Image Construction

Galaxies that are comparable or larger than the 2MASS Atlas Image size (8X16 arcmin) require multiple image mosaicing. Due to the nature of 2MASS scans and background variations, this procedure is in practice very complicated. We take advantage of the fact that for images that belong to the same scan they will have correlated background structure due to atmospheric variations ("airglow" and clouds) along the scanning direction. Hence we first fit out the background for each scan, then fit the residuals across scans. The surface fit corresponds to a cubic polynomial, with a scale length that depends on the size of the target galaxy. The fitting procedure is roughly as follows:

collect images from database
download point source catalog centered on galaxy
collect basic size and shape information from NED
determine scale size of background fit polynomial
resample ATLAS images (coadds) on a common 1" grid
correct each coadd for absolute calibration differences
mask stars and target galaxy
fit cubic polynomial surface to each column, point by point (i.e., do a running fit to each pixel along the scan direction)
fit cubic polynomial to each row residual, point by point
fit residuals across scan boundaries
remove resultant background from mosaic