Space Telescope Science Institute  Bubbles in the Visible Polarizers  6.2.1 Coronagraph Design

6.2 Coronagraphy


The ACS High Resolution Camera has a user-selectable coronagraphic mode for the imaging of faint objects (circumstellar disks, substellar companions) near bright point sources (stars or luminous quasar nuclei). The coronagraph suppresses the diffraction spikes and rings of the central source to below the level of the scattered light, most of which is caused by surface errors in the HST optics. The coronagraph was added after ACS construction began, at which point it was impossible to insert it into the aberration-corrected beam. Instead, the system is used in the aberrated beam, which is corrected after the coronagraph. While not as efficient as a corrected-beam coronagraph, especially for imaging close to the central source, it does provide a significant improvement to the high-contrast imaging capabilities of HST. Care must be taken, however, to design an observation plan that properly optimizes the coronagraph's capabilities and accounts for its limitations.

Figure 6.2: Schematic layout of the ACS HRC coronagraph.

 
The upper left inset shows a schematic of the coronagraph mechanism that can be flipped in-and-out of the HRC optical path.
 

Bubbles in the Visible Polarizers  6.2.1 Coronagraph Design
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