed previously that makes use of laser tracker surface measurements.
While the original implementation of the subpanel support system enabled the setting of individual segments to a mean surface error of around 30 micrometers RMS, this mean value was improved to around 20 μm for the entire set of 96 segments for the outer two rings, with the best segments coming in at around 18 μm RMS surface error. We believe this brings us close to the limit of achievable surface accuracy for the LMT design of laminated composite subpanels supported on simple mechanical differential adjusters.
We present an overview of primary surface improvements since 2011, and the main aspects of the LMT aperture expansion relating to the assembly and alignment of the surface segments for the outer two rings of the 50-meter primary.
The Large Millimeter Telescope (LMT) makes extensive use of 12 GHz holography during maintenance periods to finetune the alignment of primary reflector segments to the best-fit design parabola. Tracker measurements have also been used for this task, however the technique is severely limited by environmental noise and large data collection times, on the order of many hours for a single map. In 2015 we started photogrammetry trials as a complimentary measurement technique. Photogrammetry can offer reduced mapping times compared with laser trackers, and like holography, allows maps to be made at arbitrary elevation angles. Depending on the placement of reflecting targets, the technique can also provide higher spatial resolution than currently achieved using our holography system.
Accurate photogrammetry requires a robust strategy for the incorporation of multiple camera stations, a task complicated by the size of the antenna, obstructions of the surface by the sub-reflector and tetrapod legs, and the practicability of using the site tower crane as a moving camera platform. Image scaling is also a major consideration, since photogrammetry lacks any inherent distance reference. Therefore appropriate scale bars must be fabricated and located within the camera field of view. Additional considerations relate to the size and placement of reflective targets, and the optimization of camera settings. In this paper we present some initial comparisons of laser tracker, holography and photogrammetry measurements taken in 2015, showing clearly the status of alignment for distinct zones of the currently operating 32.5 m primary collecting area.
The Large Millimeter Telescope (LMT) is a single-dish fully-steerable radio telescope presently operating with a 32.5 m parabolic primary reflector, in the process of extension to 50 m. The project is managed by the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE) in México, and the University of Massachusetts Amherst, USA. A laminated surface panel from the LMT primary reflector has been subjected to a surface measurement assay at Mexico’s National Metrology Center (CENAM). Data obtained using a coordinate measuring machine and laser tracker owned by CENAM is compared with measurements using an identical model laser tracker and the photogrammetry technique, the latter systems owned and operated by the LMT. All measurements were performed within the controlled metrology environment at CENAM. The measurement exercise is intended to prepare the groundwork for converting this spare surface panel into a calibrated work-piece. The establishment of a calibrated work-piece provides quality assurance for metrology through measurement traceability. It also simplifies the evaluation of measurement uncertainty for coordinate metrology procedures used by the LMT project during reflector surface qualification.
To support higher-frequency operation, the Large Millimeter Telescope/Gran Telescopio Milimetrico (or LMT/GTM) is replacing its existing monolithic aluminum secondary mirror (M2). The new mirror is a segmented design based on the same electroformed nickel reflector panel technology that is already in use for the primary reflector segments. While the new M2 is lighter and has better surface accuracy than the original mirror, the electroformed panels are more sensitive to high temperatures. During the design phase, concerns were raised over the level of temperature increase that could occur at M2 during daytime observations. Although the panel surface is designed to scatter visible light, the LMT primary mirror is large enough to cause substantial solar heating, even at significant angular separation from the Sun.
To address these concerns, the project conducted a series of field tests, within the constraint of having minimum impact on night time observations. The supplier sent two coupon samples of a reflector panel prepared identically to their proposed M2 surface. Temperature sensors were mounted on the samples and they were temporarily secured to the existing M2 mirror at different distances from the center. The goal was to obtain direct monitoring of the surface temperature under site thermal conditions and the concentration effects from the primary reflector. With the sensors installed, the telescope was then commanded to track the Sun with an elevation offset. Initially, elevation offsets from as far as 40 degrees to as close as 6 degrees were tested. The 6 degree separation test quickly passed the target maximum temperature and the telescope was returned to a safer separation. Based on these initial results, a second set of tests was performed using elevation separations from 30 degrees to 8 degrees.
To account for the variability of site conditions, the temperature data were analyzed using multiple metrics. These metrics included maximum temperature, final time average temperature, and an curve fit for heating/ cooling. The results indicate that a solar separation angle of 20 degrees should be suitable for full performance operation of the LMT/GTM. This separation not only is sufficient to avoid high temperatures at the mirror, but also provides time to respond to any emergency conditions that could occur (e.g., switching to a generator after a power failure) for observations that are ahead of the motion of the Sun. Additionally, even approaches of 10 to 15 degrees of angular separation on the sky may be achievable for longer wavelength observations, though these would likely be limited to positions that are behind the position of the Sun along its motion.
Prior to the early science campaign of Spring 2013, the engineering team at the Large Millimeter Telescope/
Gran Telescopio Milimétrico (LMT/GTM) conducted a series of performance tests on the hexapod used
for positioning the secondary reflector (M2 mirror). The tests were of particular interest to the project due to
the high mass of the existing aluminum M2 mirror.
The testing was conducted in a lower foundation room at the LMT site on a fixture that allowed the positioner
and mirror to be oriented at both zenith and horizon orientations. In each of these positions, the repeatability of
the system zero position was tested, along with both single degree-of-freedom (DOF) and combined DOF motions.
Additionally, the tests investigated the stability of the system at constant command position to changes in the
orientation of the unit with respect to gravity. Throughout these tests, a laser tracker was used for measurement
of the position of targets on both the fixed base of the hexapod and on the outer rim of the M2 mirror. In this
way, motions of the tracker head or of the support fixture could be eliminated from the analysis.
In this paper, we present results of the accuracy and repeatability of the system, as well as comments on
the effects of the laser tracker measurement geometry with respect to the system at the zenith and horizon
The primary reflector of the Large Millimeter Telescope (LMT) Alfonso Serrano is presently composed of 84 surface panels arranged in three concentric rings, providing a 32.5 meter collecting area. Each panel comprises 8 precision composite subpanels having electro-formed nickel skins bonded to an aluminum honeycomb core. Differential thread adjusters beneath each subpanel allow for the manual removal of tip/tilt and piston errors, in addition to facilitating some fine tuning of the surface shape. An assembled panel provides a surface area of approximately 8-12 square meters.
Preparation of surface panels in 2012 and 2013 for Early Science observations made use of a Leica laser tracker. Measurement and adjustment of panels was carried out off the antenna, achieving a mean panel RMS surface error of 29.5μm for the 67 panels processed to date, with a spread of 23-37μm. A panel stability check consisting of surface walk-on tests and repeat metrology resulted in an increase in the mean surface error to 31.0μm. Following installation, in situ tracker measurements of 19 panels showed a final mean error of 45.3μm. Panels are adjusted by hand using an iterative process. In-house data processing uses fiducial marks scribed onto the subpanel molds and replicated during manufacture, to achieve accurate registration of the surface point cloud during data fitting. The number of iterations varies, depending mainly on the behavior of the differential adjusters. A well-behaved panel may be set within around 7 hours. In this paper we describe the iterative panel surface adjustment process used to date. We focus on metrology technique and data processing using the laser tracker, and present comparisons with trial photogrammetry measurements.
The Large Millimeter Telescope Alfonso Serrano (LMT) is a 50-meter (currently 32m) diameter single-dish telescope optimized for astronomical observations at millimeter wavelengths in the range 0.85 mm < λ < 4 mm. During initial operation, the LMT makes use of the central 1.7 meters of a 2.5m hyperbolic secondary reflector constructed of cast and machined aluminum. Following the first light campaign in 2011, a program of iterative surface sanding was carried out to reduce the surface error of the central area to a level compatible with that presently achieved for the primary reflector. Metrology during the sanding process was conducted using a Leica laser tracker. A total of 22 sanding iterations were interspersed with tracker measurements at differing spatial resolutions, allowing the RMS surface error to be reduced from 63 to 35 microns. Maps for the final iterations were repeated for distinct scan patterns to check for systematic variance. Since the work was carried out in early 2013, repeat measurements of the dismounted secondary have confirmed the stability of this reflector.
In this paper we present details of the surface improvement program with emphasis on the metrology techniques used throughout the process. We discuss issues such as data sampling, measurement geometry, and mirror orientation. We also consider the steps taken to ensure tight control of the sanding task itself, since this process was carried out entirely by hand. Finally we present some comparative metrology results obtained using our laser tracker and photogrammetry equipment.
The Large Millimeter Telescope Alfonso Serrano (LMT) currently has a primary reflector of 32.5m diameter composed
of 84 panels, each having a surface area of approximately 10 square meters. Each panel is supported on four electromechanical
actuators, allowing for the correction of tip-tilt, piston and twist. The actuators are designed to perform active
surface compensation of gravity deformations as a function of elevation.
Following the setting and installation of individual panels, an approximation for global alignment of the primary surface
is carried out using a total station. An RMS error of 200 - 500μm is expected for this process. Final global alignment is
conducted using holography at 12GHz for elevations corresponding to the location of geostationary sources. As an
intermediate alignment option for the antenna at zenith, the use of a laser tracker has been explored. Global alignment of
a large primary surface with a laser tracker presents the common problems related to the contact measurement of a large
object in a non-metrology environment. Key issues are the stable location of fiducial points and the relatively slow data
collection rate. Additionally the high altitude site (4600m, 15000ft) with mean temperatures around zero degrees Celsius,
presents a challenge for our interferometer-equipped trackers.
In this paper we present first results using a tracker located near the antenna vertex, and mechanical adjusters in place of
actuators. An RMS error of around 100μm was achieved. Limiting factors included inadequate fiducials and slow
mapping speed. Proposals for reduced data collection times and improved metrology robustness are presented.
This paper describes an iterative process of surface improvements made to the central 1.7m zone of a 2.5 metre
hyperbolic reflector constructed of cast and machined aluminium. Throughout all stages of the process, the mirror
surface was measured using a laser tracker, with initial maps taken by scanning the tracker target over the surface at low
spatial resolution. While the overall RMS of the full surface was in excess of 200 microns, the central area of interest
was in excess of 60 microns. The final goal of the program was to achieve 40 microns or better in this central area.
Surface maps showed a major low area on the mirror surface covering many tens of square centimeters, plus several
smaller high spots. The high spots were removed progressively by sanding with an orbital sander. Frequent pauses were made to take repeat surface measurements with the tracker.
A total of 22 grinding iterations were interspersed with tracker measurements at differing spatial resolutions, allowing
the RMS surface error to be reduced from 63 to 35 microns best measurement. Sanding periods lasted from 15 seconds
to 4 minutes at each sanding spot, while tracker measurements took approximately 15-20 minutes to acquire from 600
data points (low spatial frequency) to 6800 points at high resolution.
We present details of the surface improvement program with emphasis on the assurance of metrology integrity. We
discuss data fitting to the desired hyperbolic shape, sampling strategies, identification of sanding zones, and tracker
performance outside of operating environment specifications.
The Large Millimeter Telescope (LMT) is a 50m diameter millimetre-wave radio telescope situated on the summit of
Sierra Negra, Puebla, at an altitude of 4600 meters. The reflector surface of the LMT currently employs84 segments
arranged in three annular rings. Each segment is comprised of 8 precision composite subpanels located on five threaded
adjusters. During the current primary surface refurbishment, individual segments are aligned in the telescope basement
using a laser tracker. This allows increased spatial resolution in shorter timescales, resulting in the opportunity for
improved logistics and increased alignment precision.
To perform segment alignment an iterative process is carried out whereby the surface is measured and subpanel
deformations are corrected with the goal of 40 microns RMS. In practice we have been able to achieve RMS errors of
almost 20 microns, with 35 microns typical. The number of iterations varies from around ten to over 20, depending
mainly on the behaviour of the mechanical adjusters that support the individual subpanels. Cross marks scribed on the
reflector surface are used as fiducials, because their positions on the paraboloid are well known. Measurement data is
processed using a robust curve fitting algorithm which provides a map of the surface showing the subpanel deviations. From this map the required subpanel adjuster movements are calculated allowing surface improvement in a stepwise manner.
The Large Millimeter Telescope (LMT) currently employs a 32.5m diameter primary reflector composed of 84 surface
segments. Global alignment of the surface is carried out using the best-fit parabola. Surface alignment follows an
iterative procedure that consists of measuring the surface with a laser tracker to determine the deviations from the
theoretical surface, followed by surface adjustments at the segment level.
Global alignment of the primary surface presents many unusual problems related to the measurement of a large object in
a non-metrology environment. The LMT antenna is located at high altitude (4700m, 15000ft) in a rural setting, where
mean temperatures oscillate around zero degrees centigrade, thus presenting a challenge for traditional sensitive
metrology equipment such as the laser tracker.
Measurement of the antenna surface with the laser tracker requires the use of fiducial points that can be used to tie the
measurement of each segment position to a common reference. Several approaches to the allocation of fiducial markers
on and around the antenna are discussed in this paper.
In-house data analysis provides a surface error and detailed output for the iterative adjustment of individual segments in
order to reduce the global surface error. In this paper we discuss many aspects of the global alignment process with
particular emphasis on making optimum use of laser tracker metrology.