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Astron. Astrophys. 339, 701-708 (1998)

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Three photometric methods tested on ground-based data of Q 2237+0305 *

I. Burud 1, 2, R. Stabell 2, 3, P. Magain ** 1, F. Courbin 1, 4, R. Ostensen 1, 5, S. Refsdal 3, 6, M. Remy 1 and J. Teuber 3

1 Institut d'Astrophysique, Université de Liège, Avenue de Cointe 5, B-4000 Liège, Belgium
2 Institute of Theoretical Astrophysics, University of Oslo, Pb. 1029 Blindern, N-0315 Oslo, Norway
3 Centre 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 Tromso, N-9037 Tromso, Norway
6 Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany

Received 29 June 1998 / Accepted 24 August 1998

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 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 [FORMULA], 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[FORMULA] error below 0.04) even for frames with seeing as bad as [FORMULA].

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.

Key words: quasars: individual: Q 2237+0305 – gravitational lensing – techniques: image processing

* Based on observations obtained at NOT, La Palma.
** Maître de Recherches au Fonds National Belge de la Recherche Scientifique

Send offprint requests to: I. Burud, Liège address, (burud@astro.ulg.ac.be)

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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