Demonstration of vortex coronagraph concepts for on-axis telescopes on the Palomar Stellar Double Coronagraph

Abstract

Here we present preliminary results of the integration of two recently proposed vortex coronagraph (VC) concepts for on-axis telescopes on the Stellar Double Coronagraph (SDC) bench behind PALM-3000, the extreme adaptive optics system of the 200-inch Hale telescope of Palomar observatory. The multi-stage vortex coronagraph (MSVC) uses the ability of the vortex to move light in and out of apertures through multiple VC in series to restore the nominal attenuation capability of the charge 2 vortex regardless of the aperture obscurations. The ring-apodized vortex coronagraph (RAVC) is a one-stage apodizer exploiting the VC Lyot-plane amplitude distribution in order to perfectly null the diffraction from any central obscuration size, and for any vortex topological charge. The RAVC is thus a simple concept that makes the VC immune to diffraction effects of the secondary mirror. It combines a vortex phase mask in the image plane with a single pupil-based amplitude ring apodizer, tailor-made to exploit the unique convolution properties of the VC at the Lyot-stop plane. The prototype apodizer uses the same microdot technology that was used to manufacture the apodized pupil Lyot coronagraph (APLC) equipping SPHERE, GPI and P1640.

Characterization of microdot apodizers for imaging exoplanets with next-generation space telescopes

“…Within the range of fill factors from 0.4 to 0.7, the phase delay appears to be roughly linear for 5-10 µm dots with an average phase delay of ∼5.4 nm per 10% change in reflectance. Similar phase effects have been previously identified in microdot apodizers by Mawet et al (2014). 30 Regions where the fill factor is <0.4 were not well characterized by the interferometer because the reflected signal becomes too faint.…”

“…Similar phase effects have been previously identified in microdot apodizers by Mawet et al (2014). 30 Regions where the fill factor is <0.4 were not well characterized by the interferometer because the reflected signal becomes too faint. Residual low order surface errors dominated in the regions with 3 µm dots.…”

“…Experiments performed here expand on previous efforts to empirically characterize microdot apodizers in transmission for ground-based infrared imaging instruments [26][27][28][29][30] by including interferometric and reflectance measurements to validate models of the reflected wavefront and inform the design of coronagraphs for future space telescopes. Microdot test patterns based on square microdots in chrome deposited on anti-reflection (AR) coated glass were designed, manufactured, and characterized.…”

Characterization of microdot apodizers for imaging exoplanets with next-generation space telescopes

“…Similar phase effects have been previously identified in microdot apodizers by Mawet et al (2014). 30 Regions where the fill factor is <0.4 were not well characterized by the interferometer because the reflected signal becomes too faint. Residual low order surface errors dominated in the regions with 3 µm dots.…”

High-contrast imaging of tight resolved binaries with two vector vortex coronagraphs in cascade with the Palomar SDC instrument

“…19 Conceptually, the idea was to take advantage of the light-folding properties of vortex coronagraphs, using two of them onaxis in series to fold the diffracted light from the secondary back inside the central obscuration region of the pupil, where it can be masked most efficiently with no penalty on IWA and throughput, as successfully demonstrated on-sky. 17,20 To this purpose, the SDC instrument integrates two VVCs in cascade, based on liquid crystal Ks-band waveplates, 21 as shown of the optical layout cartoon of Figure 1. 18 SDC also provides two intermediate pupil planes (one transmissive and one reflective, see Fig.1) to insert Lyot stops or apodizing optics.…”