Below we summarize differences between the positions of (mostly compact/point-) sources derived from PSF-fit photometry and then independently derived using flux-weighted centroids in the x-y system of single exposure frames. The goal is to explore the accuracy of the PSF positioning for profile-fitting and prior band-to-band offsets for use in astrometric calibration across all bands.
Supporting plots are below. The following statistics represent trimmed-medians with uncertainties derived from half the 84th - 16th percentile range per axis divided by √N.
W1: 1609914 sources Dx = 0.00712 +/- 0.00044 arcsec Dy = 0.00354 +/- 0.00045 arcsec W2: 877749 sources Dx = -0.00817 +/- 0.00064 arcsec Dy = -0.00625 +/- 0.00061 arcsec W3: 949945 sources Dx = 0.08649 +/- 0.00133 arcsec Dy = -0.02010 +/- 0.00135 arcsec W4: 46899 sources Dx = 0.04984 +/- 0.01361 arcsec Dy = -0.11834 +/- 0.0137 arcsec
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| Figure 1a - 2D density plot of W1 'WPHot - flux weighted centroid positions' [click to enlarge] | Figure 1b - 2D density plot of W2 'WPHot - flux weighted centroid positions' [click to enlarge] |
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| Figure 1c - 2D density plot of W3 'WPHot - flux weighted centroid positions' [click to enlarge] | Figure 1d - 2D density plot of W4 'WPHot - flux weighted centroid positions' [click to enlarge] |
[Last update: 2011 April 6]