A possible advantage of telescopes with a non-circular pupil

TL;DR

This paper shows that telescopes with elongated, non-circular pupils can achieve better contrast at lower separations from bright stars, improving faint companion detection compared to traditional circular pupils.

Contribution

The study demonstrates through simulations and observations that elongated-pupil telescopes offer superior contrast at small angular separations, with robustness to measurement errors and practical rotation strategies.

Findings

Elongated pupils improve contrast at lower separations.

Simulations and real observations confirm the advantage.

Robustness to point spread function measurement errors.

Abstract

Most telescope designs have a circular-shape aperture. We demonstrate that telescopes with an elongated pupil have better contrast, at lower separations, between a bright central star and a faint companion. We simulate images for an elongated-pupil telescope and for a circular-pupil telescope of equal aperture area and integration time, investigating specifically what is the maximal contrast for finding faint companions around bright stars as a function of angular separation. We show that this design gives better contrast at lower separation from a bright star. This is shown for diffraction-limited (for perfect and imperfect optics) and seeing-limited speckle images, assuming equal aperture area and observing time. We also show the results are robust to errors in measurement of the point spread function. To compensate for the wider point spread function of the short axis, images should be taken at different rotation angles, either by rotating the telescope around the optical axis or by allowing a stationary mirror array to scan different parallactic angles with time. Images taken at different rotation angles are added using the proper image coaddition algorithms developed by Zackay & Ofek. The final image has the same contrast in all angles, rather than in specific areas of diffraction nulls. We obtained speckle observations with a small, ground based elongated-aperture telescope and show the results are consistent with simulations.