A 20 m wide-field diffraction-limited telescope

A 20 m space telescope is described with an unvignetted 1° field of view—a hundred times larger in area than fields of existing space telescopes. Its diffraction-limited images are a hundred times sharper than from wide-field ground-based telescopes and extend over much if not all the field, 40 arcmin diameter at 500 nm wavelength, for example. The optical system yielding a 1°, 1.36 m diameter image at f/3.9 has relatively small central obscuration, 9% by area on axis, and is fully baffled. Several carousel-mounted instruments can each access directly the full image. The initial instrument complement includes a 400 gigapixel silicon imager with 2 µm pixels (0.005 arcsec), and a 60 gigapixel HgCdTe imager with 5 µm pixels (0.012 arcsec). A multi-object spectrograph with 10 000 fibres will allow spectroscopy with 0.02 arcsec resolution. Direct imaging and spectroscopy of exoplanets can take advantage of the un-aberrated, on-axis image (5 nm RMS wavefront error). While this telescope could be built for operation in free space, a site accessible to a human outpost at the Moon's south pole would be advantageous, for assembly and repairs. The lunar site would allow also for the installation of new instruments to keep up with evolving scientific priorities and advancing technology. Cooling to less than 100E K would be achieved with a surrounding cylindrical thermal shield. This article is part of a discussion meeting issue ‘Astronomy from the Moon: the next decades’.

The Single-mode Complex Amplitude Refinement (SCAR) coronagraph

We present the monochromatic lab verification of the newly developed SCAR coronagraph that combines a phase plate (PP) in the pupil with a microlens-fed single-mode fiber array in the focal plane. The two SCAR designs that have been measured, create respectively a 360 degree and 180 degree dark region from 0.8–2.4λ∕D around the star. The 360 SCAR has been designed for a clear aperture and the 180 SCAR has been designed for a realistic aperture with central obscuration and spiders. The 360 SCAR creates a measured stellar null of 2–3 × 10−4, and the 180 SCAR reaches a null of 1 × 10−4. Their monochromatic contrast is maintained within a range of ±0.16λ∕D peak-to-valley tip-tilt, which shows the robustness against tip-tilt errors. The small inner working angle and tip-tilt stability makes the SCAR coronagraph a very promising technique for an upgrade of current high-contrast instruments to characterize and detect exoplanets in the solar neighborhood.

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Second-generation exoplanet imagers using extreme adaptive optics (ExAO) and coronagraphy have demonstrated their great potential for studying close circumstellar environments and for detecting new companions and helping to understand their physical properties. However, at very small angular separation, their performance in contrast is limited by several factors: diffraction by the complex telescope pupil (central obscuration and spiders) not perfectly canceled by the coronagraph, residual dynamic wavefront errors, chromatic wavefront errors, and wavefront errors resulting from noncommon path aberrations (NCPAs). These latter are differential aberrations between the visible wavefront sensing path of the ExAO system and the near-infrared science path in which the coronagraph is located. In a previous work, we demonstrated the use of a Zernike wavefront sensor called ZELDA for sensing NCPAs in the VLT/SPHERE exoplanet imager and their compensation with the high-order deformable mirror of the instrument. These early tests on the internal light source led to encouraging results for the attenuation of the quasi-static speckles at very small separation. In the present work, we move to the next step with the on-sky validation of NCPA compensation with ZELDA. With an improved procedure for the compensation of NCPAs, we start by reproducing previous results on the internal source. We show that the amount of aberration integrated between 1 and 15 cycles/pupil (c/p) is decreased by a factor of approximately five, which translates into a gain in raw contrast of between 2 and 3 at separations below 300 mas. On sky, we demonstrate that NCPA compensation works in closed loop, leading to an attenuation of the amount of aberration by a factor of approximately two. However, we identify a loss of sensitivity for the sensor that is only partly explained by the difference in Strehl ratio between the internal and on-sky measurements. Our simulations show that the impact of ExAO residuals on ZELDA measurements is negligible for integration times beyond a few tenths of a second. Coronagraphic imaging on sky is improved in raw contrast by a factor of 2.5 at most in the ExAO-corrected region. We use coronagraphic image reconstruction based on a detailed model of the instrument to demonstrate that both internal and on-sky raw contrasts can be precisely explained, and we establish that the observed performance after NCPA compensation is no longer limited by an improper compensation for aberration but by the current apodized-pupil Lyot coronagraph design. We finally conclude that a coronagraph upgrade combined to a proper NCPA compensation scheme could easily bring a gain in raw contrast of a factor of two to three below 200 mas.

Study of effect of non-central obscuration to obscured circular aperture on third order spherical aberration

In this research we have been studied the 3rd order spherical aberration for an optical system consisted of obscured circular aperture with non central circular obscuration through the calculation of point spread function (P.S.F) in presence of the obscuration in the center and comparing the obtained results with that results of moving obscuration far away from the center, where the results showed significant improvement for(P.S.F) value. The study was done of different obscurities ratios in addition to the different 3rd order spherical aberration values (W40=0.25 ,0.5 ,0.75 ,1 ).