Design and development of atmospheric dispersion corrector and fiber positioner for TMT-HROS

Abstract

Earth’s atmosphere is one of the significant factors affecting the quality of ground-based observations. For off-zenith observations with ground-based astronomical telescopes, the image size on the telescope's focal plane will increase due to atmospheric dispersion. In fiber-fed spectrographs, small-sized fiber is essential to achieve better spatial resolution. But this dispersed image causes loss of throughput at the fiber feed. Atmospheric dispersion correction is necessary to get the best throughput and resolution. High-resolution optical spectrograph (HROS) is one of the second light instruments on the TMT (Thirty-meter telescope) Nasmyth platform. One of the observing modes of HROS is a multi-object spectroscopic mode with a resolution of 20,000. Here optical fibers are used to feed light to the spectrograph slit. To place fiber on the focal plane of TMT, we need a fiber positioner, which has to move radially and tangentially along the focal plane and also has to revolve around the image plane to place fiber on any location of the object. In this work, I will present the optical design for the multi-object spectroscopic mode of the HROS, including atmospheric dispersion corrector, collimator, focusing camera, and dichroic. I will also be presenting the mechanical design of the fiber positioner.