Active surface measurements for large aperture millimeter/submillimeter-wave telescopes using a photogrammetry technique

Photogrammetry technique is widely used for the initial alignment of main-reflector panels of millimeter/submillimeter-wave telescopes by analyzing a great number of photos of the reflector at the rest state taken from different angles and distances. In this study, we investigated a possibility that the photogrammetry can be applied for real-time surface measurements which is important to realize active surface controls that improve reflector surface accuracy during scientific observations. The technique is important especially for realizing larger aperture and higher frequency telescopes. We developed a simulator to investigate the accuracy of the surface measurements with photos taken with fixed cameras mounted on the stays of the sub-reflector. As a result, we found that the accuracy of surface measurement is roughly inversely proportional to square-root of the number of fixed cameras, and the calculation time roughly proportional to the product of the numbers of cameras and measurement points. For the case of Nobeyama 45-m telescope, the accuracy of 1 mm (rms) was achieved for 164 surface points by 10 cameras with a calculation time of similar to 2 sec by a developed python code using a single-core Xeon processor. In order to improve the accuracy with a minimum number of cameras, more various camera positions (e.g., surrounding the vertex hole of the main reflector and surrounding the main reflector) should be investigated, and their combination should be optimized. Applying high-performing technologies such as multiprocessors and/or GPUs, faster calculation is to be considered.