## Overview

As a result of geometric distortion, the WFC3/UVIS CCDs and WFC3/IR detector contain pixels that vary in their area on the sky. Thus, a larger pixel collects more photons relative to a smaller pixel, leading to an overall gradient in an image of an intrinsically uniform background. The flat-fielding step in the calwf3 pipeline is designed to correct for this gradient and produce images with a flat background. As a result, surface brightness photometry of extended sources will be correct in flat-fielded data products (FLT), while the measured brightness of astronomical sources will vary depending on the position of the object on the detector.  A contour plot of the relative pixel size across the UVIS CCDs, normalized to the central pixel, is shown in the left panel of Figure 1, where the ratio of maximum to minimum pixel area is 1.074. The variation of pixel area across the IR channel is shown in the right panel, where the maximum deviation from the central value is 4.1%.

To achieve uniform aperture photometry of point sources across the detector, observers may either use FLT images, corrected by a pixel area map (PAM), or distortion-free drizzled (DRZ) images. The AstroDrizzle process of the DrizzlePac software corrects FLT images for distortion, yielding DRZ images with a flat sky and pixels with uniform area. Therefore, photometry of any source in a DRZ image will yield the same count rate irrespective of the position of the source on the image. Photometry measured on a calibration pipeline image (FLT) requires a field-dependent correction factor to achieve uniformity in the measured count rate of an object across the field.

This correction, in the form of an image, is called the Pixel Area Map (PAM) and comes from the derivatives of the geometric distortion polynomial. By multiplying the calibrated (FLT) images by the PAM, users will recover the same count rate on both types of images (FLT and DRZ) and the same zeropoint will apply to both data products:

DRZ = FLT * PAM

where the FLT image has been converted to units of count rate.

An illustration of the PAM correction is presented in Section 9.1  of the WFC3 Data Handbook.  A more detailed discussion is provided in ISR 2010-08, which highlights the unique choice for the PAM normalization that differs from previous HST instruments.