Difference imaging photometry of blended gravitational microlensing events with a numerical kernel

The numerical kernel approach to difference imaging has been implemented andapplied to gravitational microlensing events observed by the PLANETcollaboration. The effect of an error in the source-star coordinates isexplored and a new algorithm is presented for determining the precisecoordinates of the microlens in blended events, essential for accuratephotometry of difference images. It is shown how the photometric reference fluxneed not be measured directly from the reference image but can be obtained frommeasurements of the difference images combined with knowledge of thestatistical flux uncertainties. The improved performance of the new algorithm,relative to ISIS2, is demonstrated.