1999ApJ...518..153C

Since the persuasive determination of the time delay in Q0957+561, much interest has centered around shifting and subtracting the A and B light curves to look for residuals due to microlensing. Solar mass objects in the lens galaxy produce variations on timescales of decades, with amplitudes of a few tenths of a magnitude, but MACHOs (with masses of order 10^–3-10^–7 M_☉) produce variations at only the 5% level. To detect such small variations, highly precise photometry is required. To that end, we have used 150 observations over three nights to examine the effects of seeing on the light curves. We have determined that seeing itself can be responsible for correlated 5% variations in the light curves of A and B, which could masquerade as microlensing events. We have found, however, that these effects can be accurately removed by subtracting the light from the lens galaxy and by correcting for cross contamination of light between the closely juxtaposed A and B images. We find that these corrections improve the variations due to seeing from 5% to a level on the order of the photon shot noise (0.5%). We also find that this corrected aperture photometry method is more reliable at removing the errors due to seeing than is a direct implementation of point-spread function fitting photometry.