Small-grid dithering strategy for improved coronagraphic performance with JWST

Coronagraphic Target Acquisition (TA) is an important factor that contributes to the contrast performance and typically depends on the coronagraph design. In the case of JWST, coronagraphic TAs rely on measuring the centroid of the star's point spread function away from the focal plane mask, and performing a small angle ma- neuver (SAM), to place the star behind the coronagraphic mask. Therefore, the accuracy of the TA is directly limited by the SAM accuracy. Typically JWST coronagraphic observations will include the subtraction of a reference (either a reference star, or a self-reference after a telescope roll). With such differential measurement, the reproducibility of the TA is a very important factor. We propose a novel coronagraphic observation concept whereby the reference PSF is first acquired using a standard TA, followed by coronagraphic observations of a reference star on a small grid of dithered positions. Sub-pixel dithers (5-10 mas each) provide a small reference PSF library that samples the variations in the PSF as a function of position relative to the mask, thus compen- sating for errors in the TA process. This library can be used for PSF subtraction with a variety of algorithms (e.g; LOCI or KLIP algorithms, Lafrenière et al. 2007; Soummer, Pueyo and Larkin 2012). These sub-pixel dithers are executed under closed-loop fine guidance, unlike a standard SAM that executes the maneuver in coarse point mode, which can result in a temporary target offset of 1 arcsecond and would bring the star out from behind the coronagraphic mask. We discuss and evaluate the performance gains from this observation scenario compared to the standard TA both for MIRI coronagraphs.