We present wide-field near-infrared J and K _ { s } images of the Andromeda Galaxy taken with WIRCam on the Canada-France-Hawaii Telescope ( CFHT ) as part of the Andromeda Optical and Infrared Disk Survey ( androids ) .
This data set allows simultaneous observations of resolved stars and NIR surface brightness across M31 ’ s entire bulge and disk ( within R = 22 kpc ) , permitting a direct test of the stellar composition of near-infrared light in a nearby galaxy .
Our survey complements the similar Panchromatic Hubble Andromeda Treasury survey by covering M31 ’ s entire disk , rather than a single quadrant , at similar wavelengths , albeit with lower spatial resolution .
The primary concern of this work is the development of NIR observation and reduction methods to recover a uniform surface brightness map across the 3 \arcdeg \times 1 \arcdeg disk of M31 .
This necessitates sky-target nodding across 27 WIRCam fields .
Two sky-target nodding strategies were tested , and we find that strictly minimizing sky sampling latency does not maximize sky subtraction accuracy , which is at best 2 % of the sky level .
The mean surface brightness difference between blocks in our mosaic can be reduced from 1 % to 0.1 % of the sky brightness by introducing scalar sky offsets to each image .
We test the popular Montage package , and also develop an independent method of estimating sky offsets using simplex optimization ; we show these two optimization schemes to differ by up to 0.5 mag arcsec ^ { -2 } in the outer disk .
We find that planar sky offsets are not acceptable for subtracting residual backgrounds across WIRCam fields that are much smaller than the mosaic area .
The true surface brightness of M31 can be known to within a statistical zeropoint of 0.15 % of the sky level ( 0.2 mag arcsec ^ { -2 } uncertainty at R = 15 kpc ) .
We also find that the surface brightness across individual WIRCam frames is limited by both WIRCam flat field evolution and residual sky background shapes .
To overcome flat field variability of order 1 % over 30 minutes , we find that WIRCam data should be calibrated with real-time sky flats .
Due either to atmospheric or instrumental variations , the individual WIRCam frames have typical residual shapes with amplitudes of 0.2 % of the sky after real-time flat fielding and median sky subtraction .
We present our WIRCam reduction pipeline and performance analysis here as a template for future near-infrared observers needing wide-area surface brightness maps with sky-target nodding , and give specific recommendations for improving photometry of all CFHT/WIRCam programs .