Obstructed Telescopes Versus Unobstructed Telescopes for Wide Field Survey-A Quantitative Analysis

Telescopes with unobstructed pupils are known to deliver clean point-spread function (PSF) to their focal plane, in contrast to traditional telescopes with obstructed pupils. Recent progress in the manufacturing of aspheric surfaces and mounting accuracy favors unobstructed telescopes over obstructed telescopes for science cases that demand stable and clean PSF over the entire field of view. In this paper we compare the image quality of an unobstructed three-mirror anastigmat (TMA) design with that of an obstructed TMA. Both the designs have the same primary mirror, effective focal length, field of view, and detector characteristics. We demonstrate, using simulated images of faint elliptical galaxies imaged through the two designs, that both the designs can measure morphological parameters with the same precision, if the PSF is reconstructed within 12' of the source. We also demonstrate that the unobstructed design delivers desirable precision even if the PSF is reconstructed 50' away from the source. Therefore, the PSF of unobstructed design is uniform over a wider field of view compared to an obstructed design. The precision of the designs is given by the la error bars (68% confidence level) in the fitted values of the axis ratio and position angle of the simulated galaxies.