3 PHOTOMETRIC METHODS TESTED ON GROUND-BASED DATA OF Q 2237+0305

The Einstein Cross, Q 2237+0305, has been photometrically observed in four bands on two successive nights at NOT (La Palma, Spain) in Octobe r 1995. Three independent algorithms have been used to analyse the dat a: an automatic image decomposition technique, a CLEAN algorithm and t he new MCS deconvolution code. The photometric and astrometric results obtained with the three methods are presented. No photometric variati ons were found in the four quasar images. Comparison of the photometry from the three techniques shows that both systematic and random error s affect each method. When the seeing is worse than 1.'' 0, the errors from the automatic image decomposition technique and the Clean algori thm tend to be large (0.04-0.1 magnitudes) while the deconvolution cod e still gives accurate results (la error below 0.04) even for frames w ith seeing as bad as 1.'' 7. Reddening is observed in the quasar image s and is found to be compatible with either extinction from the lensin g galaxy or colour dependent microlensing. The photometric accuracy de pends on the light distribution used to model the lensing galaxy. In p articular, using a numerical galaxy model, as done with the MCS algori thm, makes the method less seeing dependent. Another advantage of usin g a numerical model is that eventual non-homogeneous structures in the galaxy can be modeled. Finally, we propose an observational strategy for a future photometric monitoring of the Einstein Cross.