– 22 – Fig. 1.— Examples of data filtering and image subtraction. All six stamps cover the same area (1.2 × 1 arcmin $^{2}$) and are centered on one of the most variable AGNs in the Bo¨ otes field, SDWFS J143032.42+332158.8, with z = 0 . 43, σ$_{12}$ = 13 . 5, and r = 1 . 0 (see § 3). Top row: stamp of the original 3.6 µ m image (a). Before we do the image subtraction, we filter the images in order to convert the Spitzer IRAC’s triangle-shaped PSFs into more Gaussian-like ones (b). Middle row: subtracted images for the 3.6 µ m channel. The AGN brightened by about 0.5 mag on a baseline of 4 years between epochs 1 (c) and 3 (d). Black (white) color here reflects the source emitting less (more) flux than in the template image defined by the average of all four epochs. All non-variable objects are subtracted out. Bottom row: subtracted images for the 4.5 µ m channel. The AGN brightened by ∼ 0 . 6 mag between epochs 1 (e) and 3 (f), consistent with what is seen in the 3.6 µ m channel.