COLD IRAC ONLY: Corrections for Intrapixel Sensitivity Variations ("the Pixel Phase Effect")
Due to the undersampled nature of the IRAC PRF, Channel 1 (3.6 µm),
and to a lesser extent Channel 2 (4.5 µm), show intra-pixel gain
variations that depend on the location of a star within a pixel.
These cause systematic errors in aperture fluxes as a function of
"pixel phase", the offset of the stellar centroid from the center
of the central pixel in which it is located, (xphase,yphase) =
(x,y) - ROUND(x,y). Early in the cryogenic mission, the point source
aperture flux was fit with a radial function in pixel phase
(Reach et al 2005). Later, Mighell et al 2008; Download
PDF, 0.7 MB*
found that the intra-pixel gain variation is better described by
a two-dimentional function of (x,y) pixel phase, primarily because
the peak of the response is not at the center of an IRAC Channel 1
At the beginning of the warm IRAC mission, we found that the pixel
phase response peak-to-peak variation increased by a factor of
about 2 in both Channels 1 and 2. The larger dynamic range enabled
us to model the intra-pixel gain in terms of the sum of gaussian
functions in measured (xphase,yphase). See the warm IRAC documentation
for more details. The pixel phase response function is normalized
such that its (integral) average across a single pixel is unity.
We display below images of the pixel phase response for channels 1 and 2. This
function was fit simultaneously with the Array
Location Dependent photometric correction function using the entire cryogenic
star dataset. An IDL function for correcting aperture photometry for both these
functions can be
downloaded here. Pixel phase correction images (the inverse of
the response) that can be applied (multiplied) to data can be downloaded in fits format
These images are subsampled in 0.01 pixel increments, starting at -0.5 and ending at
+0.5 (in both
xphase and yphase), and may be interpolated to the phase of a given stellar centroid
by the measured aperture flux.
IRAC cryogenic pixel response model,
showing intra-pixel gain variations as a function of pixel phase.
Only Channel 1 (3.6 m) and Channel 2 (4.5 m) have significant
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