Now it's time to get back to the RGB colour image.  Open the RGB raw image.  Here the Channel count is 3, and the Interleaved box should be unchecked (the 3 channels are in separate blocks).

Do the same series of Positive and Negative Curve Stretches (on the combined RGB channel) on this image, but without de-selecting the stars.  Eventually you will get a horrible looking picture like this.  Don't panic!  Histogram stretching tends to saturate colours and accentuate any colour casts there may have been in the original.  In the case of a webcam, the colour balance will almost certainly be way off.  In this case there is too much red and probably not enough green.

On this image, I did the following Curve adjustments on the individual colour channels.  I normally keep the sky background slightly blue - this is an aesthetic decision.  The background level is kept somewhat high because in the final image this level will be set by the luminosity component and the higher levels in the RGB helps to reduce colour noise in the dark parts of the image.

This part of the process is optional and really depends on how bad the noise in the luminosity image is.   I use SGBNR (Selective Gaussian Blur Noise Reduction) which, in summary, masks the lighter parts of the image, and applies a gaussian blur to the dark areas, where noise in astronomical images is usually most apparent.  The Photoshop 16 bit luminosity image is exported as a 16 bit TIFF file which SGBNR reads.   I finish all the major histogram stretching in Photoshop before I do this because when the image comes back from SGBNR it is converted to 8 bits.

SGBNR is freeware and can be downloaded from Pleiades Astrophoto.

Photoshop functions that work on 16 bit images are restricted, so before proceeding I convert both the Y and RGB images to 8 bits (Image - Mode - 8Bits/Channel).  Using the RGB image, first do a Gaussian Blur (Filter - Blur - Gaussian Blur...).  Experiment with the radius - 2 is usually fine.  The reason for this is to reduce colour noise, but overdone, colour definition can be lost.

Now select the whole of the Y image (Select - All).  Copy it and paste onto the RGB image.  Photoshop adds the Y image as a layer on top of the RGB.  The default blending mode is Normal, but select Luminosity instead.  This is now an LRGB image of M83.

If necessary, at this stage colour saturation can be enhanced.  I use a technique described by Rob Gendler, which he calls Multiple Luminance Layering.  Essentially the colour saturation of the RGB image is increased in Photoshop, the result is combined with the Luminance image at 50% opacity.  This image is then flattened, and the process repeated until the desired saturation is achieved.  In this way colour saturation can be increased without an unacceptable increase in colour noise.

Once I am happy with the colour saturation, I crop the image and use the Clone tool in Photoshop to correct trailed stars.  The image still lacked some sharpness, so I applied the Unsharp Mask filter.  Unsharp Masking tends to introduce dark halos around bright stars, so I selected these stars by clicking on the centre of a star with the Magic Wand (Tolerance = 60, 'Contiguous' unchecked).  Then I de-select the core of the galaxy and invert the selection.  The Unsharp Mask applies then only to parts of the image other than the bright stars.  Some softness was restored to the stars by re-inverting the selection, enlarging the selection by 3 pixels (to include whole stars) and applying a slight Gaussian Blur.  Some dark halos remained, so before de-selecting, I increased the brightness of the selection a bit.

Finally I added a caption and... this is the final image.