How to read the filtered images?

The original image:

• The image is the result of stacking all the available frames in one band (the I-band in most cases) after their pre-processing.
• It is an false-colour image. The colour table was individually choosen to get the best contrast.

The isophotal map:

• This is a contour map of the adaptively smoothed original image.
• The contour levels were set by hand to get more or less equidistant isophotes. No kind of calibration was done.
• The only aim is an easier imagination of the shape of the galaxy and the visiualization of possible isophote twisting, internal structure etc.

The Gradient image:

• The image represents the first derivative of the surface brightness distribution. Since the gradient is a vector, only its absolute value is coded here in false colour (red => steep gradients).
• The gradient is particulare suited to visualize sharp edges (even if they have low contrast), like shells or ripples. Because these types of structure are relatively rare in galaxies, for the majority of the objects the Laplacian image is more instructive to show the faint structures.

The Laplacian image:

• The image represents the second derivative (i.e. the curvature) of the surface brightness distribution. For convenience, the curvature is inverted: negative values (i.e. the blue part of the colour table) corresponding to concavity and positive values (i.e. the red part of the colour table) to convexity.
• The images do not retain any photometric information and must be used only to disentangle structures and to study their morphology. The Laplacian image, however, provides an independent way to estimate the size and trace the geometry of the hidden features, thus helping in masking out the structures in order to model a ``clean'' galaxy to be subtracted from the original image (cf. the case of NGC 3384, Busarello et al. 1996).
• Every convex spike - such as a star - is surrounded by a concavity. Due to the fact that the filter has a square shaped response, spikes usually have square-shaped negative artifacts. This squared shape, far from being a nuisance, helps in disentangling artifacts and real structures.
• Along the minor axis, and outside the seeing blurred galaxy core, there is almost always a negative signature produced by the concavity of the luminosity profile (cf. the r^1/4 law which is usually a good approximation of the surface brightness profile). However, since the two-dimensional curvature is the average of the one-dimensional curvatures measured in the two main curvature directions, in elongated systems the very central convex part is more extended along the major axis (in the case of high axis ratios there is no negative curvature along the major axis at all). This effect has nothing to do with what we call inner disks, which are clearly differentiated from the aforementioned fake-features by their profiles and their well-defined edges along the main axes. Since it preserves the spatial resolution, the Laplacian morphology allows us to detect edge-on or close to edge-on disks and rings as patches of enhanced emission which are more or less symmetric with respect to the nuclei of the galaxies. It should always be kept in mind, however, that the same Laplacian morphologies may be produced by completely different structures such as weak polar rings or bars.

The notes on structures:

• We describe the structures found in the filtered images for every object and give our interpretation here.
• Please, compare Capaccioli et al. 1997.

• The description of the atlas
• The Atlas cover page