Analysis - DAOPHOT was used to perform the photometry. A large threshhold (20 sigma) was used to generate the object list. Then apertures with radii ranging from 1.2 to 35 pixels were used to measure the stars. Sky annulus radii were set to 35-45 pixels. Only stars brighter than 14.0 (DAOPHOT instrumental, 25.0 zeropoint) were analyzed. The plots (below) show, for each chip, the change in magnitude going outward from one aperture radius to the next larger. Data for the individual stars are plotted as diamonds. Occasional outliers are due to neighboring stars entering the measurement aperture. The sigma clipped mean for each aperture radius is connected by the solid line.
Conclusions - These plots demonstrate that for each chip, delta MAG is below 0.01 mag by the time we get out to a 25px radius. This could change for worse seeing conditions and should be checked.
The x-axis is the measurement aperture radius in pixels. The y-axis is the change in magnitude between the aperture at the x position and the next smaller aperture (indicated at the bottom by the short vertical lines). The dot-dashed horizontal line is at the 0.01 delta MAG level. NOTE: Chip 3 was missing from this data set.
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