Analysis of Shane telescope aberration before and after collimation

摘要

The Shane 3 meter telescope was recollimated in August 1998 to remove aberrations that were perceived to affect image quality in good seeing conditions. The Lick Adaptive Optics system is capable, indirectly, of measuring the static aberrations introduced by the telescope primary and secondary mirror pair. Since A0 runs were scheduled both before and after the collimation, this allowed a comparison of the pre and post collimation telescope aberrations. The absolute calibration of the wavefront measurement is complicated by the indirect measurement technique, which uses the shape of the deformable mirror in its closed-loop corrected position as an indicator of telescope wavefront. The shape of the deformable mirror is not measured directly, but is inferred from the voltages applied to its piezoelectric actuators. The actuators are known to have temperature-dependent hysteresis and is thus a source of error in this technique. In November, an interferometer was added to the A0 system which will allow more direct measurements of corrected wavefront in future tests. Data from six measurements in July (pre-collimation) and nine measurements in September and November (post-collimation) were analyzed. The resulting phase maps show a slight difference in the wavefront with some improvement in the coma and spherical zernike terms but essentially no change (except for rotation) in astigmatism and some degradation in tricoma.