Precision Photometry for Q0957+561 Images A and B

Since the persuasive determination of the time-delay in Q0957+561, muchinterest has centered around shifting and subtracting the A and B light-curvesto look for residuals due to microlensing. Solar mass objects in the lensgalaxy produce variations on timescales of decades, with amplitudes of a fewtenths of a magnitude, but MACHO's (with masses of order $10^{-3}$ to$10^{-7}M_\odot$) produce variations at only the 5% level. To detect such smallvariations, highly precise photometry is required. To that end, we have used 200 observations over three nights to examine theeffects of seeing on the light-curves. We have determined that seeing itselfcan be responsible for correlated 5% variations in the light-curves of A and B.We have found, however, that these effects can be accurately removed, bysubtracting the light from the lens galaxy, and by correcting for crosscontamination of light between the closely juxtaposed A and B images. We findthat these corrections improve the variations due to seeing from 5% to a levelonly marginally detectable over photon shot noise (0.5%).Comment: 21 Pages with 9 PostScript figures, AASTeX 4 (preprint style