CoCo: An experiment in infrared coronagraphy at the IRTF

Imaging planets, brown dwarfs and disks around nearby stars is a challenging endeavor due to the required scene contrast. Success requires imaging down to m = 20-25 within arcseconds of stars that are 4th-6th magnitude. Light scattered and diffracted from a variety of sources increases the background flux in the area of interest by orders of magnitude masking the target objects. As first shown by M. B. Lyot in 1939 masks can be placed in the focal pane and pupil planes of a camera to occult the bright central source making it possible to image the faint extensions around it. CoCo is an experiment in using a coronagraphic camera, for IR observations, on a large telescope in an effort to understand how a coronagraph can help and how to properly design one of the new generation of large telescopes. Recent result with CoCo show a factor of 5-10 reduction in background levels in the area from 2-7 arcseconds from the central object. This paper will describe those result and summarize what has been learned towards building coronagraphic cameras for today's large telescopes.