IRAC Instrument Handbook - IRSA - California Institute of Technology

Figure C.5: The 25 PRF positions on an IRAC BCD.
C.3.8 Pixel Phase
Point Source Fitting IRAC Images
with a PRF
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IRAC Instrument Handbook
We define pixel phase as the offset between the centroid of a stellar image and the center of the pixel in
which that centroid lies. For example, an object whose centroid has pixel coordinates 128.23, 127.85 has
a pixel phase of Dx,Dy=(-0.27, 0.35). The pixel phase effect in aperture photometry in Chapter 4 is
characterized in terms of the radial pixel phase p =
we have adopted the following technique:
D + D ). To shift a PRF to a given pixel phase
i) Magnify the PRFs by a large factor, e.g. 20, using linear interpolation (so the resultant PRF sampling is
x100).
ii) Re-center the PRF by shifting it to its centroid. (Note that the estimate of the centroid of a source is
itself a function of the method used to determine the centroid, so ideally you would use equivalent
2
x
2
y