Recent Results from the MARTINI III/WHIRCAM Infra-red Adaptive Optics System

Abstract

The MARTINI-III system is an infra-red adaptive optics system designed as a semi-common user instrument for use at the Ground-based High Resolution Imaging Laboratory (GHRIL) on the William Herschel Telescope (WHT). The optical layout of the system is shown in figure 1. Light from the Nasmyth focus of the WHT is incident on a six element adaptive mirror and is reflected back onto a toroidal mirror which refocuses the light at an off-axis point. Just before this re-imaged focus the light is split by a dichroic beamsplitter and the infra-red light reflected into the infra-red camera (WHIRCAM) arm. The visible light is passed to the wavefront sensing arm where it passes through a selectable sub-aperture mask and is imaged onto the wavefront sensor (an Astromed CCD15-11 camera) by a split lens assembly. The elements of this split lens are adjustable and these are used to align the images formed by each sub-aperture in a row on a 24x4 pixel region in the corner of the wavefront sensor’s CCD chip, each spot being centred on a 4x4 pixel square. On readout the 4x4 pixels are on-chip binned into a 2x2 quad-cell, the output of which is used to calculate the X and Y wavefront slopes. The system is designed to work in two modes, one an ‘unco-phased’ mode where the mirror segments are just controlled to remove the individual wavefront tip-tilt across each segment but no attempt is made to piston the six mirrors. The other a ‘co-phased’ mode where the segment slopes are used to reconstruct the piston values of each wavefront segment. The real-time reconstruction and control is performed with a Motorola 68020 microprocessor. Pixel scales on WHIRCAM of 0.05, 0.08 and 0.24 arcseconds can be selected giving fields of 12.8, 20.5 and 61.5 arcseconds respectively. A guide star of R-band magnitude 13th or brighter is required to be centred within this field.