10⁻⁷ contrast ratio at 4.5Lambda/D: New results obtained in laboratory experiments using nano-fabricated coronagraph and multi-Gaussian shaped pupil masks

We present here new experimental results on high contrast imaging of 10^{-7} at 4.5Lambda/D (Lambda = 0.820 microns) by combining a circular focal plane mask (coronagraph) of 2.5Lambda/D diameter and a multi-Gaussian pupil plane mask. Both the masks were fabricated on very high surface quality (Lambda/30) BK7 optical substrates using nano-fabrication techniques of photolithography and metal lift-off. This process ensured that the shaped masks have a useable edge roughness better than Lambda/4 (rms error better than 0.2 microns), a specification that is necessary to realize the predicted theoretical limits of any mask design. Though a theoretical model predicts a contrast level of 10^{-12}, the background noise of the observed images was speckle dominated which reduced the contrast level to 4x10^{-7} at 4.5Lambda/D. The optical setup was built on the University of Illinois Seeing Improvement System (UnISIS) optics table which is at the Coude focus of the 2.5-m telescope of the Mt. Wilson Observatory. We used a 0.820 micron laser source coupled with a 5 micron single-mode fiber to simulate an artificial star on the optical test bench of UnISIS.