Physics – Optics
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11415s&link_type=abstract
HST Proposal ID #11415. Cycle 17
Physics
Optics
Scientific paper
The NICMOS coronagraph reduces the levels of circumstellar background light by controlling diffraction and limiting instrumental scatter. In an ideal system, both components would be minimized simultaneously when a target is centrally located in perfectly aligned image and pupil plane masks. The NICMOS coronagraphic system, however, is both imperfect and subject to change with non-elastic dewar deformations and necessarily planned optical re-alignments {expected following the SM4 power cycling of the NICMOS Cooling System} that are designed to minimize wavefront errors, not instrumental scatter and image contrast. The goal for coronagraphic imaging is to maximize the wavelength-dependent target-to-background image contrast. In the NICMOS system, this is achieved by intentionally decentering the target placement from the geometrical center of the coronagraphic optics by an amount and direction that must be determined by empirical exploration and measurement. The goal of this test is to fully map and quantify the spatially variable coronagraphic image contrast dependence on target decentering in the two most commonly used coronagraphic passbands {1.1 and 1.6 microns}. With that, the "sweet spot" for coronagraphic target placement {contrast optimization} will be {re}-determined, and the target acquisition flight S/W updated {unless the system proves stable across the NCS shut-down} to autonomously incorporate an optimal position offset from mask center when performing "Mode 2" coronagraphic target recentering slews. This proposal corresponds to SMOV activity NIC-12.;
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