Opposed Port Alignment System (OPAS): A Commercial Astronomical Telescope Modified for Viewing the Interior of the NIF Target Chamber

The National Ignition Facility (NIF) requires high resolution live images of regions inside the target chamber in order to align diagnostic instruments to fusion targets and to monitor target stability. To view the interior of the target chamber, we modified a commercial 11-inch Schmidt-Cassegrain telescope to develop the Opposed Port Alignment System (OPAS). There are two OPAS systems installed on the target chamber ports directly opposite the diagnostics. This paper describes the optical design, highlighting the two key modifications of the telescope. The first key modification was to reposition the Schmidt corrector plate and to uniquely mount the secondary mirror to a precision translation stage to adjust focus from 5.5 m to infinity. The stage is carefully aligned to ensure that the telescope’s optical axis lies on a straight line during focus adjustments. The second key modification was a custom three element lens that flattens the field, corrects residual aberrations of the Schmidt-Cassegrain and, with a commercial 1:1 relay lens, projects the final image plane onto a large format 50 mega-pixel camera. The OPAS modifications greatly extend the Schmidt-Cassegrain telescope’s field of view, producing nearly diffraction-limited images over a flat field covering ±0.4 degrees. Also discussed in the paper are the alignment procedure and the hardware layout of the telescope.