Phase-Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) on-sky demonstration with MagAO-X

Abstract

Advancing the technological development of small inner working angle (IWA) coronagraphs is essential to enabling high-contrast imaging of temperate exoplanets with future extremely large telescopes. The PIAACMC has been shown to closely approach the theoretical limit for coronagraphic throughput but its performance has not been fully characterised on-sky. This study serves as the first on-sky characterisation of contrast and IWA performance of the PIAACMC and its first technological demonstration at sub-micron wavelengths. We designed and manufactured phase-shifting focal plane masks optimised for two cases, a narrowband 875 filter (875nm, 3% band) and a broadband z' filter (908nm, 14% band). We tested the coronagraphs both with an internal source and on-sky using MagAOX, the extreme adaptive optics instrument for the Magellan Clay 6.5 m telescope at Las Campanas Observatory. We show good recovery of the off-axis light's PSF shape within 92% and 97% depending on the separation when aligning the inverse set of PIAA lenses. We demonstrate sub-lambda/D IWAs of about 0.74 lambda/D in 875 and 0.76 lambda/D in z'. We reach average raw contrasts within 1 and 5 lambda/D with the internal source of about 1.6e-3 in 875 and 1.3e-3 in z'. These are mainly limited by the focal plane mask manufacturing errors, jitter, and residual quasi-static speckles in MagAO-X. We also show on-sky average raw contrasts within 1 and 5 lambda/D of about 1.4e-2 in 875 and 7.8e-3 in z'. These are likely limited by wavefront control, low-order aberrations, and poor observing conditions. Future work will improve the design and manufacturing processes of the focal plane masks to improve robustness and reach deeper contrast, as well as integrate focal plane wavefront control for non-common path aberrations correction.

Figures (13)

• Figure 1: Schematic representation of MagAO-X's coronagraphic arm. The AO-corrected beam is sent to the NCPC deformable mirror; then, it passes through a filter wheel containing different aperture stops; a stage with the forward PIAA lenses set can be moved in or out of the beam through motorised actuators; light is then focused on a filter wheel containing different focal plane masks; another filter wheel follows with different Lyot stops; any light reflected from the focal and pupil planes is sent to separate low-order wavefront sensing cameras; another motorised stage containing the inverse PIAA lenses set can be moved in and out of the beam; finally, light is sent to the science cameras.
• Figure 2: PIAACMC optical layout. The top figures show the simulated components designed for MagAO-X, and the bottom figures show the simulated pupil or focal plane images at different positions in the system, for an on-axis source. Top figures from left to right: (1) MagAO-X's binary aperture stop used for coronagraphic observations ('bump mask'); (2) forward PIAA0, the first aspheric PIAA lens of the forward set; (3) forward PIAA1, the second aspheric PIAA lens of the forward set; (4) phase-shifting focal plane mask, a transmissive optical component to suppress on-axis light; (5) MagAO-X's Lyot stop for PIAA observations, a binary mask to block the light diffracted by the focal plane mask, modelled on the apodised pupil shape; (6) inverse PIAA1, the first aspheric PIAA lens of the inverse set. It is the same as foward PIAA1 but flipped; (7) inverse PIAA0, the second aspheric PIAA lens of the inverse set. It is the same as forward PIAA0 but flipped. Bottom figures from left to right: (A) light distribution at the MagAO-X aperture; (B) light distribution after the forward PIAA lenses set; (C) the MagAO-X point spread function (PSF) on the focal plane; (D) PSF after the focal plane mask. It doesn't change in intensity because the focal plane mask is transmissive and only acts on the phase of the light; (E) light in the pupil plane that is diffracted by the focal plane mask; (F) residual light in the pupil plane after the Lyot stop; (G) residual light in the pupil plane after de-apodisation by the inverse PIAA lenses set; (H) residual light in the science plane.
• Figure 3: Surface heights of the PIAA lenses designed for MagAO-X by warren_thesis. The dark purple line (PIAA0) is the height of the first lens of the forward PIAA set, and the light pink line (PIAA1) is the height of the second lens of the forward PIAA set. The inverse PIAA lenses set has the same surface heights with the lenses inverted.
• Figure 4: Optimised FPMs with ten concentric rings, for the narrowband 875 filter on the left, and for the broadband z' filter on the right. Note that the colourbars have different scales, and the X and Y axes are dependent on central wavelength of the filter. The axes are shown in both $\lambda$/D and physical units. Both masks have a radius of 0.75 $\lambda$/D, corresponding to 45.3 $\mu$m for the 875 filter and 47 $\mu$m for the z' filter.
• Figure 5: Mean radial profile of optimised and manufactured FPMs, for the narrowband 875 filter on the left, and for the broadband z' filter on the right. In both panels, the dashed black line (design) shows the mask design's radial profile, and the solid coloured line (manufacturing) shows the measured radial profile of the manufactured mask.