A Possible Advantage of Telescopes with a Noncircular Pupil

Most telescope designs have a circular aperture. We quantify the advantages that telescopes with an elongated pupil have over circular-pupil telescopes in terms of contrast at small 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, and we specifically investigate the maximal contrast when 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 an equal aperture area and observing time. We also show that 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 at all angles, rather than in specific areas of diffraction nulls. We obtained speckle observations with a small ground-based elongated-aperture telescope and show that the results are consistent with simulations.