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

TL;DR

This study compares three photometric analysis methods on ground-based data of Q 2237+0305, finding the deconvolution code yields more accurate results under poor seeing conditions and discussing implications for future monitoring.

Contribution

It introduces and evaluates the effectiveness of a new MCS deconvolution algorithm for photometry of gravitational lens systems, demonstrating its advantages over existing methods.

Findings

Deconvolution code provides more accurate photometry under poor seeing conditions.

No significant photometric variations detected in the quasar images.

Reddening consistent with extinction or microlensing effects.

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 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.