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• W74484507 abstract "Small time-and-initialization-dependent instabilities in the ACS coronagraph have led to revisions of the coronagraph commanding procedures and to the suggested methods for optimizing coronagraphic observations. An example optimized sequence is given. Due to the instabilities, the flat fields currently used by the pipeline are not accurate and should not be used; new flats will be generated. Also, clarification is provided on the use of the coronagraphic surface brightness profiles in the Cycle 12 ACS Instrument Handbook. Introduction The ACS coronagraph includes a set of occulting spots and a pupil mask for the suppression of diffracted light from a bright central source, such as a star or QSO. Subtraction of the residual light from the central source using the image of another star (or the same star at another roll angle, in the case of point source companion searches) is required to allow faint, nearby sources (companions, circumstellar disks) to be seen. How well the central source and reference star PSFs match depends on the stability of the system and the alignment of the sources behind the occulting spot. Motion of the coronagraphic occulting spots Monitoring of the ACS coronagraphic occulting spot positions indicates that the spots are currently drifting by about 0.8 pixels (~20 milliarcseconds) each month. Also, every time the spots are inserted into the beam, they can shift by up to 12 mas. Centroiding and slew errors may also introduce star-to-spot offsets of up to ~12 mas. Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. Instrument Science Report ACS 2002-11 All of these offsets may seem small, given that the smaller spot is 1.8 in diameter, but the distribution of residual light from the occulted central source is somewhat dependent on the location of the source relative to the spot. For instance, say a star is observed and happens to end up exactly at the center of the spot. Then another star is observed whose image will be used for PSF subtraction, but it ends up 10 mas from the spot center due to the errors identified above. When the second star is subtracted from the first, there will be visible residuals due to the differences in the PSFs caused by the star-to-spot position differences (as well as from any color and focus mismatches). Revised coronagraphic mode commanding To reduce these position-dependent errors, the ACS coronagraphic commanding sequence has been altered (which is invisible to observers), and an optimized observation strategy has been developed. While observers are not required to use this new strategy, they are strongly encouraged to do so. All coronagraphic observations begin with the acquisition and centroiding of the star (not to be confused with guide star acquisition). Before December 2002, every acquisition occurred without the spots in the beam. After the star was acquired, the spots were inserted and the star moved to the official aperture location of the spot (whether that is precisely where the spot actually was depended on how up-to-date the aperture locations were, due to spot drift). The spots were moved out of the beam when the next non-coronagraphic observation was taken or another coronagraphic acquisition was done (such as after rolling the telescope around the star). So, the spots could end up in a slightly different position when a new target (say, a reference PSF star) was observed, even if it was in an adjacent orbit. Acquisitions are now done with the spots in the beam (the acquisition box has been shifted to avoid the spots). The spots are only moved out when the next non-coronagraphic image is taken. So, when one observes multiple targets in adjacent orbits with the coronagraph, and does not take any direct images, the spots will stay in place, removing one source of star-to-spot position error. Of course, this means that only that particular sequence of target observations are internally consistent with respect to the spot-insertion offset. Also, the centering box size used during acquisition has been expanded to 5 by 5 pixels, which provides slightly more accurate positions." @default.
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• W74484507 date "2002-12-01" @default.
• W74484507 modified "2023-09-27" @default.
• W74484507 title "ACS Coronagraph Update for Cycle 12 Proposers" @default.
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