Aims. We seek to understand the impact on shape estimatorsobtained from circular and elliptical shapelet models under tworealistic conditions: (a) only a limited number of shapelet modes areavailable for the model, and (b) the intrinsic galactic shapes are notrestricted to shapelet models. Methods. We create a set of simplistic simulations, in which thegalactic shapes follow a Sérsic profile. By varying the Sérsic indexand applied shear, we quantify the amount of bias on shear estimatesthat arises from insufficient modeling. Additional complications fromPSF convolution, pixelation, and pixel noise are also discussed. Results. Highly elliptical galaxy shapes cannot be accuratelymodeled within the circular shapelet basis system and are biasedtowards less elongated shapes. This problem can be cured by allowingelliptical basis functions. Another problem occurs for galaxies whoseradial profile differs strongly from the Gaussian weighting functionemployed in both the circular and the elliptical shapelet bases. Forgalaxies with large Sérsic indices, shear estimates from circularshapelets are biased low even for small apparent ellipticities becauseof the preference for round models, and shear estimates from ellipticalshapelets depend critically on accurate ellipticity priors. Independentof the particular form of the estimator, the bias depends on the trueintrinsic galaxy morphology, but also on the size and shape of the PSF.Conclusions. As long as the issues discussed here are notsolved, we question that the shapelet method can provide weak-lensingmeasurements for all observable galaxies (independent of their Sérsicindex) with an accuracy demanded by upcoming missions and surveys. Anaccurate and reliable calibration, specific for the dataset underinvestigation, would be required but is difficult to infer consistentlyfrom the data. Key words: gravitational lensing: weak - techniques: image processing - methods: numerical
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