Photogrammetric surface measurement of the KOSMA 3m-telescope

We have used digital photogrammetry to accurately measure the surface of the KOSMA 3m-telescope's primary mirror. The method uses a large number of optical photographs of the telescope, taken from many different viewing angles to reconstruct the three-dimensional mirror surface. Thin retro-reflective targets are applied to the mirror in the places of interest. With a large format, high resolution metric CCD-camera aseries of pictures is taken under many different viewing angles. A computer program compares the image data and constructs a three dimensional model of the target positions. We used approximately 100-230 targets distributed over the primary mirror and about 50 exposures to reconstruct the KOSMA telescope surface. The measurement accuracy is approximately 10 mum (RMS). The measured mean deviation between the initial surface setup and the ideal parabola was confirmed independently by planetary observations at 345, 492, 660, and 810 GHz. The frequency dependence of the beam efficiencies, derived from scans on Jupiter, follows the Ruze-formula for an initial surface error of 35 +/- 5 microns. This error was reduced by subsequent adjustments using surface maps of the deviations derived from the photogrammetric data sets. New observations of Jupiter to confirm this improvement are pending.