High-order adaptive optical system for Big Bear Solar Observatory

A high-order Adaptive Optical (AO) system for the 65 cm vacuum telescope of the Big Bear Solar Observatory (BBSO) is presented. The Coudé-exit of the telescope has been modified to accommodate the AO system and two imaging magnetograph systems for visible-light and near infrared (NIR) observations. A small elliptical tip/tilt mirror directs the light into an optical laboratory on the observatory's 2^nd floor just below the observing floor. A deformable mirror (DM) with 77 mm diameter is located on an optical table where it serves two wave-front sensors (WFS), a correlation tracker (CT) and Shack-Hartman (SH) sensor for the high-order AO system, and the scientific channels with the imaging magnetographs. The two-axis tip/tilt platform has a resonance frequency around 3.3 kHz and tilt range of about 2 mrad, which corresponds to about 25″ in the sky. Based on 32 × 32 pixel images, the CT detects image displacements between a reference frame and real-time frames at a rate of 2 kHz. High-order wave-front aberrations are detected in the SH WFS channel from slope measurements derived from 76 sub-apertures, which are recorded with 1,280 × 1,024 pixel Complex Metal Oxide Semiconductor (CMOS) camera manufactured by Photobit camera. In the 4 × 4 pixel binning mode, the data acquisition rate of the CMOS device is more than 2 kHz. Both visible-light and NIR imaging magnetographs use Fabry-Pérot etalons in telecentric configurations for two-dimensional spectro-polarimetry. The optical design of the AO system allows using small aperture prefilters, such as interference or Lyot filters, and 70 mm diameter Fabry-Pérot etalons covering a field-of-view (FOV) of about 180″ × 180″.