Diffraction-limited imaging on large telescopes

In this paper we describe an instrument to obtain diffraction limited images on a large telescope using aperture masking. Detailed images of giant stars have been routinely obtained on the 4.2-m William Herschel Telescope (WHT) in La Palma with this technique. When using this method one of the major causes of data loss is the difficulty in detecting the secondary mirror support spiders crossing, and thus obscuring part of the aperture mask. To overcome this problem a system for continuously monitoring the front face of the aperture mask has been devised. Exposure times have been significantly reduced by placing the image in a corner of the CCD chip, the size of which is determined by the seeing, and by compressing the image to a single row of pixels. In order to perform this technique the fringes must be accurately aligned with the columns of the CCD chip. To eliminate the extra losses in throughput due to an optical image rotator the CCD camera, rather than the image, is rotated using a motorized mount. The aperture mask can be rotated independently to various position angles with respect to the object so that full UV coverage can be obtained.