Inspired by Howard Trottier's picture of this galaxy group, I decided to try it myself. The large elliptical galaxy is NGC 5557.  From its outer shells and tidal tails, it's clear that it had just gone through a violent merging and relaxing process.  The two smaller spiral galaxies, NGC 5544/5545 (together aka. Arp 199) are actively interacting with each other too. The three galaxies have similar redshifts (within 200 km/s) so they belong to the same group.  The two smaller galaxies will sooner or later be devoured by NGC 5557.

This is an LRGB composition. I spent x hr on the L filter, and y hr on the Johnson-Cousins RVB filters. Because the RVB images are also very deep, at the end I stack them together with the standard L to form a "super L." This improves the quality of the luminance layer a bit. The stacking was done in PI. The LRGB composition and subsequent processing were done in PS.

One thing I learned during the processing is that it is a good idea to run PI's local normalization before stacking on multi-night deep imaging like this.  This is because as the target rises and falls, the direction of the gradient changes relative to the target. If the exposures with many different gradients are stacked, the background of the final stacked image will have a very complicated shape, unlike that in each individual exposure there is just a very simple gradient. As a result, background modeling and removal can hardly be perfect in such a stacked multi-night image. The imperfect gradient removal can hamper the detection of faint diffuse features like the tidal tail here. On the other hand, if we use local normalization (or the normalize scale gradient script), all exposures will be forced to have the same gradient as the reference exposure. In that reference exposure, the gradient can be easily modeled by a simple function (like a first degree polynomial in ABE). This makes things much easier.

For fun, I also uploaded a high-contrast inverted B&W version so you can better see the faint tidal features.