6 Concluding remarks

This paper has been devoted to the description of techniques and the presentation of data about the B-R colour distributions in E-type galaxies, as measured from rather good resolution frames. The scientific results have been given in the three previous paper of this series and again summarized in the Introduction.

Therefore the concluding remarks will be devoted to technical matters.

1.

It has been proven that colour measurements can be extended inside the "seeing'' degraded regions of galaxian images, if a suitable treatment is applied to equalize the different PSF's in the two frames involved in any colour. This treatment may be a convolution of the "best'' frame by a correcting function, or eventually the deconvolution of the "worse'' by this same function. This procedure is successful only if the two frames have similar "seeing'', so that the correcting function is narrow enough, and well measured from a suitable star in the field.

2.

The short discussion of available colour surveys of E-type galaxies in the above section, suggests that the data are not in a well defined and unique colour system, and eventually suffer from serious calibration errors for a number of objects. A reconsideration of the data, and perhaps new observations, may be needed to obtain a fully reliable set of colours and their gradients, in the usual optical colours, for a significant sample of E-type objects.