Near-focus active optics: An inexpensive method to improve millimeter-wavelength radio telescopes

The application of active and adaptive optics allows the construction of large diameter lightweight optical telescopes for observations below the seeing limit of the atmosphere. Active wavefront correction in a Cassegrain/Gregory-type radio telescope can be made with a deformable main reflector or deformable subreflector. Here we suggest the possibility of correcting spatially large-scale wavefront deformations with a small size corrector located near the focus of the telescope. Using representative examples of the Institut de Radioastronomie Millimetrique (IRAM, Spain) 30-meter diameter, millimeter-wavelength telescope, we calculate the improvement expected from the correction of (1) the systematic component in homology deformations, (2) the large-scale residual errors of a reflector adjustment, and (3) the beam degradation experienced in observations with a wobbling subreflector. The improvement in surface/wavefront precision obtained from piston correction with a corrector of some 50 elements is of the order of 30-40%. We investigate in particular the systematic component of homology deformations, their representation by low-order Zernike polynomials, and their elimination by near-focus correction. We study in detail the homology deformations of the IRAM 30-m reflector.