Figure 1: Grey-scale representation of the phase distribution across the aperture, obtained in June with the new coherent holography equipment. The map size is ; this is the most detailed map of the 30m antenna obtained so far. The grey scale contrast has been set high to enhance details. Panel rings are clearly visible. The test panel, displaced on purpose, is visible above and to the right of the center.
For several years, IRAM has been performing holographic measurements of the surface of the 30m telescope, and executing adjustments of the panels (actually, frames) in an effort to improve the surface accuracy and thereby increase the main-beam efficiency and decrease the uncontrolled signal pickup from the error beam.
The method used was the so-called phase retrieval method, where the essential phase information is not measured directly, but retrieved from two or more power pattern measurements performed with the telescope focussed and defocussed. The solution for the phase is obtained by an iterative algorithm.
After a period of significant progress in the early years of the telescope, the accuracy of the surface seems to have reached an asymptotic level of (rms, illumination weighted), while an analysis of the method's accuracy, both from theory and from then internal consistency of results, indicates that a measurement accuracy of should be obtained.
IRAM has therefore decided 18 months ago to perform measurements by the coherent holography method, where the phase of the signal collected by the main reflector is measured against that of a reference receiver looking directly at the satellite beacon (Italsat), which is radiating at 39 GHz, and is ideally located close to 45 elevation.
The main contributors to this project were J. Lamb (project management, optics, sensitivity analysis, data acquisition), F. Matiocco (receiver construction), and D. Morris (data reduction), with support from a number of people of the Granada and Grenoble technical staffs.
The first field test took place at the end of June. The secondary mirror was dismounted and the dual receiver was installed at the prime focus. The electronic noise contributes less than rms to the phase noise, which is dominated by atmospheric fluctuations. Some time was spent to sort out interface problems in data acquisition, but eventually a quite promising map was obtained. For the first time a map of the reflector was obtained, where individual panels can unambiguously be seen (see Fig. 1, where panel rings can be clearly seen, especially the test panel above and right of center).
A further test is scheduled in September, with several goals :