The Einstein Cross, Q~2237+0305, has been photometrically observed in four
bands on two successive nights at NOT (La Palma, Spain) in October 1995. Three
independent algorithms have been used to analyse the data: an automatic image
decomposition technique, a CLEAN algorithm and the new MCS deconvolution code.
The photometric and astrometric results obtained with the three methods are
presented. No photometric variations were found in the four quasar images.
Comparison of the photometry from the three techniques shows that both
systematic and random errors affect each method. When the seeing is worse than
1.0", the errors from the automatic image decomposition technique and the Clean
algorithm tend to be large (0.04-0.1 magnitudes) while the deconvolution code
still gives accurate results (1{sigma} error below 0.04) even for frames with
seeing as bad as 1.7". Reddening is observed in the quasar images and is found
to be compatible with either extinction from the lensing galaxy or colour
dependent microlensing. The photometric accuracy depends on the light
distribution used to model the lensing galaxy. In particular, using a numerical
galaxy model, as done with the MCS algorithm, makes the method less seeing
dependent. Another advantage of using 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.Comment: 9 pages, accepted for publication in A&