Optical designs of the LGS WFS system for GMT-LTAO

Abstract

The Laser Tomographic Adaptive Optics system for Giant Magellan Telescope (GMT) uses a single conjugated deformable mirror, the segmented Adaptive Secondary Mirror (ASM), to correct atmospheric wavefront aberrations with the help of a constellation of six laser beacons equally spaced on the sky. We will present different approaches for the design of the Laser Guide Star (LGS) Wave Front Sensor (WFS) system for GMT to cover all sodium emission altitudes and telescope elevations, from 80 km to 200 km range and how the preliminary design was derived from these approaches. The designed LGS WFS system includes a defocus-compensation mechanism working with a simple zooming optics to achieve the pupil image with constant pupil size, nearly constant wavefront correction, as well as pupil distortion correction. Either a trombone-mirror structure or a direct LGS-WFS translation is used for the defocus compensation, when conjugating the LGS altitudes in the sky. In the designs, a zooming collimator images the ASM in the GMT at the exit pupil of the LGS WFS system, where the designed lenslet-array is tailored for the selected CCD format for the required plate scale on the sky. Additionally, we have proposed a novel and simple solution for pupil-image segmentation when working with smaller CCD arrays. This novel solution consists of a single multi-aperture blaze grating for pupil segmentation in the system.