  We observed Proxima Centauri with the Spitzer Space Telescope InfraRed Array
Camera (IRAC) five times in 2016 and 2017 to search for transits of Proxima
Centauri b. Following standard analysis procedures, we found three asymmetric,
transit-like events that are now understood to be vibrational systematics. This
systematic is correlated with the width of the point-response function (PRF),
which we measure with rotated and non-rotated Gaussian fits with respect to the
detecor array. We show that the systematic can be removed with a novel
application of an adaptive elliptical-aperture photometry technique, and
compare the performance of this technique with fixed and variable
circular-aperture photometry, using both BiLinearly Interpolated Subpixel
Sensitivity (BLISS) maps and non-binned Pixel-Level Decorrelation (PLD). With
BLISS maps, elliptical photometry results in a lower standard deviation of
normalized residuals, and reduced or similar correlated noise when compared to
circular apertures. PLD prefers variable, circular apertures, but generally
results in more correlated noise than BLISS. This vibrational effect is likely
present in other telescopes and Spitzer observations, where correction could
improve results. Our elliptical apertures can be applied to any photometry
observations, and may be even more effective when applied to more circular PRFs
than Spitzer's.
