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April 22, 2018

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Recently on HiCAT, we obtained a very first dark hole with a broadband light source, combining a segmented aperture, a coronagraph (Apodized Pupil Lyot Coronagraph or APLC), and wavefront control (the Speckle Nulling method, now running on demand in any configuration). 

Once the full APLC was set, with a segmented aperture (segment gap, central obscuration, and spiders), the experiment had three steps:

  • Conducted the Speckle Nulling procedure with our monochromatic light source (638nm) to create a DM command for a dark zone.
  • Switched the light source to broadband, loaded the dark zone command, and computed the contrast.
  • Switched to another tunable light source, took data at a range of wavelengths from 600nm to 680nm, and computed the contrast at each wavelength.

fig5_triple_frame_broadband_darkzone_v5

In the final configuration (see image above), the broadband contrast is 6.3*10^-6 in a 6% bandpass, while the monochromatic contrast is 1.7*10^-6. Please note the superposition of the "natural" circular dark zone of the APLC and of the deeper dark zone (nonsymmetrical) from the wavefront control. 

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