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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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Receivers
Mirrors
Telescopes
Motion measurement
Metrology
Laser metrology
