Five years (1992-1997, ~2650 images) of optical observations of thegravitational lens system QSO 0957+561 were analyzed using a newphotometry package developed by Jan Teuber and me. My supervisor wasProf. Rolf Stabell, and the CCD images were kindly provided by Prof.Rudy Schild.
The fully automated program was written in the IDL environmentand incorporated all operations needed to produce the final lightcurves of the quasar images; from source detection, field starphotometry, calibration and finally, photometry of the quasarimages. We also corrected for the light-contamination by the lensgalaxy and for the crosstalk between the closely juxtaposed quasarcomponents. Both aperture photometry and PSF-fitting photometry wereperformed.
From the brightness data, the time delay between the twin images wasdetermined utilizing two different methods: dispersion estimationtechnique and chi-square minimization. We inferred a time delay of~425 days, with an estimated 1-sigma formal error of about 1.2 days(which is probably too optimistic).
The effects of gravitational microlensing on the measured brightnessof the two quasar images were also briefly investigated. Microlensing-inducedvariations with a time scale of a year and amplitude of ~0.05magnitudes were observed, and a few other tentative fluctuations inthe microlensing curve had shorter time scales.