The Large Millimeter Telescope (LMT) Alfonso Serrano is a bi-national (Mexico and USA) telescope facility constructed on the summit of Sierra Negra, at an altitude of 4600m, in the Mexican state of Puebla. The LMT is a 50-m diameter single-dish radio-telescope designed, constructed and optimized to conduct scientific observations using heterodyne and continuum receivers, as well as VLBI observations, at frequencies between ~70 and 350 GHz.
The LMT has an active surface control-system to correct gravitational and thermal deformations of the primary reflector to enable both night-time and daytime observations. We describe the current status and technical performance of the LMT, the instrumentation development program, and an on-going series of engineering and technical upgrades that will increase the optical efficiency and sensitivity of the telescope which will improve the overall scientific productivity and operational efficiency of the LMT.
The Large Millimeter Telescope (LMT) Alfonso Serrano is a 50m-diameter single-dish radio telescope constructed at an altitude of 4600 meters on the summit of Volcan Sierra Negra, an extinct volcano in the Mexican state of Puebla. The LMT is a bi-national scientific collaboration between Mexico and the USA, led by the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE) and the University of Massachusetts at Amherst. The telescope currently operates at wavelengths from 4mm to 1mm, and during the dry winter months the LMT site provides the highest levels of atmospheric transmission and potential future access to submillimeter observing windows. This paper describes the current status and scientific performance of the LMT, the suite of scientific instrumentation and future engineering upgrades that will optimize the optical efficiency of the telescope and increase its scientific productivity.
With the completion of the full 50 m primary reflector surface of the Large Millimeter Telescope/Gran Telescopio Milimetrico (LMT/GTM), the project also upgraded the primary surface actuators. These actuators were custom-designed by ADS International in Valmadrera, Italy to meet the accuracy, load, and physical size requirements necessary for robust operation. Specifically, the actuators had to provide precise and repeatable positioning, support both operational and survival loading conditions for even the largest surface segments, and still fit within the geometric constraints imposed by interior angles of the backup structure truss members. Factory and laboratory testing confirmed that the actuators should meet the requirements. As reported in earlier papers, the LMT/GTM site poses particular challenges for electromechanical devices. As expected for a mountaintop site (4,600 m), the low atmospheric density reduces cooling effectiveness for motors and drives. To add to the challenge, the ambient temperature hovers near freezing and there is significant precipitation during the summer. This results in frequent freeze/thaw cycles. The constant formation and either sublimation or melting of ice has been an operational challenge for many devices at the LMT/GTM. Because of the large number of primary surface actuators (720 in total), it is particularly important that these units, their drive control boxes, and their cable connections be able to meet all specifications even under the site conditions. To confirm the suitability of the actuators, the LMT/GTM procured an initial set of sixteen actuators for testing at the site. After laboratory testing, the actuators were installed into the outer two rings of the telescope and cycled from February 2016 through January 2017. This extended testing provided direct operational experience over a wide variety of weather conditions. The program was long enough to provide confidence in the actuator design. With the first article testing complete, the project ordered a production run of the actuators. The installation of these actuators met the telescope completion plan requirement of bringing the LMT/GTM to a 50 m active surface telescope for the 2017-18 scientific observing season. This paper presents the final report on the first article testing program, as well as a summary of the characterized performance of the production actuators prior to installation.
As part of the telescope completion plan, the Large Millimeter Telescope (LMT/GTM) project replaced the hexapod positioner for the secondary (M2) mirror. The new hexapod was provided by Symetrie of Nimes, France. The particular challenges for the LMT/GTM hexapod are that it is both large and precise. After completion of the fabrication and internal contractor verification of the system, the project conducted a series of characterization tests, both at the fabrication facility and at the telescope site. During the factory tests, the project team tested the hexapod in both vertical and horizontal positions, verifying the motion range, accuracy, repeatability, and velocity, all at the maximum operational payload. Additionally, the team performed verification checks on the stiffness of the hexapod. The results were excellent, with calibrated errors of less than 5 microns in the translation degrees of freedom and less than 1 arcsec in the rotations, which was at the limit of the metrology of the tests. Following the successful factory test, the system was transported to the telescope site and the tests were repeated. While the calibration step was not performed during the site tests, the raw results were comparable to the factory values, clearing the way for installation on the telescope. The new hexapod was installed, along with a new M2 mirror, in the Fall of 2017 in advance of the LMT/GTM’s first observing season as a 50 m telescope. This paper presents the test program, the metrology approach, the characterization tests, the calibration method, and the final factory acceptance test results.
Following the magnitude 7.1 earthquake in central Mexico on 19 September 2017, personnel from the Large Millimeter Telescope/Gran Telescopio Milimetrico (LMT/GTM) performed a visual inspection of the foundation and structure of the LMT/GTM. Though no damage was found, the project also conducted a measurement of the system using the precision tiltmeters that are permanently installed at the elevation axis. Fortunately, a series of baseline tests had been run in 2013 after the installation of the first tiltmeter, including a comparison to the original survey results. To match the fastest of these tests, the site crew ran the telescope through ±360° of azimuth rotation at moderate slew speeds in each direction.
The most important result is that there have been no statistically significant changes in the tilt variation of the alidade since either the original track surveys during initial construction or the tiltmeter tests in 2013. This conclusion is based on comparisons of tilt results averaged over 2 degree bins in the data. The result confirms that there was no apparent change to system performance.
As the LMT/GTM has moved to final completion as a 50 m diameter telescope, the scientific and instrumentation teams have requested information concerning the actual motions between the reinforced M3 platform of the telescope and the receiver cabin floor. To provide some bounding information on these effects, the LMT/GTM engineering and metrology teams developed a test program to measure these effects by means of a laser tracker. Two sets of tests were performed. The first focused on the relative motions between the M3 platform, the M4 mirror, and the receiver cabin floor. The second was directed at measuring the effective stiffness of the floor under load.
In the first tests, a laser tracker was employed to measure groups of targets on the M3 platform, the M4 mirror, and the receiver cabin floor. The baseline distances were then compared continuously for several hours. In this test, the M4, which is supported directly from the M3 platform, was found to be more stable than the receiver cabin floor. In most cases, the errors were consistent with thermal variations in the structure. The most dramatic change was observed near sunset, with position drift rates of about 300 μm/hr. Later at night, the M4 position stabilized, but the receiver cabin still sometimes showed position variations of over 100 μm/hr. These results put a bound on the maximum allowable time between checking the pointing and focus of the telescope.
The second tests measured the stiffness of the receiver cabin floor by measuring the underside of the platform from the floor below while weights were placed at different locations in the testing area of the floor above. As expected, the largest deflections were measured when the load was placed at the center of the floor grating between the mid-span of the smallest floor structure I-beams, with a stiffness of 14 N/μm. The stiffness was about 10% higher (just under 16 N/μm) directly at the smaller I-beams near their mid-span. A more dramatic difference was measured for loads near a main structural cross beam. In that case, targets that connected to the beam itself were found to have a stiffness of nearly 34 N/μm, more than twice the mid-span stiffness. However, in that location, the stiffness for loads in the middle of the floor grating increased only to 17 N/μm, because the flexibility is dominated by the floor grating itself. Comparison of the unloaded condition of the structure after each test showed slow drifts of the relative positions of the platforms, consistent with the thermal drift hypothesis supported by the first tests.
This paper presents the tests and analysis, together with the detailed results of the receiver room motion and floor stiffness.
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