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tokens: 12 of the exposures do not approach the “saturation” threshold (blue dots), while 14 of the exposures have one pixel with a significant nonlinearity correction applied (red dots). Aperture photometry of the individual HST -calibrated FLT files shows a a bimodal pattern with a systematic difference of 0.2 mag. The bimodality is a result of the dither pattern: in the exposures with a correction applied, the PSF of the star is centered perfectly on a single pixel while in the other exposures the center falls in a gap between pixels, lowering the peak flux in any one pixel below the nonlinearity threshold. Figure 1. Aperture photometry of the SNAP observations of XZ Cyg. Each point corresponds to a single frame, points are color-coded based on their dither position. The ∼0.2 mag difference caused by the dither pattern is a result of an overcorrection for nonlinearity in a single bright pixel at dither point a. Although the stars are well isolated, the presence of significantly nonlinear pixels without a proper correction means that we cannot apply simple aperture photometry to our direct SNAP and GO observations. Our solution to this situation is to mask pixels in frames with poor nonlinearity corrections and then undertake PSF photometry. 3.1. Direct Imaging Photometry Photometry was carried out with DAOPHOT (Stetson 1987 , 1994 ) on all individual direct imaging “FLT” frames with the HST Pixel Area Map correction applied. Because we cannot generate a unique PSF for each frame, we used DAOPHOT to generate and apply one PSF for use in the 64 × 64 images and then generated a separate PSF for the 128 × 128 images. To generate the PSF, we used stars that are “clean” (i.e., images that do not have pixels that approach the nonlinear regime): 156 images for the 64 × 64 subarray and 208 for the 128 × 128 subarray or 26 images of six and eight stars each, respectively. To determine which parameters to assign to our model PSF, we generated several thousand individual PSF models with PSF radius and fitting radius varied in integer pixel increments from 2 to 17 and the seven different PSF functional forms available in DAOPHOT. We then compared the magnitude derived from each model PSF for each star with aperture photometry and chose PSF parameters that, on average, best reproduced the aperture magnitudes. We assigned a PSF radius of 14 pixels to the 64 × 64 PSF model and 17 pixels for the 128 × 128 PSF model and used a fitting radius of 12 pixels and a Moffat function with β = 3.5 for both PSF models. We also use the “clean” images to test the effect of pixel masking on our PSF photometry. We chose the brightest pixel in each image, masked it, and redetermined the magnitude using the same range of PSF parameter values. Regardless of the parameters used, masking pixels did not significantly impact DAOPHOT PSF photometry measurements. When masking stars in our 64 × 64 images, we found a systematic offset of −0.0017 ± 0.0022 mag, and for 128 × 128 images we