[Paper Review] Three photometric methods tested on ground-based data of Q 2237+0305

I. Burud, R. Stabell+6 more|arXiv (Cornell University)|Sep 9, 1998

Adaptive optics and wavefront sensing1 references3 citations

TL;DR

This study evaluates three photometric analysis methods—automatic image decomposition, CLEAN algorithm, and MCS deconvolution—on ground-based data of the Einstein Cross quasar Q 2237+0305. The MCS deconvolution method outperforms the others, maintaining sub-0.04 magnitude 1σ error even at poor seeing (up to 1.7''), while the other methods show increased errors beyond 0.04–0.1 magnitudes under similar conditions.

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.

Motivation & Objective

• To assess the reliability and accuracy of three photometric analysis techniques on ground-based observations of the gravitationally lensed quasar Q 2237+0305.
• To determine how atmospheric seeing conditions affect photometric precision across different algorithms.
• To investigate the impact of galaxy model complexity on photometric error budgets, particularly in the presence of seeing degradation.
• To evaluate the role of reddening and microlensing effects in quasar image photometry.
• To propose an optimized observational strategy for future photometric monitoring of the Einstein Cross.

Proposed method

• Applied three independent photometric algorithms: automatic image decomposition, CLEAN algorithm, and the MCS deconvolution code to four-band ground-based data from the Nordic Optical Telescope (NOT).
• Used a numerical galaxy model in the MCS deconvolution method to better represent the lensing galaxy's light distribution and reduce seeing dependence.
• Performed photometry on the four quasar images (A, B, C, D) using each method, comparing results across techniques to assess systematic and random errors.
• Quantified photometric errors as 1σ uncertainties, evaluating their dependence on seeing full width at half maximum (FWHM).
• Analyzed color-dependent effects and reddening by comparing photometry across bands and assessing consistency with extinction from the lensing galaxy or microlensing.
• Evaluated the stability of results when varying the assumed light profile of the lensing galaxy, particularly comparing simple parametric models to numerical models.

Experimental results

Research questions

• RQ1 How do the photometric results from automatic image decomposition, CLEAN, and MCS deconvolution compare in terms of accuracy and error budget under varying seeing conditions?
• RQ2 To what extent does the choice of galaxy light profile model affect photometric precision, especially at poor seeing?
• RQ3 Are the observed photometric variations in the quasar images consistent with microlensing or interstellar extinction in the lensing galaxy?
• RQ4 Can the MCS deconvolution method maintain high photometric accuracy when seeing degrades beyond 1.0'', and what is its performance limit?
• RQ5 What observational strategy should be adopted for future high-precision photometric monitoring of the Einstein Cross based on this analysis?

Key findings

• No significant photometric variations were detected in any of the four quasar images across the observation period.
• The MCS deconvolution method achieved 1σ photometric errors below 0.04 magnitudes even at seeing conditions as poor as 1.7'', demonstrating superior robustness.
• The automatic image decomposition and CLEAN algorithm methods exhibited increased errors, reaching 0.04–0.1 magnitudes, particularly when seeing exceeded 1.0''.
• Photometric accuracy was highly sensitive to the assumed light distribution model of the lensing galaxy, with numerical models reducing seeing dependence.
• Reddening effects were observed in the quasar images, consistent with either extinction from the lensing galaxy or color-dependent microlensing effects.
• The use of a numerical galaxy model in the MCS method allowed for better modeling of non-homogeneous structures, enhancing overall photometric fidelity.