  Telescopes with unobstructed pupil are known to deliver clean point spread
function (PSF) to their focal plane, in contrast to traditional telescopes with
obstructed pupil. Recent progress in the manufacturing 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 same precision, if the PSF is
reconstructed within 12 arc-minutes of the source. We also demonstrate that,
the unobstructed design delivers desirable precision even if the PSF is
reconstructed 50 arc-minutes away from the source. Therefore the PSF of
unobstructed design is uniform over a wider field-of-view compared to an
obstructed design. The image quality is given by the 1$\sigma$ error-bars (68%
confidence level) in the fitted values of the axis-ratio and position-angle of
the simulated galaxies.
