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

Abstract

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 1σ error-bars (68% confidence level) in the fitted values of the axisratio and position-angle of the simulated galaxies.

Gravitation and the Universe from large scale-structures

“…In order to get the highest image quality for the images, an off-axis mirror will be the preferred solution. Such solutions have been investigated and their advantages have been quantified [22,35].…”

Next-generation telescopes with curved focal surface for ultralow surface brightness surveys

“…In order to design the ideal survey for this type of observations, one would choose: a place with the lowest skybackground level; a telescope without obscuration, as any obscuration or spider causes diffraction and displace more power in the point spread function (PSF) towards the wings (Singaravelu & Cabanac 2014;Hugot et al 2014;Muslimov et al 2017a); a fast optical system with a small f /D (where f is the focal length and D is the diameter of the entrance pupil), as for extended objects the imaging speed depends on the focal ratio (not on the aperture size, as for point-source objects); it should have either only refractive elements with anti-reflection coatings (e.g. Abraham & van Dokkum 2014), or superpolished reflective surfaces, and no dust contamination.…”

Fast, wide-field and distortion-free telescope with curved detectors for surveys at ultralow surface brightness

“…[9][10][11]). In particular, off-axis optical designs such as the Three Mirror Anastigmat (TMA) with aspherical [12][13][14], or freeform mirrors surfaces [15,18] yield a very high performance over an extended field of view (FoV). These designs were analysed in detail and compared with other schemes such as the Schmidt and SEAL telescopes [16,17], and led to the conclusion that the modified reflective Schmidt design fulfils all the requirements in terms of optical performance and, in addition, is made up of easily manufacturable optical components.…”