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Satellite Spots

An in depth discussion of the satellite spots can be found in GPI Observational Calibrations VIII: Characterization and Role of the Satellite Spots. by Wang et al.

Coronagraphic observations pose a unique problem for differential spectrophotometry and astrometry of an exoplanet relative to its primary, which is occulted. To overcome this obstacle, GPI includes a square grid in a pupil plane which acts as a two-dimensional amplitude grating. The grid is superimposed on the the pre-occulter pupil apodizer. Diffraction of starlight from this grating injects first-order diffraction spots into the field of view for a given wavelength.

In each wavelength channel in spectral mode, this creates reference spots that we term satellite spots. In this mode the approximate magnitude difference with the star is 9.3±0.5 magnitudes.

In broadband polarimetry mode, the satellite spots become streaks extending radially outward from the location of the star. In this mode the apprximate magnitude difference with the star is 9.5±0.5 magnitudes.

These satellite spots preserve the information needed to reconstruct the spectrum and location of the occulted star: the diffraction pattern is centered on the true stellar position and is imprinted with an attenuated version of the stellar spectrum.

The usage of the satellite spots is in general transparent to the user and requires in most cases no adjustment or affect the feasibility of the observations, with the exception of the case of an extremely bright nebula/background emission surrounding the star. Please contact the GPI instrument scientist in this case before submitting a proposal for a feasibility check.