Introduction

Cometary grains evolve with the distance from the nucleus and are not the same in the regular coma as in jets, fans, shells, and in other specific cometary features. Nonimaging (aperture) observations refer to averages across the coma and can therefore only provide average characteristics for fresh, evolved, regular and peculiar particles mixed together. They cannot be used to obtain realistic characteristics of any specific comet dust populations. Although the importance of images in studying cometary dust has never been brought into question, studies usually concentrate on brightness images. These reveal the dust-filling factor and are therefore well suited to study dust distributions throughout the coma and yield at least relative dust concentrations. Here, we take advantage of the fact the dust concentration dependence can be removed through a simple normalization inherent in brightness ratios such as polarization or colour. We analyze colourimetric and polarimetric images of comets Hyakutake, Tabur and Hale-Bopp to reveal properties and evolution of cometary dust.

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