The Challenge of WIRCam Photometry of the Andromeda Galaxy Jonathan Sick, Queen's University I will discuss the engineering challenges of our program to produce the first high-resolution surface brightness map and resolved stellar catalog in the near-Infrared of the nearest neighbouring spiral galaxy, the Andromeda Galaxy (M31). CFHT/WIRCam J and Ks photometry of M31 can provide the best assessment of stellar populations for an external spiral galaxy because stars are resolved throughout the disk, and with little dust obscuration. The comparison of the integrated light to the resolved stellar population for discrete regions of M31 will permit a much needed calibration of NIR stellar population models and mass estimators. Yet M31 is also a formidable target since its large angular size makes straightforward sky subtraction nearly impossible. From our original 2007 WIRCam campaign we learned that even 5 minute time lags between a sky sample and a target image will create 10% uncertainty in the true sky level. By studying the behaviour of sky fields and flat fields, we believe that spatial structure may be as significant as temporal variations in sky estimates. We completed a 2009 WIRCam campaign with a nodding program designed to minimize both spatial and temporal sky variations. I will show a preview of those results, and sketch the final path towards high performance sky estimation. This talk will provide insight into the challenges and lessons learned in developing a user-level pipeline for a large-area WIRCam survey.