P27	July 1998

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
² Institute of Theoretical Astrophysics, University of Oslo, Pb. 1029 Blindern, 0315 Oslo, Norway
³ Center for Advanced Study, Drammensveien 78, N-0271 Oslo, Norway
⁴ URA 173, CNRS-DAEC, Observatoire de Paris, F-92195 Meudon Principal Cédex, France
⁵ Department of physics, University of Tromsø, Norway
⁶ Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany

^* Maître de Recherches FNRS, Belgium

⁺ Based on observations obtained at NOT, La Palma.

Abstract

     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 from the three methods are presented. No photometric variations were found in the four quasar images. Comparison of the photometry obtained with the three methods show 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 gives more accurate results (1  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.