Individual Star Fitting and Removal

Typically 1500 SMEI data frames from each camera are combined for a single orbit into a composite surface-brightness skymap. Individual stars transiting near the center of a camera's field of view (FOV) have a characteristic fish-shaped point-spread function (PSF) which varies considerably in size as the star moves across the FOV narrow dimension. A composite skymap standard-star PSF for each camera was constructed using a dozen isolated bright stars. The accompanying figure illustrates the resulting PSF for each of the three cameras.

At an angle θ from the center of the FOV, the appropriate PSF is narrowed in the horizontal dimension here by a factor cosine(θ). In order to minimize compromising the photometric specification for SMEI (0.1% differential photometry per square degree of sky), individual point sources are removed from the composite skymaps. Details of the fitting procedure are found in Hick, Buffington and Jackson (2007). This fitting and removal procedure not only generates considerably cleaner skymaps, but also generates precision photometric timeseries for the 5593 cataloged objects that have been removed.