The term "post-processing" comes from the film world and is applied fairly loosely in the CCD world to any image processing or edting that is accomplished after the initial final processing has been completed. Usually, such work is done in general digital graphics programs such as Photoshop or Paint Shop Pro and serves the purpose of adjusting the format or editing remnant artifacts of the image. Modern graphics programs are very powerful, however, and can be used to drastically change the luminance and chrominance relationships in color images, repair gradients, sharpen, smooth, etc. Photoshop, for example, now performs many functions on 16-bit image layers, which leads to utility for some high-resolution processing and editing.
a. Resampling and Cropping. The final image may not be the desired overall size (in pixel dimensions) that you want or it may have unesthetic framing or edges due to imperfect overlap of LRGB frames. Resampling and cropping are two functions performed very effectively by graphics packages. Resampling increases or decreases the number of pixels in one or both axes of an image using a method which correlates adjacent pixel values to create a smooth pixel-to-pixel transition. It can shrink or enlarge images very esthetically. Cropping basically cuts off one or more undesired edges of an image.
b. Brightness, Contrast, and Color Adjustments. Graphics programs generally have very effective tools for "tweaking" the brightness, contrast, and color balance of images. Obviously, if the initial processing has been done correctly in terms of white-balanced color calibration and linear stretching, little adjustment needs to be done in these areas; however, if you desire to produce color balance "to taste" rather than according to a more scientific calibration methodology you will find graphics program tools very much to your liking.
c. Sharpening and Smoothing. Graphics programs have very effective filtering tools for sharpening and smoothing composite images. The unsharp mask in Photoshop is extraordinary. Sometimes, iterating mild sharpening and smoothing filters can produce a very esthetic blend of high-resolution features with a smooth background.
d. Noise Filters (despeckle, median, etc.) and Pixel Editing. Similarly, the noise filters in graphics programs can be highly effective. These include a variety of tools based on median filtering and other algorithms. If previous processing has not eliminated all the "speckles" or artifacts in an image, these filters or pixel editing functions can be useful. Although one must be very careful with editing functions (after all, we aren't just "creating" images...right?), they can be very handy in esthetically eliminating blooming artifacts and other unrealistic and undesired pixel values.
e Faded Minimum Filter (PhotoShop). One post-processing tool stands alone, in my opinion, in its effectiveness in improving the quality of color composite images. This is the Faded Minimum Filter in Photoshop. This is an erosion-type filter, which acts to reduce the size of high-frequency areas in an image, such as stars. Images with somewhat bloated stars due poor seeing, focus, or tracking can benefit greatly from the application of this tool. In addition, it employs a variety of user-selectable modes and a wide range of opacity, which in combination can improve the appearance of the image background and highlights. I recommend using the smallest radius filter (one pixel) and the Hard Light Mode. Experiment and enjoy!
f. File Formats. Any graphics package will load and save a variety of file formats. For retention, display, and delivery of a small file size with a high quality appearance, save your post-processed results as a JPEG file with moderate file compression.
g. Creating mosaics. Finally, although many would not consider it to be post-processing, the creation of mosaics is often best accomplished in graphics programs, especially Photoshop. Rob Gendler is a very fine imager and expert Photoshop user who published an article in Sky and Telescope in 1999 on piecing together mosaics in Photoshops. See the final draft of that article on Rob's website at http://www.robgendlerastropics.com/Article3.html . Jan Wisniewski is another expert creator of mosaics. See his article on mosaic images at http://www.astrosurf.com/jwisn/assembling.htm .
Above all, ENJOY!