NICMOS Coronagraphy

Abstract

ABSTRACTThe NICMOS coronagraphic capability is a unique resource. It has been used to search for and/or confirm the presence of planetary or brown dwarf companions around nearby stars, to probe the structure of circumstellar disks, and to study the host environment of quasars with damped Lyα systems. These and other programs are expected to continue with NICMOS, using the recently installed NICMOS cooling system. In this paper we present an operational overview of NICMOS coronagraphy from its inception on 1998 February 6 through 1998 December 18. During this time interval, which corresponds to H ubble Space Telescope (HST) cycles 7 and 7N, coronagraphic observations of 329 targets for 22 programs were obtained. The flight software was enhanced to locate the position of the hole as well as the target during onboard acquisitions. Coronagraphic images are affected by changes in the point‐spread function (PSF) due to thermally induced HST focus changes, NICMOS cold‐mask movement, and light scatter around the coronagraphic hole. Proper PSF subtraction requires a well‐planned observing strategy that is appropriate for the target, and a matched PSF target. For example, for point sources, observations with the same filter taken back‐to‐back in the same orbit with a roll of the spacecraft between observations yields good results. For circumstellar disks, contemporaneous observations of a matched PSF target are required.